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Biochemistry ; Questions and Answers
BOOK · JANUARY 2013
DOI: 10.13140/RG.2.1.3676.0168
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Nalluri Mallikarjuna Rao
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Questions and Answers
Dr. N. MALLIKARJUNA RAO
BIOCHEMISTRY - Questions and Answers
Questions and Answers
BIOCHEMISTRY - Questions and Answers
BIOCHEMISTRY - Questions and Answers
Questions and Answers
Dr. N. MALLIKARJUNA RAO
Professor & HOD
Department Of Biochemistry
Vishnu Dental College, Bhimavaram - 534202.
BIOCHEMISTRY - Questions and Answers
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BIOCHEMISTRY : Questions and Asnwers
Dr. N. MALLIKARJUNA RAO
© 2013 SEEKAY Publications
First Edition : 2013
ISBN : 978-81-924169-3-9
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BIOCHEMISTRY - Questions and Answers
Preface
This book is written to help student in their preparation for
examinations. It meets needs of first year M.B.B.S., B.D.S.,
B.Sc.(N), B.P.T., M.Sc (Medical) and second year B.Pharm
students. Topics prescribed by Various Health Science,
Universities in India Vijayawada are included in the book. In
this book questions and answers are given for 21 topics.
Complex pathways are presented in a easy to remember way.
This book is written in such way that learning of questions and
answers given in each chapter makes student to acquire concept
or theme of that topic simultaneously. The book contains 495
questions. Of this answers are provided to 249 questions
remaining are model questions. Answers to 54 essay questions,
110 short questions and 85 very short or brief questions are
given in this book. Answers are given in simple language with
necessary diagrams or illustrations. Model questions given
enhances students ability to answer questions with alteration.
I am grateful to Sri K. Prasanna Kumar of Seekay Publications
for publishing the book.
BHIMAVARAM
DR. N. MALLIKARJUNA RAO
BIOCHEMISTRY - Questions and Answers
BIOCHEMISTRY - Questions and Answers
Contents
1. Cell, Membrane and Transport
001
2. Carbohydrates
007
3. Proteins, Plasma Proteins, Aminoacids and Peptides
013
4. Lipids
029
5. Enzymes
039
6. Nucleotides and Nucleic acids
058
7. Biological oxidation
068
8. Carbohydrate Metabolism
076
9. Lipid Metabolism
102
10. Protein and Amino acid Metabolism
120
11. Porphyrin and hemoglobin Metabolism
141
12. Nucleotide Metabolism
150
13. Replication, Transcription and Translation
159
14. Vitamins
173
15. Minerals
189
16. Water, electrolytes and acid –base balance
199
17. Nutrition and Energy Metabolism
205
18. Hormones
211
19. Organ function Tests
219
20. Xenobiotics
225
21. Cancer
229
BIOCHEMISTRY - Questions and Answers
CHAPTER - 1 | Cell Membrane & Transport
Chapter
1
Cell Membrane & Transport
1. Describe common structural and functional features of eukaryotic cell.
A. 1. Though mammals contain many types of cells which differ in function, shape, size
etc., they have common features.
2. All types of cells contain nucleus, membrane and sub cellular components etc.
3. Each cell component has uniqe structure and function.
Nucleus
1. It is located in the centre of most of the cells. It is surrounded by double layered membrane
in which pores are present.
2. Pores present in the membrane permits exchange of material between nucleus and other
structures of cell.
3. The outer membrane of nucleus is continuous with other membrane.
4. Chromosomes are present in the nucleus of human and other mammalian cells.
5. Chromatin is the substance present in chromosomes.
6. Chromatin is nucleoprotein which consist of DNA and proteins.
7. Nucleus also contain some amount of RNA.
8. DNA and RNA present in nucleus are carriers of genetic information.
NUCLEUS
Outer Nuclear Membrane
Chromosome
Inner
Nuclear Membrane
Nuclear Pore
Mitochondria:
1. Like nucleus it is also surrounded by double layered membrane.
2. The inner membrane forms folds which are named as cristae.
3. Knob like structures are present in cristae.
4. Matrix is the name given to space within inner membrane.
001
BIOCHEMISTRY - Questions and Answers
5. Number of mitochondria varies from one organ to other.
6. Mitochondria is the power house of the cell.
7. Size and shape of mitochondria depends on the function of organ in which they are present.
8. Electron transport chain, citric acid cycle, β-oxidation, ketone body formation, pyruvate
oxidation, few of heme biosynthesis and urea cycle enzymes are present in mitochondria.
MITOCHONDRIA
Outer
Membrane
Chistae
Knob
Inner Membrane
Matrix
Endoplasmic reticulum: This membranous net work is divided into smooth endoplasmic
reticulum and rough endoplasmic reticulum.
Smooth endoplasmic reticulum: It is also
known as microsomal fraction of cell. It appears
smooth due to the absence of ribosomes. It is
site of hydroxylation reactions of drugs and
steroids etc.
Rough Endoplasmic Reticulum
Nucleus
Smooth
Endoplasmic
Reticulum
Rough endoplasmic reticulum: It is
continuation of outer nuclear membrane. It
appears rough due to presence of ribosomes. It
Nucleus
Ribosome
is the site of protein synthesis.
Golgi complex: It is another membranous net
work present in cell. It is involved in secretion
of proteins, formation of other cellular
components and in glycosylation of proteins.
002
Golgi
Complex
CHAPTER - 1 | Cell Membrane & Transport
Lysosomes: They are vesicle like membrane surrounded structure present in cytoplasm. They are
involved in hydrolysis of internalized foreign molecules as well as endogenous substances. Since
lysosomes are involved in the removal of endogenous substances they are called as suicide bags of
cell.
Peroxisomes: Are membranous vesicles found in cytosol. They are involved in hydrogen peroxide
metabolism.
Cytosol: Soluble portion of the cell is called as cytosol. It contains enzymes of glycolysis, HMP
shunt, aminoacid and fatty acid activation, fattyacid synthesis, and few enzymes of porphyrins
and urea synthesis.
2. Write note on chemical constituents of cell.
A. Chemical constituents of life forms (Cells):Cells contain various organic as well as
inorganic molecules and water.
a. Organic substances : They form major part of cell. There are two type of organic
molecules. Macro molecules are nucleic acids, proteins, lipids and carbohydrates.
Amino acids, fatty acids, peptides, vitamins, monosaccharides, nucleotides, hormones
and coenzymes are small organic molecules.
b. Inorganic molecules : They are present as anions and cations. They are sodium,
potassium, calcium, magnesium, bicarbonate, chloride, phosphate etc.
c. Water: It is the most predominant molecule of cell.
3. Write about structure of cell membrane.
A. 1. Membranes are non covalent assemblies of lipids and proteins with carbohydrates
attached.
2. They are gel or semi fluid or semi solid structures.
3. Membrane lipids are organized in a bilayer form in which proteins are embeded. 4.
The two sides of membrane are different i. e. molecular composition of cytosolic side
of membrane differs from extra cellular side.
Membrane lipids :
1. Lipids present in membrane are
PROTEIN
Protein
Lipid
phospholipids, cholesterol and glycolipids.
2. Phosphalipids and glycolipids form
Membrane
Bilayer
Cytosol
membrane bilayer.
3. The proportion of phospholipid and
glycolipid in membrane is different in
membranes.
4. Membrane lipids are in constant motion.
003
BIOCHEMISTRY - Questions and Answers
Membrane proteins:
1. There are two types of membrane proteins.
2. They are peripheral membrane proteins and integral membrane proteins.
3. The protein content is different in membranes.
4. The peripheral membrane proteins are present on membrane surface.
5. The integral membrane proteins occupy membrane bilayer.
Fluid mosaic model:
1. It is model proposed for membrane structure.
2. Membrane is of fluid in nature.
3. Lipids forms bilayer.
4. The membrane proteins float in the lipid bilayer.
5. Membrane proteins interact extensively with lipids present in bilayer.
6. Surface of the membrane appears as that of mosaic surface.
Mosaic Surface
Lipid Bilayer
Proteins
Protein
Fluid Mosaic Model
4. Describe transport of molecules across cell membrane with examples.
A. For the trans port of molecules across membrane several mechanisms exist.
Membrane transport: Two or more types of transport mechanism are involved in
movement of molecules across membrane.
They are A. Simple or passive diffusion B. Mediated transport
A. Simple diffusion: It is transport of molecules down the concentration gradiant. It
does not require either energy or carrier.
Examples : Absorption of xylose and mannose.
B. 1. Facilitated or mediated transport: This type of transport requires carrier molecule.
The carrier molecule is responsible for moving molecules from out side of cell to in
side or vice versa. It does not require energy.
Mechanism of transport of molecules by carrier involves conformational change in carrier
molecule. The carrier molecule exist in two states and has binding site for solute molecule. In
the native state the binding site of carrier molecule is exposed to high concentration of solute.
004
CHAPTER - 1 | Cell Membrane & Transport
The solute molecule binds to carrier molecule at its binding site. This is followed by
conformational change in the carrier molecule which exposes solute to low concentration.
Solute molecule is released and carrier molecule comes back to native state.
Membrane
Conformational
Solute
Outside
inside
Change
Carrier
Examples:
Binding Site
1. Glucose uptake by adipocytes, erythrocytes
2. Fatty acid uptake by enterocytes
3. Transport of glucose from enterocyte into blood
2. Active transport: It transport solute molecules against concentration gradient i. e. from low
concentration to high concentration. It is accompanied by hydrolysis of ATP.
Examples:
1. Na+/K+ – ATPase
2. Ca2+-ATPase of muscle.
3. H+/K+-ATPase of stomach.
3. Secondary active transport: In this type of transport energy required for movement of
solute molecule is derived from movement of another solute molecule down concentration
gradient. Hence it is called as cotransport. Carrier is symporter.
Examples:
1. Glucose uptake by enterocyte
2. Aminoacid uptake by enterocyte
5. Define ionophores and ion channels. Give examples.
A. Ionophores: Ionophores form pores in membrane which allows movement of ions across
membranes.
Examples: 1. Gramicidin. 2. Valinomycin. 3. Diphtheria toxin.
Ion channels: Ion channels are pores (channels) present in membrane that allow
movement of ions across membrane.
Examples : 1. Sodium (Na+) channel. 2. Pottasium (K+) channel. 3. Calcium (Ca2+) channel.
4. Cholirde (Cl-)channel.
6. Write differences between facilitated transport and active transport.
A. Differences between facilitated transport and active transport:
005
BIOCHEMISTRY - Questions and Answers
Facilitated transport
Active transport
1. Transport molecules down the concentration
1. Transport molecules against gradient.
concentration gradient.
2. Requires no energy.
2. Requires energy.
3. Carrier is saturated
3. No carrier saturation.
4. Influenced by hormones
4. Not under hormonal influence.
7. What are the functions of cell membrane?
A. 1. Membranes separates cell from its surroundings.
2. Shape of cell depends on membrane.
3. Cell interacts with environment through the membrane.
4. Membranes act as permeability barriers.
5. Membranes are involved in energy production.
6. Flow of molecules form cell into surroundings and vice versa is regulated by
membranes.
7. Formation of various cell organelles requires membrane.
Other model questions are
8. Write note on mitochondria structure and functions.
9. Write briefly on nucleus/ nucleolus.
10. Write about cytomembranes of a eukaryotic cell.
11. Define facilitated transport and active transport. Give examples
for each.
12. Write about membrane lipids and membrane proteins.
13. Explain features of fluid mosaic membrane model with help of a
diagram
14. Facilitated transport.
006
CHAPTER - 2 | Carbohydrates
Chapter
2
Carbohydrates
1. Classify carbohydrates. Give examples for each class. Add note on the
function of each example.
A. Carbohydrates classification: Carbohydrates are classified into
a. Monosaccharides.
b. Oligo saccharides,
c. Polysaccharides based on their carbon chain length.
Monosaccharides:
1. Monosaccharides are carbohydrates which
H–C
can not be hydrolyzed to small molecules.
O
H – C – OH
CH2OH
C
O
2. Monosaccharide containing three to seven
CH2OH
carbons with functional aldehyde or keto
group are present in nature.
Glyceraldehyde
CH2OH
Dihydroxy Acetone
3. They are aldotriose, keto triose, aldo tetrose,
keto tetrose, aldopentose, ketopentose,
aldohexose, ketohexose and aldoheptose, ketoheptose.
4. Glyceraldehyde and dihydroxy acetone are aldotriose and ketotriose respectively. The
phosphorylated forms are metabolic intermediates.
5. Erythrose is an example for aldotetrose and erythrulose is an example for ketotetrose.
Erythrose phosphate is metabolic intermediate.
6. Aldopentose and ketopentose are ribose and
ribulose respectively. Ribose is constituent of
nucleic acids. Ribulosephosphate is metabolic
CHO
CH2OH
H – C – OH
C–O
HO – C – H
HO – C – H
intermediate.
7. Aldohexoses are glucose, galactose and mannose.
Fructose and sedoheptulose are ketohexose and
ketoheptose respectively.
H – C – OH
H – C – OH
H – C – OH
H – C – OH
CH2OH
CH2OH
8. Glucose is present in our blood and gives rise to
energy on oxidation.
Glucose
Fructose
007
BIOCHEMISTRY - Questions and Answers
9. Galactose is a constituent of lactose and has function like glucose.
10. Phosphorylated sedoheptulose is metabolic intermediate.
Oligosaccharides :
They consist of few monosaccharides.
They are disaccharides, trisaccharide etc.
Monosaccharide
Monosaccharide
Glycosidic Bond
Disaccharide
Monosaccharide
Monosaccharide
Monosaccharide
Tri Saccharide
Glycosidic Bond
Disaccharides:
1. Disaccharide consist of two monosaccharide units.
2. Glycosidic bond joins individual monosaccharides. Maltose, lactose and sucrose are examples.
Name
Composition
Linkage
Source
Lactose
Glucose+ Glucose
α (1→4)
Malt, barley
Maltose
Glucose+Galactose
β (1→ 4)
Milk
Sucrose
Glucose+Fructose
α, β (1→ 2)
Sugarcane, honey, fruit juices.
Functions: All disaccharides yields energy after their hydrolysis to constituent
monosaccharides.
Polysaccharides:
1. Polysaccharides are made up of more than ten monosaccharide units.
2. They are polymers of monosaccharides.
3. They are divided into
a. Homopolysaccharides
b. Heteropolysaccharides.
Homopolysaccharides:
1. They are made up of only one type of monosaccharide.
2. So building block of homopolysaccharide is only one type.
3. They are glycogen, starch, cellulose, inulin, dextrin etc.
Starch:
1. It consist of two components. A major amylose and minor amylopectin components.
008
CHAPTER - 2 | Carbohydrates
2. Amylose is a linear polymer of glucose in which monomeric glucose units are joined by α (1, 4)
linkages. It has helical secondary structure.
3. Amylopectin has branched structure.
4. In the linear part glucose units are joined by α (1, 4) linkage. At the branch point glucose units
are held by α (1→6) linkage.
5. For every 20-30 glucose units a branch point is present in amylopectin.
6. The secondary structure of amylopectin is random coil due to branches.
7. Starch is common polysaccharide in our diet. It is a storage polysaccharide present in our food
stuffs like rice, wheat, pulses, tubers, grains etc.
AMYLOPECTIN
Glu
Glu
Glu
Glu
Glu
Glu
Glu
a (1 Z 4)
Glycosidic Bond
Glu
Glu
Glu
Glu
Glu
Glu
Glu
Glu
Glu
Glu
a (1 Z 6) Glu
Glycosidic Bond
Glu
a (1 Z 4)
Glycosidic Bond
Glu
Glu-Glucose
Glycogen:
1. The structure of glycogen is like that of amylopectin part of starch.
2. Glucose units are held by α (1→4) likages in straight chain part and at branch point α (1→6)
glycosidic bond is present between glucose units.
3. Though the glycogen structure is similar to amylopectin the number of branch points are
more.
4. Branching occurs for every 6 glucose units.
a (1Z6) linkage
a (1Z4) linkage
Glycogen
009
BIOCHEMISTRY - Questions and Answers
5. It is present in humans and other mammals.
6. It is also known as animal starch because in animals it serve as reserve food or stored material
7. It is present in liver and skeletal muscle in more amounts.
Heteropolysaccharides:
1. They are made up of more than one type of monosaccharide.
2. Usually a disaccharide which is made up of more than one type of monosaccharide serve as
building block or repeating unit.
3. Hyaluronicacid, heparin, chondroitin sulfate, keratan sulfate etc. are examples for
heteropolysaccharides.
4. Their composition and functions are given below.
Name
Composition
Functions
1. Hyaluronicacid
- ( — β-glucuronicacid-
Lubricant in synovial fluid
N-acetylglucosamine-)n –
and in eye.
2. Chondroitinsulfate – (–β- glucuronicacid-N- acetyl
3. Heparin
Structural component of
Glucuronicacid sulfate-)n —
bone, tendon and cartilage
– (–Iduronicacid- glucosamine sulfate –
Anti coagulant
Glucuronicacid – glucoosamine sulfate-)n –
4. Dermatan sulfete
– (– Iduronicacid- N-acetyl
Component of bone & skin
Galactosamine sulfate–)n –
5. Keratan sulfate
– (– galactose-N-acetyl
Components of cartilage &
Galactosamine sulfate-)n –
loose connective tissue
2. Define carbohydrates. Write their biological functions.
A. Carbohydrates are defined as poly hydroxy alcohols with functional aldehyde or keto
group.
Functions:
1. They are major energy source for man.
2. They function as reserve food material in man and plants.
3. They are components of connective tissues, bone, cartilage, skin, membrane and
nerve tissue.
4. They are components of blood group substances, nucleic acids etc.
5. Carbohydrate derivatives are vitamins, antibiotics and drugs
3. Define enantiomers or optical isomers. Give examples.
A. Enantiomers (optical isomers): Optical isomers of a compound are called as enantiomers. D
and L glucose are examples for optical isomers.
010
CHAPTER - 2 | Carbohydrates
4. Define epimers and anomers. Give examples.
A. Epimers:
1. They differ in the configuration of –OH and H groups on 2nd, 3rd and 4th carbon atoms of
monosaccharide.
2. Glucose and galactose are epimers.
3. They differ in the configuration of –OH and H groups on fourth carbon atom.
4. Like wise glucose and mannose.
Anomers:
1. Anomers differ in configuration of –OH and-H groups on first or anomeric carbon of sugar.
2. α –glucose and β – glucose are two anomers of glucose.
3. In α- glucose –OH is present on right side whereas in β –glucose it is present on left side.
5. Write note on mutarotation.
A. Mutarotation:
1. Due to the presence of asymmetric carbon glucose exhibits optical activity and
rotates plane polarized light.
2. Optical rotation of a solution containing α –D-glucose is +112◦.
3. But on standing the rotation decreases and reaches +52. 5◦ and no more change
occurs due to equilibrium.
4. β –D-glucose also exhibits this change in optical rotation when allowed to stand in a
solution.
5. This compound initially show +19◦ of rotation and it gradually increases to +52. 5◦.
6. This phenomenon is called as mutarotation.
7. It is due to change of glucose form pyranose ring form to open chain form.
α-D-Glucose
+120
↔
◦
D –Glucose ↔
+52. 5
β-D- Glucose
◦
Pyranose ring form Open chain form
+19◦
Pyranose ring form.
6. Write briefly about paper chromatography.
A. 1. It is most widely used separation technique.
2. It is used for the separation of closely related compounds from mixture.
3. It is based on partition principle of the compounds to be separated between two phases.
4. The mixture to be separated is applied on whatman No:1 filter paper over a short
distance from one end.
5. The paper serve as support for stationary phase of solvent system.
011
BIOCHEMISTRY - Questions and Answers
6. The solvent system consist of n-butanol, aceticacid and water in the ratio of 4:1:5.
7. The paper is dipped in the solvent system and solvent is allowed to flow over applied
sample.
8. The water is absorbed by filter paper and serve as stationary phase.
9. The organic solvent that moves over the paper is known as mobile phase.
10. Compounds which are more soluble in organic solvent move faster.
11. The relative mobility of the compounds during chromatography depends on the
partition coefficients of the compounds in two solvent phases.
12. So similar compounds which have different partion coefficients move to different
extents.
13. The ratio of the distance moved by compound to the distance moved by solvent is
known as Rf values.
14. Rf values are different for different solvent systems.
15. Compounds are identified by staining.
Sample Application
16. Aniline or silver nitrate are used to stain
Galactose
carbohydrates after separation.
17. Among carbohydrates glucose moves faster followed by
galactose.
Glucose
18. Paper chromatography is also used for separation of
Fructose
Solvent Front
amino acids.
Other model questions are
7. Write a note on monosaccharides.
8. Define disaccharides. Give examples.
9. How sucrose and maltose differ with respect to a. Structure b.
Source c. Function
10. Write briefly about structure and functions of starch and glycogen.
11. What are polysaccharides? Classify. Give examples.
12. Write briefly about mucopolysaccharides
13. Write four functions of carbohydrates.
14. Write composition and functions of a. Hyaluronicacid b. Heparin.
15. Write composition and function of lactose.
012
CHAPTER - 3 | Proteins, Plasma Proteins, Peptides & Aminoacids
Chapter
3
Proteins, Plasma Proteins, Peptides & Aminoacids
PROTEINS
1. Classify proteins based on composition giving examples for each class.
A. Proteins classification, based on composition. According to this proteins are
classification into
a. Simple proteins.
b. Conjugated proteins.
c. Derived proteins.
a. Simple proteins:Are those proteins which yields only aminoacids on hydrolysis.
Ex:Trypsin, plasma albumin, pepsin etc.
b. Conjugated proteins:
1. Are those proteins that yields aminoacids and other organic or inorganic molecules
or non protein molecules on hydrolysis.
2. The non protein molecule is called as prosthetic group.
3. Usually conjugated proteins are named according to the name of non protein.
4. Some examples are tabulated below.
Conjugated protein
Non protein part
Examples
1. Heme proteins
Heme
Hemoglobin
2. Glycoproteins
Carbohydrate
Immunoglobulins
3. Flavoproteins
FMN or FAD
Succinate dehydrogenase
4. Nucleoproteins
Nucleicacid
Chromatin
5. Phosphoproteins
Phosphorus
Casein
6. Lipoporteins
Lipids
Various classes of lipoproteins
like VLDL, HDL
7. Metalloproteins
Metals
Cytochromes
c. Derived Proteins: Are those proteins that are derived from partial hydrolysis of
simple or conjugated proteins. Gelatin, Peptone and proteose are examples.
013
BIOCHEMISTRY - Questions and Answers
2. Classify proteins on the basis of solubility giving examples for
each class.
A. Classification of proteins based on their solubility. According to this proteins are
classified into
a. Albumins.
b. Globulins.
c. Glutelins.
d. Protamins.
e. Histones.
f. Prolamines.
g. Sclero proteins.
a. Albumins:Are those proteins that are soluble in water as well as salt solutions. Egg
albumin, plasma albumin and lactalbumin are examples.
b. Globulins: Are proteins weakly soluble in water but soluble in salt solutions.
Ovoglobulins, plasma globulin and lactoglobulins are examples.
c. Glutelins: Are proteins soluble in mild acids and alkalis. zein, glutenin and oryzenin
are examples.
d. Protamines: Are proteins soluble in water and ammonia. Fish proteins like salmine and
sturine are examples.
e. Histones: Are those proteins which are soluble in water and dilute acids. Histones
present in chromosomes are examples.
f. Prolamines:Are proteins insoluble in water and alcohol but soluble in dilute alcohol.
Plant proteins zein and gliadin are examples.
g. Sclero Proteins: Are proteins insoluble in water, acids and alkalis. Animal proteins
keratin, elastin and collagen are examples.
3. Describe structural organization of proteins.
OR
What is primary, secondary, tertiary and qua ternary structure of proteins?
Explain each one giving examples. .
A. Primary structure of proteins :
1. Aminoacid sequence of a protein is known as primary structure of protein.
2. Peptide bonds and disulfide bonds are involved in primary structure.
H2N
Ala
Gly
Phe
Ala - Alanine
Gly - Glycine
Phe - Phenyl Alanine
Lys - Lysine
014
Tyr
His
Leu
Peptide Bond
Val
Gly
Lys
Leu
COOH
Tyr - Tyrosine
His - Histidine
Val - Valine
Leu - Leucine
CHAPTER - 3 | Proteins, Plasma Proteins, Peptides & Aminoacids
Primary structure of insulin:
1. It consist of two polypeptide chains.
2. They are A chain and B chain.
3. Inter chain di sulfide bonds links two chains.
4. Further an intra chain disulfide bond is present in A chain.
5. Glycine is the N-terminal aminoacid and aspargine is the C-terminal amino acid in
A chain.
6. In the B chain alanine is C-terminal amino acid and phenyl alanine is the
N-terminal aminoacid.
Intrachain Disultide Bond
S
S
|
|
CYS
CYS
CYS
|
S Inter
| Chain Disulfide Bond
S
|
CYS
CYS - Cysteine
CYS
|
S
|
S
|
CYS
A Chain
B Chain
INSULIN
Secondary structure of protein:
1. Two dimentional folding of polypeptide chain is known as secondary structure of
protein.
2. The folding of protein chain can be ordered or disordered.
3. The ordered secondary structures are α-helix and β-pleated sheet.
4. The disordered secondary structures are random coil and reverse turn or β-turn.
H2N
Alpha (α)Helix
It is the secondary structure found in α-Keratin of hair, nails and
epidermis of the skin.
Structural features of α-helix:
a. Coiling of polypeptide or protein chain along long axis produce α-
COOH
helix.
b. α-helix is stabilized by intra chain hydrogen bonding.
c. Peptide bonds are involved in hydrogen bonding.
d. C=O and –N-H groups of peptide bond participate in hydrogen bonding.
015
BIOCHEMISTRY - Questions and Answers
e. There are 3. 6 aminoacids in one turn of α-helix.
f. Peptide bonds that are four aminoacid residues away participate in hydrogen bonding
i. e. -N-H of second aminoacid residue and –C=O of sixth aminoacids are involved in
hydrogen bonding.
g. α-helix of fibrous proteins is right handed.
h. α-helix is destabilized by hydrophobic aminoacids.
I. In contrast aromatic aminoacids stabilizes α-helix.
j. α-helical regions are found in several other proteins.
Beta (β)Pleated Sheet
When two or more polypeptide chains line side by side along long axis beta pleated sheet is
formed. Adjacent segments of a protein or polypeptide chain may also form secondary
structure.
Polypeptide Chain 1
Polypeptide Chain 2
Beta Pleated Sheet (Parallel)
Structural features of β-Pleated Sheet
a. Polypeptide chains are fully extended along long axis in beta pleated sheet.
b. Inter chain hydrogen bonds stabilizes beta pleated sheet.
c. Based on direction β-pleated sheet is divided into i. Antiparallel β-pleated sheet and ii.
Parallel β-pleated sheet.
d. In antiparallel β-pleated sheet polypeptide chains run in opposite direction.
e. In parallel β-pleated sheet polypeptide chains run in same direction.
f. Pleated sheet is found in many proteins. Albumin and hemoglobin of blood contains βpleated sheet.
g. Antiparallel β pleated sheet is found in β-Keratin of silk fibroin, spider web and amyloid
protein found in the brain of Alzheimer's disease patients.
h. β-pleated sheet content varies among proteins.
Tertiary structure of protein:
1. It is formed due to three dimentional folding of polypeptide chain of protein in space.
2. Tertiary structure of protein contains ordered and disordered secondry structures i.
e. α-Helix, β-pleated sheet, random coil conformation etc.
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CHAPTER - 3 | Proteins, Plasma Proteins, Peptides & Aminoacids
Forces involved in maintenance:
1. Several non covalent bonds stabilizes tertiary structure.
2. Usually it refers to native conformation of a protein.
3. Internal hydrogen bonds, electrostatic, hydrophobic and van der Waals interactions
are bonds that keep tertiary structure intact.
4. In the case of proteins that are made up of only one polypeptide chain tertiary structure
is the final level of protein structure.
Quaternary structure of protein:
1. Proteins which are made up of more than one polypeptide chain contains quaternary
structure.
2. Such proteins are known as oligomeric proteins and constituent polypeptide chains are
referred as sub units or protomers.
COOH
Sub Unit
NH2
Quaternary Structure
Tertiary
Structure
Hemoglobin, creatine phosphokinase. Lactate dehydrogenase etc are examples for
proteins with quaternary structure. Hemoglobin and lactate dehydrogenase are made
up of four subunits whereas creatine phosphokinase contains two sub units.
4. Define proteins. Write their functions.
A. Proteins are polymers of aminoacids. The aminoacids are joined by peptide bonds.
Hence they are also called as polypeptides.
Amino Acid1
Amino Acid2
Peptide Bond
Amino Acid3
Amino Acid4
Amino Acidn
Protein (Polypeptide)
Functions of Proteins:
1. Proteins are present in body. They are structural components of tissues, cells etc.
2. Proteins function as hormones
3. Proteins functions as enzymes
4. Proteins regulate gene expression.
5. Proteins are involved in muscle contraction
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BIOCHEMISTRY - Questions and Answers
6. Proteins perform transport functions.
8. Proteins are used as nutrients.
7. Proteins act as buffers.
9. Proteins act as reservoir of minerals
10. Proteins act as infective agents.
5. Classify proteins based on their shape.
A. Protein classification on the basis of shape. According to this proteins are divided into a.
Fibrous proteins b. Globular proteins.
a. Fibrous Proteins: Are proteins in which polypeptide chains are elongated. Keratin,
collagen and elastin are examples.
b. Globular Proteins: Are proteins in which polypeptide chains are folded into globular
or spherical shape. Hemoglobin, albumin and trypsin are examples.
6. Write briefly about protein denaturation.
A. 1. Denaturation of protein is loss of native conformation.
Heat
Protein
Denatured Protein
2. Denatured proteins exhibit properties which are not shown by native protein.
3. They are
A. Loss of biological function.
B. Solubility changes.
C. Susceptible to enzyme action.
D. Increased chemical reactivity.
E. Dissociation of subunits in case of oligomeric protein.
Methods of protein denaturation
By several ways proteins are denatured. They are
1. By exposing protein to extreme acidic or alkaline PH. 2. High temperature. 3. Use of
detergents like sodium dodecyl sulfate (SDS). 4. By treating with strong acids like
trichloroacetic acid (TCA), Tungstic acid and picric acid. 5. Exposing to ultraviolet
light. 6. Using salts like urea and guanidinium chloride at high concentration. 7.
Vigorous shaking. 8. Ultrasonication. 9. Heavy metal exposure like lead, arsenic,
mercury etc. 10. By organic solvents like acetone, alcohol etc.
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CHAPTER - 3 | Proteins, Plasma Proteins, Peptides & Aminoacids
Clinical Importance:
1. Protein denaturation is part of estimation of blood constituents.
2. Plasme protein separation involves protein denaturation.
3. Isolation of protein or enzyme from mixture of proteins involves denaturation.
Examples for protein denaturetion:
1. Exposure of egg albumin to high temperature leads to formation of coagulum.
2. Sweet tasting protein monellin loses its property on denaturation.
7. Write methods for determination of protein primary structure.
A. Primary structure of protein is determined by
a. Sanger's method.
b. Edman's method.
Sanger's method:
1. In this 1-fluoro-2, 4-dinitrobenzene (FDNB) is used to determine primary structure of
protein.
2. FDNB reacts with free aminogroup of protein to produce yellow 2, 4 – dinitrophenyl
residue of aminoacids which are identified after chromatographic separation.
3. Since FDNB reacts with other amino acids only one aminoacid is determined at a time with
this method
FDNB + Protein
Identification of
amino Acid
Dinitrophenyl Derivative
Chromatography
Edman's meathod:
1. In this method also primary structure is elucidated from N-terminus.
2. However complete sequence of protein is obtained by repeating several times with
Edman's reagent.
3. Unlike Sanger's method Edman's reagent reacts with only one aminoacid and rest of the
aminoacids remain intact.
4. Edman's reagent (Phenylisothiocyanate)reacts with free aminogroup in presene of acid to
produce phenylthiohydantoins which are estimated by using chromatography.
PLASMA PROTEINS
8. Describe composition and functions of various plasma proteins.
A. Several structurally and functionally different proteins are present in plasma. They are
albumin and various components (fractions) of globulins.
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BIOCHEMISTRY - Questions and Answers
Albumin: It contributes osmotic pressure in blood. It is involved in maintenance of blood
volume. One gm of albumin can hold 18ml of fluid in blood. It is involved in transport of several
substances. Further it binds to various substances and drugs. Fatty acids and bilirubin are
transported by albumin. Several hormones also transported by albumin Sex hormones and
glucocorticoids are transported by albumin. Albumin function as buffer. Peripheral tissues
use albumin as nutrient.
Alpha, (α1) alpha2 (α2), beta (β)and gamma (g) globulins are components of globulin fraction of
plasma. Further each of subglobulin fraction consist of several proteins.
α1-globulins
α1-antitrypsin and α1-acid glycoprotein are principle components of this fraction. Other
components are α-lipoprotein, prothrombin, α1-fetoprotein, thyroxine binding and retinol
binding proteins.
α1-antitrypsin: It accounts for more than 90% of α-globulin fraction. It is an inhibitor of trypsin,
chymotrypsin, elastase etc. It prevents action of proteases on pulmonary tissue and other
tissues. Lack of α1-antitrypsin results in emphysema.
α-Lipoprotein: It is involved in transport of lipid (cholesterol)from peripheral tissues to liver
for removal.
Prothrombin: It is one of the blood clotting factors and involved in blood coagulation.
α1-fetoprotein:As the name implies it is the protein present in foetal blood and its presence in
adult blood indicates liver cancer. It is considered as tumor marker for liver cancer. Thyroxine
and retinol binding proteins are involved in the transport of thyroxine and vit. A respectively.
α2-globulins: α2-macroglobulin, haptoglobulin, erythropoietin, ceruloplasmin and pseudo
choline esterase are present in this fraction.
α2-Macroglobulin: It is an inhibitor of proteases. It combines with proteases to form complex
which is then easily removed from circulation.
Haptoglobulin: It is involved in the transport of hemoglobin. It combines with hemoglobin to
from complex.
Erythropoietin: It is required for formation of reticulocytes.
Ceruloplasmin: It is also known as ferrooxidase. It is a copper containing protein.
Pseudo choline estrase: It is an enzyme present in blood.
β-globulins
β-lipoprotein, transferrin and complement-3 are components of this fraction.
β-Lipoprotein : It is involved in the transport of lipids from liver to peripheral tissues.
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CHAPTER - 3 | Proteins, Plasma Proteins, Peptides & Aminoacids
Transferrin: It is involved in the transport of iron.
Complement-3:It is one of the component of complement system.
γ-globulins
Immunoglobulins are major component of this fraction. C-reactive protein is another
component of this plasma protein fraction.
C-Reactive protein: It is produced in inflammatory condition.
Immunoglobulins
They are involved in defence function. They are antibodies present in serum. They are
produced when foreign molecules or antigens enters inside body.
Structure:
1. Generally an Immunoglobulin is made up of 4 polypeptide chains. The molecular weight
of this is about 150000 daltons.
2. Two types of polypeptide chains are present. Two heavy or H chains and two light or L
chains.
3. Each H chain molecular weight is 50, 000 and contains 450 aminoacids.
4. Molecular weight of L chain is about 25, 000 and contains 220 aminoacids.
5. The H chains contains variable region at N terminus [VH] and three constant regions at
C terminus [CH1, CH2 and CH3].
6. In the L chain one variable region (VL) at N terminus and constant region (CL) at C
terminus exist.
7. The aminoacid sequence varies in variable regions of H and L chains and largely
depends on class or type of immunoglobulin.
8. However constant regions of H and L chain aminoacid sequence is constant or same in
various types of immunoglobulins
9. The variable regions recognizes antigens.
Shape
1. Overall shape of immunoglobulin is that of Y.
2. Two H chains intertwines to form base of Y.
3. Arm of the Y is formed by joining L chains to H chains.
4. Most of the immunoglobulins contains carbohydrate in CH2 region.
5. Several intra and inter chain disulfide bonds maintain Y shape.
–S-S–
Light
Chain
immunoglobulins are classified into three major classes and two
Heavy
Chain
minor classes. Ig G, Ig A and IgM are major classes. Ig and IgE are minor
IMMUNOGLOBULIN
Classification: Based on composition of H and L chains
classes. Not only composition, size, shape, distribution and function
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BIOCHEMISTRY - Questions and Answers
also varies among various classes of immunoglobulins. Each class of immunoglobulin contains
unique H chain based on which they are named. The different H chains are g (Gamma), α
(Alpha), μ (mu), δ (Delta) and ε (Epsilon). However in all five classes of immunoglobulins only
two types of L chains are found. They are κ (kappa) and λ (lambda).
1. Ig G Class
1
Structure : It consist of two g type H chains and two
L chains of K or lambda type. So it is designated as
g2 L2 or g2 K2 or g2λ2.
Function: It is major immunoglobulin of serum. It is
2
the major antibody of new born. Ig G binds to foreign
cells or antigens which increases their susceptibility
for elimination.
Ig G dimer
2. IgA Class
Structure: It consist of two alpha type H chains and two κ or λ type L chains. Hence it is
designated as α2 L2. It may exist as multimer of the basic unit. Polypeptide chains like SC
and J are also found. They are involved in joining of monomers.
Function: It accounts about 10-20% of immunoglobulins. It is chief of antibody of mucosal
cells, secretions of lungs and gut where it combines with antigen thus protecting them from
harmful antigens.
3. Ig M Class
1
Structure: It consist of two μ type H chains and two L
chains. Hence it is designated as μ2L2. This basic unit
exist as multimer like Ig A class. Most common
5
2
occurence is in the pentameric form (μ2 L2) 5. SC and J
components also may occur.
Function: Ig M on B- Lymphocytes act as receptor for
antigens. Complement fixation requires Ig M. About 5
4
3
– 10% total immunoglobulins is Ig M type.
4. Ig D Class
Ig M Pentamer
Structure: It is made up of two δ type H chains and two C chains. It is designated as δ2 L2.
Function: It is involved in alternate pathway of complement fixation. It accounts only 0.
5% of total immunoglobulins.
5. Ig E Class
Structure: It is made up of two ε type H chains and two L chains. It is designated as ε2 L2.
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CHAPTER - 3 | Proteins, Plasma Proteins, Peptides & Aminoacids
Function:It is involved in anaphylactic response. Among all classes of immounoglobulins it
is least concentrated. However in allergic reactions its concentration may increase. They
may be found in mucous secretions of lung and gut.
9. Write briefly about Bence-Jones protein.
A. Bence –jones proteins are found in urine of multiple myeloma patients. They are derived
from immunoglobulin light chain. They are detected in urine based on their behaviour on
heating. These proteins precipitate at 40- 600C and dissolves at boiling point. Further
cooling re precipitates and boiling re dissolves.
10. Write a note on acute phase proteins.
A. 1. α1-antitrypsin, haptoglobulin, ceruloplasmin, complement -3, fibrinogen and Creactive proteins are known as acute phase proteins.
2. In acute inflammation their concentration in plasma increases.
3. Interleukin released by macrophases at site of injury induces synthesis of these
proteins by liver.
4. In plasma levels of these proteins during inflammation raises at different rates. Creactive protein raises initially.
5. This is followed by raise in α1-anti trypsin. At the end complement-3 level raises.
PEPTIDES
11. Define peptide. How they are formed ? Give examples.
A. 1. Peptides are compounds containing peptide bonds.
2. Peptides are formed due to inter action between carboxyl group of one aminoacid
with amino group of other aminoacids.
3. Peptide bond formation involves loss of one water molecule.
4. Glutathione, thyrotrophin releasing hormone, enkaphalins, oxytocin, vasopressin
are examples for peptides.
H2N – CH – COOH – H2N – CH – COOH
|
|
R2
R1
H2O
H2N – CH – CONH – CH – COOH
|
|
R1
R2
Peptide Group
12. Define dipeptide, tripeptide and penta peptide. Give examples.
A Dipeptide: A dipeptide is made up of two aminoacids which are joined by single peptide
bond. carnosine and anserine are examples.
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BIOCHEMISTRY - Questions and Answers
AminoAcid1
AminoAcid2
Dipeptide
Peptide Bond
Tripeptide : It is composed of three aminoacids. Two peptides bonds connect these
aminoacids. Glutathione and thyrotrophin releasing hormone are examples.
AminoAcid1
AminoAcid2
AminoAcid3
Tripeptide
Peptide Bond
Pentapeptide : Five aminoacids are linked by four peptide bonds. Enkaphains are examples.
13. Write the glutathione composition, short form and functions.
A. Glutathione : It consist of glutamate, cysteine and glycine. It is written as glutamatecysteine- glycine. G-SH is short form. It is a reducing agent. It undergo dimerization on loss
of hydrogen. G –S-S-G is oxidized form. It is involved in the maintenance of -SH groups on
proteins on reduced form. In red blood cells (R. B. C.) it is involved in the elimination of
hydrogen peroxide. It participates in detoxification. It is involved in hormone secretion and
apoptosis.
14. Define cyclic peptide and toxic peptide. Give examples.
A. Cyclic peptide (s):It is formed when amino and carboxyl terminals of the peptide are joined
by peptide bond. Antibiotic gramicidin –S and tyrocidin are examples.
Toxic peptides: Are peptides acting as toxins. α-Amanitin is toxic peptide present in mush
rooms which is responsible for mush room poisoning.
AMINOACIDS
15. Classify aminoacids based on side chain and ring structure giving
examples.
A. Aminoacids are classified into seven major classes based on side chains.
a) Aliphatic aminoacid s: Are those which contain aliphatic side chains. Glycine, alanine,
valine, leucine and isoleucine are examples for aliphatic aminoacids. The latter three
aminoacids are also known as branched chain aminoacids.
H
|
H2N – C – COOH
|
H
H
|
H2N – C – COOH
|
CH3
H
|
H2N – C – COOH
|
CH
Glycine
Alanine
CH3 CH3
H
|
H2N – C – COOH
|
CH2
|
CH
Valine
CH3 CH3
Leucine
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CHAPTER - 3 | Proteins, Plasma Proteins, Peptides & Aminoacids
b) Hydroxy aminoacids :Are those aminoacids that contain sulfhydryl groups in side
chain. Serine and threonine are examples for hydroxyl aminoacids.
H
|
H2N – C – COOH
|
CH – CH3
|
CH2
|
CH3
H
|
H2N – C – COOH
|
CH2 – OH
H
|
H2N – C – COOH
|
CH – OH
|
CH3
H
|
H2N – C – COOH
|
CH2 – SH
H
|
H2N – C – COOH
|
CH2
|
CH2 – S
|
CH3
Isoleucine
Serine
Threonine
Cysteine
Methionine
c) Sulfur containing aminoacids : These aminoacids contain sulfhydryl groups in side
chain. They are cysteine, methionine and cystine.
H
|
H2N – C – COOH
|
CH2 – COOH
H
|
H2N – C – COOH
|
CH2 – CONH2
H
|
H2N – C – COOH
|
CH2
|
CH2 – COOH
H
|
H2N – C – COOH
|
CH2
|
CH2 – CONH2
Aspartate
Aspargine
Glutamate
Glutamine
d) Acidic aminoacids : Side chains of these aminoacids contain acidic groups or their
amides. They are glutamate, glutamine, aspartate and aspargine.
H
|
H2N – C – COOH
|
CH2
|
NH
CH2
|
CH2 – NH – C – NH2
Arginine
H
|
H2N – C – COOH
|
CH2
|
NH
N
H
|
H2N – C – COOH
|
CH2
|
CH2
|
CH2
|
CH2 – NH2
Histidine
Lysine
e) Basic aminoacids: Basic groups are present in side chains of these aminoacids. They
are arginine, lysine, hydroxyl lysine and histidine.
H
|
H2N – C – COOH
|
CH2
H
|
H2N – C – COOH
|
CH2
OH
Phenyl Alanine
Tyrosine
H
|
H2N – C – COOH
|
CH2
|
N
H
Tryptophan
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BIOCHEMISTRY - Questions and Answers
f) Aromatic aminoacids : Aromatic rings are present in the side chains of these
aminoacids. They are phenylalanine, tyrosine and tryptophan.
OH
N
H
COOH
Proline
N
COOH
H
Hydroxy Proline
g) Iminoacids: Are those aminoacids in which amino group is replaced by imino group.
They are proline and hydroxy praline.
16. Define an aminoacid. Write their functions.
A. Aminoacids are acids containing aminogroups. They are building blocks of proteins and
peptides present in humans and other living organisms.
Amino Group
H
|
H2N – C – COOH
|
R
Acid Group
R-Side Chain
Amino Acid
Functions: Free aminoacids are found in blood and cells of humans. Hormones, purines,
pyrimidines, heme, some vitamins, creatine etc found in body are derived from aminoacids.
17. Classify aminoacids based on reaction in solution.
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CHAPTER - 3 | Proteins, Plasma Proteins, Peptides & Aminoacids
A. Semi essential aminoacids: Semi essential aminoacids are synthesized in the body to some
extent. They are histidine and arginine.
Unusual or rare aminoacids or Non protein aminoacids: These aminoacids are not found in
proteins. But they have other functions. Examples are i). Intermediates of urea cycle i. e.
ornithine. citrulline and argininosuccinate. ii). taurine. iii). Gamma aminobutyric acid
(GABA). iv). Beta (β)-alanine. v). Pantothenic acid.
20. Explain charge or acid base properties of aminoacids.
A. 1. Depending on pHof surroundings an aminoacid can exist as cation or positively charged
molecule, anion or negatively charged molecule and zwitter ions.
2. Zwitter ion carries no net charge It contains equal number of positive charges and
negative charges.
3. Further aminoacids act as acids or bases. When alkali is added aminoacid act as acid by
donating proton. Aminoacid act as base by accepting a proton from acid.
4. At nutral pH aminoacid functional groups amino and carboxyl groups exist in ionized
form.
5. The amino group exist in protonated -NH3+form and carboxyl group in the dissociated
–COO-form this is known as zwitter ionic form.
6. In strong acidic conditions –COOH remains undissociated i. e. aminoacid exist as
cation.
7. In strong alkaline condition proton from –NH3+is lost i. e. aminoacid exist as anion.
Anion
←
H
AlkalineP
Zwitterion
Neutral PH
→
cation
acidicPH
8. The PHdependence of charge of aminoacid is used for separation of aminoacids.
21. Define isoelectric point of aminoacid. How it is determined?
Write its importance.
A. 1. At iso electric point aminoacid exist as zwitter ion.
H
|
H2N – C – COO–
|
R
+ H |
H3N – C – COO–
|
R
+ H |
H3N – C – COOH
|
R
2. The isoelectric point of an aminoacid having one carboxyl group and one amino group is
obtained by dividing Pkvalues of these groups with 2.
3. At isoelectric point aminoacids or proteins have minimum solubility.
4. This is exploited for separation of proteins or aminoacids from mixture.
22. Define Pk values of amino acid.. Write the significance of it.
A. 1. It is PH at which un dissociated and dissociated forms of a group are present in equal
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BIOCHEMISTRY - Questions and Answers
amounts.
2. For example Pk of amino group of aminoacid is designated as Pkam. It is PHat which
dissociated (-NH2) and undissociated (-NH3+) are found in equal amounts.
3. Like wise Pk of acid groups of aminoacid is designated as Pka.
4. The Pk values indicates strength of groups.
5. Low Pkvalues indicates more ionizing power. High Pk values indicates less ionizing power.
Other model questions are
23. Write a note on immunoglobulins.
24. Write briefly about conjugated proteins.
25. What are derived proteins ? Give examples.
26. What is meant by secondary structure of proteins? Give examples.
27. Write about quaternary structure of proteins giving examples.
28. Write about structural feature of α –helix.
29. Write a note on ß -pleated sheet.
30. Draw general structure of immunoglobulins. Label its parts.
31. Write briefly about Ig E and Ig M.
32. Write briefly about tertiary structure of protein.
33. Define primary structure of protein. Write a method for its determination.
34. Write a note on albumin.
35. Write very briefly about aromatic aminoacids.
36. Write note on α 1- globulins.
37. Write clinical importance of alpha fetoprotein and macroglobulin.
38. Name sulphur containing aminoacids and basic aminoacids.
39. Classify immunoglobulins. Give structural and functional aspect of each class.
40. Name three charged forms of aminoacid. When the aminoacid assumes these states?
41. Write a note on α 2- globulins.
42. Write a note on ß-globulins.
43. Write a note on the general structure of immunoglobulins.
44. Non protein aminoacids
45. Protein structure
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CHAPTER - 4 | Lipids
Chapter
4
Lipids
1. Classify lipids. Give examples for each class along with functions.
A. Classification: Based on composition lipids are classified into
I. Simple lipids
II. Compound lipids and
III. Derived lipids.
I. Simple lipids : Esters of fatty acids with alcohol are known as simple lipids. Fats and
waxes are simple lipids.
a. Fats:
1. Are esters of fatty acids with glycerol.
2. Triglycerides, diglycerides and mono glycerides are fats.
3. Triglyceride is also called as tri acyl glyccrol.
4. In triglycerides three fatty acids are esterified to three hydroxyl groups of glycerol.
5. In diglycerides two of the hydroxyl groups of glycerol are esterified with glycerol.
6. Only one fatty acid is esterified to any one of hydroxyl group of glycerol in
monoglycerides.
Esterbond
CH2 – OH
|
CH – OH
|
CH2 – OH
CH2 – O – COR1
|
CH – O – COR2
|
CH2 – O – COR3
CH2 – O – COR1
|
CH – O – COR2
|
CH2 – OH
CH2 – O – COR1
|
CH – OH
|
CH2 – OH
Glycerol
Triglyceride
Diglyceride
Monoglyceride
COR1, COR2, COR3 - Acyl Groups
Functions:
1. They are mainly involved in storage function.
2. Adipose tissue present under skin contains triglycerides. In the abdomen, thighs and in
mammary gland, adipose tissue containing triglycerides is present.
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BIOCHEMISTRY - Questions and Answers
3. Obese people contain more triglycerides.
4. Women contain more triglycerides than men.
5. In hibernating animals, seals and penguins triglycerides are more.
6. Fat under the skin has dual roles. It function as energy store as well as insulator
against cold.
b. Waxes: Are esters of fatty acids with long chain alcohols. Wool and bees wax are waxes
known well. Wool is ester of fatty acid with long chain alcohol lanosterol and agnosterol.
Bees wax is an ester of fatty acid with myricyl alcohol.
Functions:
1. Waxes form protective layer over the skin, fur, feathers of animals. Shiny appearance
of fruits, leaves of plants are due to waxes.
2. Waxes are hard at low temperature and soft at high temperature.
3. Wool a wax of animal origin is used as protection against low temperature or cold.
Woolen clothing protect us from cold for this reason.
4. Waxes act as water barrier for animal, plants, birds etc.
II. Compound lipids: Are esters of fatty acids with alcohol containing additional groups
and nitrogenous bases. They are further subdivided based on alcohol present. They are
glycerophospho lipids and sphingolipids. In glycero phospholipids glycerol is alcohol
and sphingosine is alcohol in sphingolipids.
A. Glycerophospholipids:
1. In which two fatty acids are esterified to two hydroxyl groups and nitrogenous base
bearing phosphate is esteri fied to third hydroxyl group of glycerol.
2. Glycerophospholipid lacking nitrogenous base is known as phosphatidicacid.
3. Some glycerophospholipids are considered as derivatives of phosphatidic acid and they
are named accordingly.
4. Phosphatidyl choline, phosphatidyl serine, phosphoatidyl ethanolamine and
phosphatidyl inositol are examples for glycerophospholipids.
5. Due to the presence of phosphate they are often referred as phospholipids.
i) Phosphatidyl choline: It consist of glycerol, two fatty acids esterified to first and
second hydroxyl groups. Phosphate is esterified to third hydroxyl group.
Nitrogenous base choline is esterified to phosphate. Lecithin is the alternate name
for this glycerophospholipid.
ii) Phosphatidyl serine : It is an aminophospholipid. Serine an aminoacid is attached to
phosphate which is esterified to third hydroxyl of glycerol. First and second
hydroxyl groups of glycerol are esterified with two fatty acids. Cephalin is alternate
name for this phospholipid.
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CHAPTER - 4 | Lipids
iii) Phosphatidyl inositol : Sugar alcohol inositol is esterified to phosphate of
phosphatidic acid.
CH2 – O – COR1
|
CH – O – COR2
|
CH2 – O – Phosphocholine
CH2 – O – COR1
|
CH – O – COR2
|
CH2 – O – Phosphoserine
CH2 – O – COR1
|
CH – O – COR2
|
CH2 – O – Phospho
Ethanolamine
Phosphatidyl Choline
Phosphatidyl Serine
Phosphatidyl Ethanolamine
Functions:
1. Phosphatidyl choline is major lipid present in cell membrane. It is also present in egg
yolk and plasma lipoproteins.
2. Cephalin is also component of cell membrane, lipoproteins and nervous tissue.
3. Cell membrane contains phosphatidyl inositol.
4. Inositol triphosphate (IP3) which is involved in signal transducution is derivative of
phosphatidyl inositol.
B. Sphingolipids: They consist of an aminoalcohol sphingosine, fatty acid, nitrogenous
base and additional groups.
They are subdivided into
a. Sphingomyelins.
b. Glycolipids
a. Sphingomyelins:
1. They are made up of fatty acid linked to sphingosine by amide bond and
phosphoryl choline which is esterified to sphingosine.
2. Due to presence of phosphate sphingomyelins are also considered as
phospholipids.
Sphingomyelin
Fatty acid
Sphingosine
Phosphate
Choline
Functions:
1. Sphingomyelins occur in myelin sheath of nervous tissue.
2. They are most abundant sphingolipids.
3. They are also present in grey matter.
4. Cell membrane also contain sphingomyelin.
b. Glycolipids: They are subdivided into two groups.
1. Cerebrosides.
2. Gangliosides.
031
BIOCHEMISTRY - Questions and Answers
Cerebrosides:
a. They consist of sphingosine, fatty acid and carbohydrate or sugar.
b. Usually they are named according to sugar present.
c. For example if glucose is the sugar present in a cerebroside then it is called as
glucocerebroside.
d. Similarly galacto cerebroside contain galactose sugar.
e. In some cerebrosides sulfate is esterified to sugar moiety.
f. They are known as sulfatides or sulfolipids.
Cerebroside
Fatty acid
Sphingosine
Sugar
Sulfolipid
Fatty acid
Sphingosine
Sugar
Sulfate
Gangliosides:
a. They are most complex of all compound lipids.
b. They are made up of sphingosine, fattyacid, oligosaccharide and sialic acid.
c. The oligo saccharides contain aminosugar and acetylated aminosugars.
Ganglioside
Fattyacid
Sphingosine
Oligosaccharide
Sialicacid
Functions:
1. White matter of the brain and myelin sheath of nerves contain cerebrosides.
2. Grey matter contain gangliosides.
3. Gangliosides serve as receptors for toxins, hormones etc.
4. Cerebrosides and gangliosides are also present in non neural tissues.
5. Gangliosides are also involved in cell cell recognition, growth and differentiation
and carcinogenesis.
III Derived lipids : Hydrolysis of simple and compound lipids produce derived lipids. Fatty
acids, steroids, fat soluble vitamins and glycerol are examples for derived lipids.
Fatty acids : Hydrolysis of triglycerides yield fatty acids. They are acids containing
long hydrocarbon chain. Many fatty acids are identified in nature. They are subdivided
into
a. Saturated fatty acids.
b. Unsaturated fatty acids based on nature of hydrocarbon chain.
a. Saturated fatty acids:
1. The hydrocarbon chain of these fatty acids is saturated.
2. No double bonds occur.
3. Saturated fatty acids containing up to 20 carbons are identified.
4. More important are palmitic acid, stearic acid and arachidonic acids.
032
CHAPTER - 4 | Lipids
CH3 – CH2 – CH2 – CH2 – – – – – – CH2 – COOH
Hydro Carbon Chain
Acid Group
Saturated Fatty Acid
b. Unsaturated fatty acids:
1. They contain double bonds in hydrocarbon chain.
2. Unsaturated fatty acids containing up to 30 carbons are identified.
3. They are subdivided into mono unsaturated fatty acids and polyunsaturated
fatty acids (PUFA) based on number of double bonds.
4. Mono unsaturated fatty acids are palmitoleic acid and oleic acid. They
contain one double bond.
5. Poly unsaturated fatty acids are linoleic, linolinic and arachidonic acids.
They contain many double bonds.
CH3 – CH2 – – – – CH = CH – CH2 – – – – CH = CH – – – – CH2 – COOH
Double Bond
Unsaturated Fatty Acid
Functions:
1. Fatty acids are source of energy for humans like glucose.
2. Fatty acids are components of nervous tissue, lipoproteins etc.
3. Poly unsaturated fatty acids are essential fatty acids.
4. They are required for the synthesis of eicosanoids.
5. They are also components of cell membrane.
Steroids : They contain complex fused ring system which is also known as steroid nucleus.
Fused ring system contains four rings collectively known as cyclopentanoperhydrophenan
threne ring.
Cholesterol is an example for steroid which is steroid alcohol.
Functions :
1. It is most abundant steroid in animals.
2. About 200g of cholesterol is present in human adult.
3. Nervous tissue is rich in cholesterol.
4. Egg yolk is also rich in cholesterol.
5. Cholesterol is used for the formation of vitamins
and steroid hormones.
6. Vit. D is derivative of cholesterol.
HO
Cholesterol
033
BIOCHEMISTRY - Questions and Answers
7. Glucocorticids, mineralo corticoids, male sex hormones, female sex hormones are
derivatives of cholesterol.
2. Define lipids. Write briefly about their functions.
A. Lipids are organic substances soluble only in organic solvents like chloroform, ether and
benzene but insoluble in water.
Functions:
1. Lipids are structural components of cell membrane and nervous tissue.
2. Lipids present in myelinated nerves act as insulators for propagation of
depolarization wave.
3. Lipids present under skin act as thermal insulator against cold.
4. Lipids are energy source for man like carbohydrates.
5. Lipids like steroids function as hormones.
6. Lipids present around kidney act as padding and protect kidney from mechanical
injuries.
7. Lipids serve as vitamins.
8. Lipids are part of lipoproteins present in blood plasma.
9. Absorption of fat soluble vitamins requires lipids.
10. Essential fatty acids a kind of lipids are essential for life.
11. Lipids act as microbicides and fungicides.
12. Some lipids function as surfactants.
13. Lipids are involved in immune response.
14. Lipids act as mitogens.
15. Some lipids serve as precursors for the formation of complex lipids.
16. Due to its high energy and water output on oxidation mammals including humans
prefer to store energy in the form of lipid only
3. Write a note on structure and function of lyso phospholipids.
A. Partial hydrolysis of glycerophospholipids yield lysophospholipids. Hence they contain
only one acyl group instead of two acyl groups and phosphorylated nitrogenous base.
Functions : They are produced as intermediates during phospholipid biosynthesis. Lyso
lecithin a derivative of lecithin is present in cobra venom. It is a strong hemolysing agent.
4. Write a note on plasmalogens.
A. They are also glycerophospholipids. They contain unsaturated fatty alcohol in the place of
first fatty acid at first hydroxyl group. Because of this an ether linkage is found on first
carbon instead of usual ester linkage. Phosphorylated nitrogenous bases are usually
choline, serine, ethanolamine etc.
034
CHAPTER - 4 | Lipids
Functions:
1. They are structural components of tissues like brain, muscle and heart.
2. Platelet activating factor is a plasmalogen.
3. In cancer cells plasmalogen content is more.
5. Write a note on dipalmitoyl lecithin.
A. 1. It consist of two palmitic acid residues esterified to first and second carbon atoms of
glycerol and phosphocholine on third carbon.
2. In the lung it serve as surfactant.
3. It is involved in the maintenance of shape of alveoli of lungs.
4. It is synthesized only after 30 weeks of gestation.
5. Hence its deficiency occurs in premature infants and causes respiratory distress
syndrome (RDS).
6. Write the composition and clinical importance of Cardiolipin.
A. 1. Cardiolipin is a double phosphoglycerolipid.
2. Two phosphatidic acids are esterified to first and third carbons of glycerol.
3. It is structural component of inner mitochondrial membrane.
4. It shows immunological properties.
5. It is found in cardiac muscle hence the name.
6. It is useful in the diagnosis of syphilis.
7. What are lipoproteins ? Write their composition and general structure.
A. 1. Lipoproteins are lipid and protein complexes present in plasma.
2. The protein part of lipoprotein is called as apolipoprotein or apoprotein.
3. Non covalent bonds keep lipid and apoprotein together.
Composotion:
1. Triglycerides, free and esterified cholesterol and phospholipids are major lipids
present in lipoproteins.
2. However proportions of these lipids in various classes of lipoproteins differs.
3. Composition of apoprotein differs among lipoproteins.
4. Also proportions of proteins in various classes of
lipoproteins differs.
5. Five types of apoproteins are known so far.
6. They are apoprotein A, or apo A, apoB, apoC, apoD,
Apoprotein
CORE
and apo E. ApoF, apoG and apo H are also found.
7. Some of them has subtypes also.
Lipids
Lipoprotein
8. ApoB is largest of all.
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BIOCHEMISTRY - Questions and Answers
Structure:
1. Lipoproteins have spherical to oval shaped structure.
2. Non polar hydrophobic lipids lies in the central core.
3. They are surrounded by more polar hydrophilic apolipo proteins and lipids.
4. The outer polar coat solubilizes inner non polar lipids in aqueous environment of
plasma.
8. Write separation, classification and functions of lipoproteins.
A. Separation and classification:
1. Lipoproteins are separated by ultracentrifugation and electrophoresis.
2. Ultra centrifugation separates lipoproteins based on their density.
3. Density of a lipoprotein is inversely related to lipid content.
4. So higher the lipid content of lipoprotein then lower its density.
5. Ultracentrifugation separates lipoproteins into 4 classes.
6. They are1. Chylomicrons. 2. Very low density lipoproteins (VLDL). 3. Low density
lipoproteins (LDL) and 4. High density lipoproteins (HDL).
7. Electrophoretic separation of lipoproteins is based on differences in mobilities.
8. The plasma lipoprotein electrophoresis gives four bands which corresponds to
chylomicrons, α-lipoprotein, preβ-lipoprotein and β-lipoprotein.
LDL
VLDL
HDL
Lipoprotein
Electrophore sis
Chylomicrons b-Lipo
Protein
Preb
a-Lipo
Lipo Protein
Protein
Functions:
1. Chylomicrons are involved in the transport of dietary triglycerides from intestine to
liver.
2. Very low density lipoproteins (VLDL)are involved in the transport of endogenous
triglycerides from liver to peripheral tissues.
3. Low density lipoproteins (LDL) are involved in the transport of cholesterol from liver to
peripheral tissues.
4. High density lipoproteins (HDL) are involved in the transport of cholesterol form
peripheral tissues to liver.
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CHAPTER - 4 | Lipids
5. Some apoproteins have functions other than structure. They act as activators or
inhibitors of enzymes of lipid metabolism.
9. What are prostaglandins? Name them. Write their functions.
A. Prostaglandins (PG) are derived from prostanoic acid. It is a cyclic compound with two side
chains. The cyclic ring is cyclopentane ring. Many types of prostaglandins are found. They
differ in substituent groups on cyclo pentane ring. Some known prostaglandins are PGA,
PGB, PGC, PGD, PGE, PGF, PGG and PGH.
COOH
Cyclo
Pentane
Ring
Sidechain 1
Sidechain 2
CH3
Prostanoic Acid
Functions:
1. Prostaglandins have several effects on cardiovascular system.
a. They act on heart and increases cardiac output and myocardial contraction.
b. They are involved in maintenance of arterial pressure and vascular tone.
c. Some prostaglandins act as antihypertensive agents. They lowers blood pressure.
2. Prostaglandins act on central nervous system. They are involved in sedation and
tranquilizing effect in cerebral cortex.
3. Prostaglandins influences excretory functions of kidneys. They facilitates elimination
of sodium, potassium and chloride ions. They also influences urine volume.
4. Prostaglandins act on respiratory system.
a. They dilates bronchi. b. They act as anti asthmatics. c. They relieve nasal
congestion.
5. Prostaglandins act on digestive system.
a. They decrease acid secretion in stomach.
b. They are useful in peptic ulcer treatment.
6. Prostaglandins have actions on reproductive system.
a. They cause contraction of uterine muscle.
b. They are useful in inducing abortions.
c. They have role in fertility.
7. Prostaglandins play role in metabolism. Through cAMP they mediate their action.
cAMP level alteration affects lipid as well as carbohydrate metabolism.
8. Some prostaglandins are involved in inflammation.
9. Haematopoietic system also influenced by prostaglandins.
a. They inhibit platelet aggregation.
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BIOCHEMISTRY - Questions and Answers
b. Some promote clot formation.
c. Some cause platelet aggregation.
10. Prostaglandins promotes tooth movement by increasing resorption.
10. Define micelles, mixed micelles and Liposome and lipid bilayer.
Write the importance of each one.
A. These liquid structures are generated by amphipathic molecules which contain both
hydrophobic as well as hydrophilic parts.
Micelles : Are formed when amphipathic molecules are present beyond critical
concentration in aqueous medium. They are sphere shaped aggregates of amphipathic
molecules. Bile salts form micelles which are required for lipid digestion.
Mixed micelles: Are formed when micelles of one type of lipids combines with other lipids.
In the intestine bile salt micelles combines with products of lipid digestion to form mixed
micelles. Mixed micelle formation is essential for digestion and absorption of lipids.
Liposome: Is formed when a lipid bilayer cyclizes i. e. two ends of lipid bilayer joins. They
are used as carriers of drugs or genes in case of gene therapy.
Lipid bilayer:Is formed when phospholipids are present in water and oil mixture. Cell
membrane is a lipid bilayer.
Other model questions are
11. Write briefly about triglycerides.
12. Write the importance of cholesterol.
13. What are glycerophospholipids? Give examples. Mention their functions.
14. Write a note on phospholipids.
15. Write the composition and function of sphingomyelin.
16. What are glycolipids? Give examples.
17. Write composition and function of gangliosides.
18. What are derived lipids? Give examples.
19. Classify fatty acids. Give examples for each class.
20. Write the functions of prostaglandins.
21. Essential fatty acids.
22. Eicosanoids
038
CHAPTER - 5 | Enzymes
Chapter
5
Enzymes
1. Classify enzymes. Give examples for each class along with reaction
and cofactors involved.
A. Classification: Based on the type of reaction they catalyzes enzymes are classified into
six major classes. All classes of enzymes with examples are given below.
1. Oxidoreductases: They oxidizes or reduces substrates using an hydrogen acceptor or
donor.
Glutamate dehydrogenase is an example which catalyzes below given reaction.
Glutamate+ NAD+H2O → α-ketoglutarate +NADH+H+ + NH4.
Succinate dehydrogenase that catalyzes below given reaction is another example.
Succinate +FAD →Fumarate + FADH2.
2. Transferases: They transfer group between substrates.
Transaminase catalyze transfer of amino group from one aminoacid to ketoacid as shown
below.
Alanine+ α-Ketoglutarate→ Pyruvate + Glutamate
Glucokinase catalyses transfer of phosphate from ATP to glucose as shown
Glucose +ATP→ Glucose-6-phosphate + ADP.
3. Hydrolases: These enzymes hydrolyzes glycosidic bond or ester bonds etc.
Amylase catalyzes hydrolysis of glycosidic bonds of starch.
Amylase
Starch +H2O
Hydrolytic products.
Pepsin catalyzes hydrolysis of peptide bonds of proteins
Pepsin
Protein+H2O
Hydrolytic products.
4. Lyases:They catalyzes splitting of substrates by using mechanism other then hydrolysis
and generates double bonds in products HMG- CoA lyase is an example.
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BIOCHEMISTRY - Questions and Answers
HMG-CoA lyase
HMG-CoA
Acetoacetate+Acetyl-CoA.
Citrate lyase is another example.
Citrate+ ATP+CoA→Oxaloacetate +Acetyl-CoA+ADP+Pi
5. Isomerases: They catalyzes formation of functional, optical and geometrical isomers.
Phosphohexose isomerase inter converts functional isomers.
Glucose-6- phosphate→Fructose -6-phosphate.
Maleyl acetoacetate cis-trans isomerase catalyzes inter conversion of geometric isomers.
Maleyl acetoacetate→ Fumaryl acetoacetate.
6. Ligases:These enzymes catalyzes formation of new compounds by linking two compounds
using energy.
Arginino succinate synthase is an example.
Citrulline+ Aspartate+ ATP→ Argininosuccinate+ AMP+PPi
Propionyl –CoA carboxylase is another example.
Propionyl-CoA+CO2+ ATP →D-Methyl malonyl- CoA+ADP+Pi
2. Define enzymes. Write an enzymatic reaction and properties of
enzymes.
A. Enzymes are biological catalysts. They fasten the chemical reactions in living
organisms.
An enzyme catalyzed reaction consist of substrate, enzyme and product. Substrate is
substance on which enzyme act.
Substrate
Enzyme Product.
Enzyme properties:
Enzymes are proteins and they are not consumed in the reaction. Enzymes are usually
high molecular weight substance. Molecular weight of enzymes ranges form thousands to
millions. Enzymes are able to cut big molecules to small molecules. Conversely enzymes
form big molecules by joining small molecules. Enzymes are more efficient than man made
catalysts and they have enoromous power of catalysis.
3. Explain how enzymes accelerate reactions in living organisms?
A. 1. Enzymes accelerate reactions like that of catalyst because enzymes are catalysts.
2. The acceleration of reaction by catalyst is explained with transition state theory.
3. When enough energy is supplied reactant of a reaction is converted to product.
040
CHAPTER - 5 | Enzymes
4. It involves formation of transition
state of reactant.
Transition
State
5. Usually transition state is unstable so
reactant get converted to stable
product.
6. In presence of catalyst reactant
E
N
E
R
G
Y
attains transition state much easily
and requires less energy. 7. In
Non Enzyme
catalyzed
Enzyme
Catalyzed
Ground State
presence of enzymes transition state
is attained very rapidly and requires
Uncatalyzed
A
C
T
I
V
A
T
I
O
N
O
Progress of Reaction
very less energy.
8. The amount of energy required by reactant to attain transition state is known as
activation energy.
9. Thus enzymes accelerate reactions by lowering activation energy.
4. Write about nomenclature and EC number of enzymes.
A. Enzyme name consist of two parts. The first part indicates substrate name the second part
ends with “ase” and indicates type of reaction enzyme catalyzes.
EC Numbre : It is a enzyme code number given to an enzyme. It has four digits. The first
digit indicates major class, second digit indicates sub class, third digit refers to sub
subclass and final digit indicates specific enzyme.
5. Define active site of enzyme. Write its characteristics.
A. Active site: It is part of the enzyme that is needed for enzyme action or catalysis.
Characteristics of active site: It has two parts.
a. Catalytic site : Part of active site that brings about catalysis.
b. Binding site : Part of active site that binds to
substrate. Aminoacids that makes active site are far
E
away in the absence of substrate. In the presence of
N
substrate active site aminoacid that are apart comes
Z
closely and orient in specific manner to form precise
Y
active site. Active site is three dimentional and are
M
clefts within enzyme molecule. Serine, histidine,
E
Binding Site
Catalytic Site
aspartate, cysteine, glutamate etc usually make up
active site.
6. Write about active site models of an enzyme.
A. Two models are proposed for active site of enzyme.
041
BIOCHEMISTRY - Questions and Answers
1. Lock and key model: As the name implies shape of the active site and substrate are
complementary like that of lock and key in this model. Complementary nature of active
site and substrate shape allows formation of tight enzyme substrate complex to yield
product and free enzyme. However this model fails to explain reversible enzyme
catalyzed reactions due to rigid shape of active site.
Active
site
+
Enzyme (E)
+
Substrate (S)
(ES) Complex
Enzyme
Product
2. Induced fit model: In this model rigid nature of active site is avoided. Enzyme active
site is flexible in this model. Further in the absence of substrate active site is not in
proper form. Binding of substrate to enzyme induces conformational change in enzyme
molecule. As a result precise active site forms to favour tight binding between enzyme
and substrate and catalysis. Since enzyme is unstable in induced conformation it
returns to native state in the absence of substrate. This model allows formation of
enzyme product complex to favour the formation of substrate in the case of reversible
enzyme catalyzed reactions.
Active
site
+
Enzyme (E)
+
Substrate (S)
(ES) Complex
Enzyme
Product
7. Explain influence of various factors on enzyme catalyzed reaction
with suitable diagrams and examples.
A. Enzyme catalyzed reactions are affected by many factors.
They are
1. Substrate concentration.
2. Temparature
3. Hydrogen ion concentration.
4. Enzyme concentration.
5. Cofactors and inhibitors.
042
CHAPTER - 5 | Enzymes
1. Substrate concentration: Initial velocity (VO) of enzyme reaction increases
proportionately in the beginning with increasing substrate concentration (S). Further
increase in substrate concentration leads to slight increase in initial velocity and
reaches maximum (Vmax). Beyond that increase in substrate concentration has no effect
on velocity of enzyme reaction. The plot of (S) versus VO is a rectangular hyperbola. It is
known as Michaleis plot.
Vmax
Vmax
2
Vo
O
Km
(S)
Michaleis-Menton Equation:It is mathematical expression for Michalies plot relating
substrate concentration, initial velocity and maximum velocity.
Vmax (S)
VO =
Where Km= Michaleis constant.
Km+ (S)
From this equation Michaleis constant is obtained. From Michaleis plot substrate
concentration that produces maximum velocity is difficult to obtain. But at least
substrate concentration that produces half maximal velocity is possible to know. So by
substituting this in Michaleis – Menton equation we get.
Vmax
Vmax (S)
=
2
Km+ (S)
On cross multiplication
Km+2 (S)=S
i. e. Km= (S).
Michaleis constant: It is substrate concentration that produces half maximal velocity.
Km significance:
a. Measurment of enzyme activity requires knowledge of Km. It provides substrate
concentration range for proper measurement of enzyme activity.
b. Km indicates affinity of enzyme towards substrate. Km and affinity are inversely related.
High Km indicates low affinity and low Km indicates high affinity.
c. Km values of enzyme are needed for use as drugs and reagents.
043
BIOCHEMISTRY - Questions and Answers
2. Temperature: Enzymes work optimally at a particular temperature. Above or below that
temperature enzyme exhibits low activity.
Optimum Temperature: It is temperature at which enzymes are optimally active. For most of
the enzymes. Optimum temperature is temperature of cell where it exist. Hence optimum
temperature for most of the mammalian enzymes is 37◦ C.
Enzyme activity increases as temperature is increased until optimum temperature is reached.
Beyond that enzyme activity decreases with increasing temperature. Plot of enzyme activity
versus temperature is bell shaped curve. Some of plant derived enzymes and enzymes of
thermophilic bacteria have optimum temperature close to boiling point.
100
Optimum
100
Optimum
Temperature
PH
Enzyme
Enzyme
Activity
Activity
50
50
0
37
Temperature (°C)
70
0
7
PH
14
3. Hydrogen ion concentration: Like optimum temperature enzymes requires a particular PH for
optimum activity. This is known as optimum PH. For most of the enzymes optimum PH ranges
from 5-8 or PHof body or cell in which it occurs. However enzymes with alkaline optimumPH or
acidic optimum PHare known. When PH and enzyme activity are plotted a bell shaped curve is
obtained.
4. Enzyme concentration : The rate of product formation in an enzyme catalyzed reaction is
proportional to concentration of enzyme. The plot of enzyme concentration and rate of product
formation is straight line passing through origin.
5. i. Inhibitors: These substances if present in enzyme catalyzed reaction they inactivate
enzyme. As a result rate of product formation may decrease or not occur.
ii. Cofactors: Several enzymes can work only in presence of some non protein molecules. In
the absence of these molecules enzyme catalysis may be slowed down or not take place.
COMPETITIVE INHIBITION
1. It is a kind of reversible enzyme inhibition.
2. It occurs in presence of competitive inhibitor.
3. The competitive inhibitor is structurally similar to the substrate. Hence it competes with
substrate to bind at active site.
044
CHAPTER - 5 | Enzymes
4. Binding of inhibitor at active site blocks formation of product.
5. By increasing substrate concentration this type of enzyme inhibition is masked.
6. In presence of competitive inhibitor Km of an enzyme increases i. e, affinity decreases. However
Vmax is not altered.
7. The interaction of enzyme inhibitor and substrate is shown as equation below.
E+S
ES
E+I
EI
X
E +P
I = Inhibitor
E+P
P = Product
Example: Classical example for competitive inhibition is inhibition of succinate
dehydrogenase by Malonate which is structurally related to substrate succinate.
Vmax
Substrate
Only
Inpresence of
competitive
inhibitor
Vmax
2
Vo
O
Substrate
Competitive
Inhibitor
Competitive Inhibition
Km Km (S)
COOH
|
Succinate
CH2
Dehydrogenase
Succinate |
+ FAD
Fumarate + FADH2
COOH
CH2
(-)
|
(-) Inhibition
|
CH2
COOH
Competitive
Malonate |
Inhibtor
COOH
Applications
Competitive inhibitors are used in medicine as 1) Antibiotics, 2). Anti cancer agents 3). Drugs for
treating metabolic diseases.
Antibiotics : Competitive inhibitors used as antibiotics to treat bacterial infections are mainly
sulfonamides or sulfa drugs. Most of these drugs contains sulfanilamide an analogue of p- amino
benzoic acid. For growth bacteria need vitamin folic acid. p- amino benzoic acid is required for
formation of folic acid. Sulfonilamide competitively inhibit enzyme involved in synthesis of folic
acid using p- amino benzoic acid. This results in block in folic acid formation. Lack of folic acid
leads to arrest of bacterial growth.
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BIOCHEMISTRY - Questions and Answers
þ-Amino benzoic Acid + Precursor
Block in folic Acid
Formation
(-)
Sulfonila Mide
Arrest of Bacterial
Growth
Anti cancer agents: Several competitive inhibitors are used as anti cancer agents. Folic acid
analogs are most notable among them. Rapidly growing cancer cells requires folicacid for nucleic
acid formation. Dihydrofolate reductase is competitively inhibited by folic acid analogs like
aminopterin and amethopterin. They are used in the treatment of blood cancer. Inhibition of
dihydrofolate reductase results in block in folic acid formation. This in turn affect nucleic acid
synthesis. Lack of nucleic acids leads to arrest of cancer growth.
Reductase
Folic Acid (F)
FH4
Block in Nucleic
Acid Formation
(-)
Aminopterin
Arrest of Cancer
Growth
Competitive inhibitors in treatment of metabolic diseases: Competitive inhibitors are used in the
treatment of gout, atherosclerosis, hypertension etc. i) Gout is disease due to excessive production
of uric acid. It is treated using allopurinol, a competitive inhibitor of enzyme xanthine oxidase
involved in uric acid production. Inhibition of xanthine oxidase leads to decreased uric acid
production.
Hypoxanthine
xanthine
oxidase
(-)
Allopurinol
xanthine
x anthine
oxidase
(-)
Allopurinol
Reduced
Uric Acid
Formation
Gout Cured
ii) Lovastation is competitive inhibitor of enzyme HMG - CoA reductase involved in cholesterol
production. In atherosclerosis cholesterol is present in excess. When used lovastatin blocks
cholesterol production. This leads to arrest of advancement of atherosclerosis.
046
CHAPTER - 5 | Enzymes
HMG-CoA
Reductase
HMG-CoA
(-)
Lova Statin
Block in
Mevalonate
Formation
Reduced
Cholesterol
Formation
Arrest of
Atherosclerosis
Progress
iii) Captopril, lisinopril and enalapril are competitive inhibitors of angiotensin converting
enzyme involved in blood pressure regulation. They are used in the treatment of hypertension.
High Blood Pressure
or
Hypertension
Angio tensin converting
Enzyme
Normal
Blood Pressure
(-)
captopril, Lisinopril
NON COMPETITIVE INHIBITION
1. It is another type of enzyme inhibition.
2. Most of the cases are irreversible enzyme inhibition.
3. Non competitive inhibitors are not structural analogs of substrates.
4. They bind enzyme at site other than active site. Hence no competition occurs between
substrate and inhibitor to bind at active site.
5. Substrate can bind to enzyme inhibitor complex. Rate of formation of product from these
complexes is affected.
6. So in non competitive inhibition Km remains same but Vmax is altered.
7. The interaction of enzyme, substrate, inhibitor is written as
E+S
ES+I
ESI
E+S
ES
E+P
E+I
EI+S
EIS
E+P (SLOW); I=Inhibitor, P=Product.
E+P (SLOW).
Vmax
Substrate
Only
Vmax
In presence of
Non Competitive
Inhibitor
Vmax
2
Vmax
2
Vo
O
E
N
Z
Y
M
E
Substrate
Non Competitive Inhibitor
Km
(S)
047
BIOCHEMISTRY - Questions and Answers
Examples
Several non competitive inhibitors irreversibly inactivate enzymes. So they are often known as
048
CHAPTER - 5 | Enzymes
transfer of one carbon units between substrates. Formimino group of formiminoglutamate is
transferred to FH4as shown below.
Formiminoglutamate+FH4
Glutamate + Formimino FH4.
Formimino group of formiminoFH4 is later transfered to other substrates.
d. Coenzymes involved in transfer of groups: Methyl cobamide coenzyme of vit. B12 is involved in
methyl group transfer. CoenzymeA coenzyme of pantothenic acid is involved in CoA transfer
reactions. Methionine synthase transfers methyl group of methyl cobamide as shown below.
Homocysteine +methylcobamide
Methionine + cobamide
Acyl-CoA synthatase catalyzes transfer of CoA to fatty acid as shown below.
Fatty acid +CoA+ATP
Acyl-CoA+AMP+PPi
e. Nucleotide coenzymes : Many nucleotides function as coenzymes. They are ATP, GTP, CTP,
ADP, GDP, CDP, PAPS and SAM.
Metals
Enzymes in which metal is part of enzyme molecule are known as metalloenzymes and metal is
attached through coordinate bond. More over metal takes part in catalysis. Removal of metal
leads to loss of catalytic activity of enzymes. Cytochrome oxidase, catalase, succinate dehydrogenase are examples for iron metallo enzymes.
Metal dependent enzymes
Are those enzymes in which metal is not part of enzyme molecule but it is required for catalysis. It
act as bridge between enzyme and substrate. In the absence of metal enzyme is unable to form
enzyme substrate complex. Hexokinase, galactokinase and pyruvate kinase are dependent on
magnesium for activity.
Metal activated enzymes
In presence of metals activity of these enzymes is increased to many folds. In the absence of metal
they catalyze reaction but at low rate. Chloride is an activator of amylase and angiotensin
converting enzyme. Calcium is an activator of trypsin.
8. Define isoenzymes. Give examples. How they are separated?
A. Isoenzymes are multiple forms of an enzyme. They catalyze same reaction but differ in
physicochemical properties. They may occur among organs, species. They are present in
blood and other fluids. Isoenzymes of several dehydrogenases, transaminases and
phosphatase are identified.
Lactate dehydrogenase Isoenzymes
Lactate dehydrogenase is a oligomeric enzyme. It is a tetramer. Made up of two types of sub
049
BIOCHEMISTRY - Questions and Answers
units. Isoenzymes of lactate dehydrogenase differ in quaternary structure or sub unit
composition. Sub units present in lactate dehydrogenase are H and M type. Different
isoenzymes of lactate dehydrogenase and their composition is given below.
Name of isoenzyme
Sub unit composition
LDHI
H4 o H H H H
LDH2
H3M or H H H
LDH3
H2 M2 or H H M M
LDH4
HM3 or H MMM
LDH5
M4 or MMMM
Separation of isoenzymes: Electrophoresis is used for separation of lactate dehydrogenase
isoenzymes. When serum is subjected to electrophoresis at PH8. 6 the five isoenzymes of lactate
dehydrogenase separates into 5 bands. The five isoenzymes bands corresponds to LDH1, LDH2
LDH3, LDH4, and LDH5.
Alkaline phosphatase Isoenzymes: Iso enzymes of alkaline phosphatase are tissue or organ
specific. Four organ specific isoenzymes are known. They can be separated on electrophoresis.
The four organ specific isoenzymes are derived from bone, intestine, liver and placenta. These
isoenzymes of alkaline phosphatase differ in composition. They are glycoproteins. The
carbohydrate content of isoenzymes is different.
Creatine phosphokinase Isoenzymes: Three isoenzymes exist for creatine phosphokinase.
Creatine phosphpokinase is a oligomeric protein contain two sub units. Isoenzymes of creatine
phosphokinase differ in quaternary structure. Sub unit composition varies among isoenzymes of
creatine phosphokinase. Two types of sub units are found in isoenzymes. They an M and B. The
subunit composition of three isoenzymes is given below.
Isoenzyme
Subunit Composition
Ck1
BB
Ck2
MB
Ck3
MM
9. Write about regulation enzyme activity by covalent modification.
A. 1. By covalently attaching group to enzyme molecule its activity is regulated.
2. Phosphate and nucleotide are groups used to regulate enzyme by covalent attachment.
3. Serine residue of enzyme molecule is site of phosphorylation. Tyrosine residue of
enzyme molecule is site of nucleotide attachment.
4. Phosphorylation is catalyzed by protein kinase and adenyl transferase catalyzes
nucleotide attachment.
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CHAPTER - 5 | Enzymes
5. Glycogen synthase and glutamine synthatase are two enzymes whose activity is
regulated by attachment of phosphate and nucleotide respectively.
Glycogen synthase (High active)+ATP
Glutamine synthetase (More active)+ATP
Phosphorylated glycogen synthase (Less active)+ADP
Glutamine synthatase-AMP (Less active)+PPi
10. Write an essay on clinically important enzymes.
A. Estimation of enzymes in blood and other body fluids in normal and disease
conditions is important for diagnosis and prognosis. Under normal conditions blood
contains some enzymes. These enzyme are divided into a. Functional enzymes. b. Non
functional enzymes.
Functional enzymes: These enzymes are present in significant amounts in blood because
they have physiological function.
Non functional enzymes: These enzymes are present in blood only in small amounts
under normal conditions. But concentration of these enzymes increases when organs are
damaged due to disease or injury. The amount of enzyme present is proportional to extent
of disease. Hence estimation of enzyme in blood is used to confirm diagnosis that is
made by physical examination. Further estimation of enzymes in blood is also used to
know effectiveness of treatment. There fore measurement of enzyme levels is both
diagnostic as well as prognostic importance.
In the case of secretory enzymes block in secretory route causes increase in levels of these
enzymes.
Apart from serum, cerebrospinal fluid (CSF), Synovial fluid, peritonial fluid and
amniotic fluid are used for measurement of enzyme levels.
Some routinely measured clinically important enzymes levels in normal condition and
associated pathological states are given below.
1. Aminotransferases or Transaminases: Two important aminotransferases or
transaminases are aspartate transaminase (AST) and alanine transaminase (ALT). The
normal AST level in blood is about 3-20 units /litre (U/L). ALT normal level is 4-20U/L.
These enzymes are also known as SGOT (serum glutamate oxaloacetate
transaminase)and SGPT (Serum glutamate pyruvate transaminase). These two
enzymes differ in distribution among tissues. Heart is rich in AST. However liver
contains both of them in equal amounts. There fore in acute infective hepatitis both
enzymes are elevated. The levels of these enzymes reaches peak value following infection
and return to normal level in a week. AST level is increased in myocardial Infarction or
heart attack and hence it is estimated in diseases specific to heart. ALT level rises in
diseases of liver because ALT is more in liver only. Some of the liver diseases associated
with raise in ALT level are alcoholic cirrhosis, biliary obstruction, cancer and toxic
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BIOCHEMISTRY - Questions and Answers
hepatitis. In lung disease both transaminases in serum are elevated. Skeletal muscle is
another organ that contain significant amount of ALT. Hence ALT level increases in
diseases affecting skeletal muscle like muscular dystrophy and muscle injury.
2. Alkaline Phosphatase: Normal level of this enzyme in blood is 20 -90 U/L. Rickets,
obstructive jaundice, hyper parathyroidism, bone cancer and cancer are some diseases
associated with increased level of this enzyme in blood. Liver secretes this enzyme into
bile. So block to flow of bile causes increase in blood level of this enzyme. Hence in
obstructive jaundice level of enzyme increases by ten fold. In intestinal disorders, lung
and kidney damage. Leukaemia, congestive heart failure and Hodgkins disease also
the enzyme level is more.
3. Lactate Dehydrogenase (LDH) : Normal level of this enzyme is 70 to 90 U/L. Serum LDH
level increases mainly in heart attack or myocardial infarction. The level of this enzyme
in serum increases within 24 hrs of heart attack and reaches maximum level in 2 to 3
days and returns to normal in 7 days. Acute hepatitis, pernicious anaemia,
megaloblastic anaemia, muscular dystrophy and blood cancer levels of this enzyme is
more.
4. Creatine Phosphokinase (CPK) : Normal level of this enzyme is 12 to 60 U/L. Skeletal
muscle contains more of this enzyme. Hence it is elevated in skeletal muscle diseases
like muscular dystrophy, muscle injury and Polio myositis. Severe muscular exercise
may raise plasma CPK level. It is also elevated in other than diseases of skeletal muscle
like hypothyroidism, tetanus, etc.
5. Acid Phosphotase : This enzyme is concentrated in prostate gland. Normal level of this
enzyme is 2. 5-12U/L. Its level is mainly elevated in prostate cancer. In bone diseases
and breast cancer also level of this enzyme is increased.
6. Gamma glutamyltranspeptidase (GGT): Normal level of this enzyme in plasma is upto
30U/L. Like alkaline phosphatase this enzyme is secreted into bile. Hence in liver
disease like obstructive jaundice, alcoholic cirrhosis level of this enzyme is increased.
In brain lesions level of this enzyme is elevated.
7. Isocitrate dehydrogenase (ICDH): Apart from plasma this enzyme is found in
cerebrospinal fluid (CSF) also. Hence measurement of this enzyme is useful in diseases
affecting brain. In the plasma normal level of this enzyme is upto 5U/L. In
inflammatory conditions level of this enzyme is elevated. In acute infective hepatitis
and toxic hepatitis level of this enzyme is increased. However in obstructive jaundice
level of this enzyme is more elevated than in brain tumors.
8. Amylase: This is a secretory enzyme. It is secreted by pancreas and parotid gland.
Normal level of this enzyme in plasma is 800- 1800U/L. It is mainly elevated when
there is block in its secretory route. So in acute pancreatitis and parotitis level of this
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CHAPTER - 5 | Enzymes
enzyme is more. Further in intestinal obstruction and mumps also the enzyme level is
increased in plasma.
9. Lipase:It is secreted by pancreas along with amylase. The level of this enzyme in
plasma is about 150U/L. It is mainly increased in diseases affecting pancreas like acute
pancreatitis and cancer of the pancreas. In other abdominal diseases also level of this
enzyme is elevated. They are abdominal lesions, peritonitis, intestinal obstruction,
perforated peptic ulcer etc. ,
11. Write a note on clinically important isoenzymes.
A. High plasma enzyme level may not indicate severity of disease and organ involved because
plasma enzyme is derived from several tissues. But iso enzyme level indicates organ
involved in disease because isoenzymes are organ specific. Further like enzymes
distribution of isoenzymes among organs varies. If an organ is diseased more isoenzyme of
that organ enters plasma. Estimation of that isoenzyme level in plasma is used to confirm
organ affected. Thus isoenzyme estimation is useful in differential diagnosis.
1. Isoenzymes of Lactate Dehydrohydrogenase or LDH isoenzymes; The five isoenzymes of
LDH differ in their distribution. Each isoenzyme has unique source. The proportion of
isoenzymes in serum is also different. Heart is rich in LDH1 so LDH1 in serum is mostly
derived from heart. Likewise LDH5 in serum is derived from skeletal muscle because it
is rich in LDH5. Liver contains LDH2 to LDH5in different proportions.
LDH1 level in serum increases when heart muscle is damaged as occurs in myocardial
infarction. Hence measurement of LDH1 isoenzyme in serum is more sensitive index of
myocardial damage than total LDH activity. Like wise LDH5 is more sensitive index of
skeletal muscle damage.
2. Isoenzymes of creatine phosphokinase or CPK isoenzymes: Plasma CPK activity is
contribution of three isoenzymes i. e. CPK1, CPK2, CPK3. The CPK2 isoenzyme accounts
for about 2% of total CPK in normal people. But it increase by ten times (20%) with in
few hours of myocardial infarction. There fore CPK 2 estimation serve as better index of
heart attack.
3. Isoenzymes of alkaline phosphatase: Plasma alkaline phosphatase activity is due to
four of its isoenzymes. The four isoenzymes of alkaline phosphatase are organ specific.
They are derived from bone, liver, placenta and intestine. Alkaline phosphatase
isoenzymes measurement is useful in differential diagnosis. In metastatic carcinoma
liver lesions are differentiated from bone lesions by measuring alkaline phosphatase
isoenzymes.
12. Write a note on allosteric enzymes.
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BIOCHEMISTRY - Questions and Answers
A. 1. Allosteric enzymes consist of many sub units.
2. Their activity increases in presence of activator and decreases in presence of
inhibitors.
3. Activators and inhibitors binds at allosteric other than substrate binding site.
4. Further allosteric inhibitors are not structurally related to substrates of allosteric
enzymes.
Kinetics
1. Allosteric enzymes exhibit kinetics different from classical Michaelis- Menton
kinetics.
2. A sigmoidal shape curve is obtained when substrate concentration and initial
velocity are plotted instead of rectangular hyperbola.
3. Further the curve shifts to right in presence of allosteric inhibitor and to left in
presence of allosteric activator.
4. The sigmoidal curve also indicates a rapid change in initial velocity in presence of
substrate alone, allosteric inhibitor and allosteric activator.
Example
Allosteric Activator
Substrate only
Allosteric Inhibitor
V
O
(S)
1. Aspartate carbamoyltransferase (ACT) is classical example for allosteric enzymes.
2. It catalyzes formation of carbamoyl aspartate from carbamoyl phosphate and
aspartate as shown below.
Carbamoyl phosphate + aspartate
carbamoyl aspartate +phosphate
3. CTP is allosteric inhibitor and ATP is allosteric activator.
4. ATP converts less active ACTase to high active form.
5. In contrast CTP converts high active form to less active form. ATP and CTP bind at
separate sites other than substrate binding sites.
13. Write a note on pro enzymes.
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CHAPTER - 5 | Enzymes
A. 1. Pro enzymes are inactive precursor forms of enzymes.
2. They are also known as zymogens.
3. Limited proteolysis removes few amino acid residues or a portion of pro enzyme
molecule which results in conversion of pro enzyme to active enzyme.
4. Enzymes of protein digestion and blood clotting factors are synthesized in inactive
proenzyme forms.
5. They are converted to active form when physiological need arises.
6. At acidic PH pepsinogen is converted to pepsin in stomach. It involves cleavage of
peptide bonds.
Pepsinogen → Pepsin
7. Pencreatic proteases are produced in proenzymes form.
8. Enterokinase initiates conversion of these proenzymes to enzyme. It converts
trypsinogen to trypsin initially.
9. The remaining pancreatic proenzymes are converted to enzymes by trypsin. It converts
chymotrypsinogen, proelastase, pro phospholipase, and procarboxypeptidase to
chymotrypsin, elastase, phospholipase and carboxypeptidase respectively.
10. In presence of factor X and V prothrombin is converted to thrombin which in turn
converts fibrinogen to fibrin during blood clotting.
Enterokinase
Trypisn
Trypsinogen → Trypsin Chymotrypsinogen → Chymotrypsin
Proelastase → Elastase
Prothrombin → Thrombin
Pepsingen → Pepsin
pH 1.5
14. Explain enzyme induction and repression with examples.
A. Induction:In presence of an inducer synthesis of inducible enzymes is more. It is known as
induction.
Example: Usually in E. coli lactase is produced in small amounts. If E. coli is grown in
lactose containing medium synthesis of lactase increases. So lactose acting as inducer
increases synthesis of lactase which is an inducible enzyme.
Repression: In presence of repressor synthesis of enzymes required for repressor formation
is blocked. This is known as repression.
Example: When histidine is present in S-typhi medium synthesis of enzymes involved in
formation histidine is blocked. So histidine acting as repressor blocks its own synthesis.
15. Explain co operativity phenomenon.
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BIOCHEMISTRY - Questions and Answers
A.
1. Cooperativity phenomenon is proposed to explain this rapid change in velocity of
allosteric enzyme.
2. According to this the allosteric enzyme exist in two forms a 'T' tensed less active
state and 'R' relaxed high active state.
3. Binding of substrate to 'T' form is slow and causes conformational change to 'R'
form.
4. Further binding of substrate to 'R' form is rapid.
5. Allosteric inhibitor stabilizes enzyme in T form where as allosteric activator
stabilizes enzyme in R form.
Other model questions are
16. Define cofactors. Classify. Give examples. Write reaction for each
example.
17. Explain competitive inhibition with examples.
18. Describe enzyme inhibition.
19. How competitive inhibitors are useful in clinical medicine?
20. Write a note on influence of substrate concentration on enzymatic
reaction.
21. Define Km. Write its importance.
22. Explain effect of the following on enzymatic reaction
A. Hydrogen ion concentration. B. Temperature.
23. Write about competitive inhibitors used as chemotherapeutic
agents.
24. In what diseases following enzymes level in blood is more
A. SGOT
B. Alkaline phosphatase
25. Write about enzyme poisons.
26. Write an essay on clinical enzymology.
27. Write a note on non competitive inhibition.
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CHAPTER - 5 | Enzymes
28. Define proenzyme, isoenzyme and coenzyme. Give examples for
each.
29. Write the effect of substrate concentration on enzyme reaction.
30. What is meant by feedback inhibition. Give examples.
31. Write about cardiac enzymes and isoenzymes.
32. Define metalloenzymes, metal dependent enzymes and metal
activated enzymes. Give examples for each.
33. Define functional enzymes and non functional enzymes. Give
examples.
34. Write briefly about LDH isoenzymes.
35. Write the diagnostic importance of serum transaminases.
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CHAPTER - 6 | Nucleotides and Nucleic Acids
Chapter
6
Nucleotides and Nucleic Acids
NUCLEOTIDES
1. Define nucleoside. Give examples.
A. Nucleoside: It consist of nitrogenous base and sugar. Nucleosides are classified based on
nitrogenous base present. They are purine nucleosides and pyrimidine nucleosides.
Purine nucleoside: They contain purine bases adenine and guanine. Adenosine is
nucleoside of adenine and guanosine is the nucleoside of guanine.
Adenosine
Adenine
Ribose
Guanosine
Guanine
Ribose
Pyrimidine nucleosides : These nucleosides are composed of pyrimidine bases.
Cytosine, Uracil and thymine are pyrimidine nitrogenous bases. Cytidine is nucleoside of
cytosine. Uridine and thymidine are nucleosides of uracil and thymine respectively.
Cytidine
Thymidine
Cytosine
Thymine
Ribose
Uridine
Uracil
Ribose
Ribose
2. Define nucleotide. Give examples.
A. Nucleotides: They are phosphorylated nucleosides. A nucleotide consist of nitrogenous
base, sugar and phosphate.
Nucleotide
Purine or Pyrimidine base
Sugar
Phosphate
Purine nucleotides:In these nucleotides nitrogenous base is purine. They are adenosine
monophosphate (AMP), adenosine diphosphate (ADP)and ATP. Like wise guanosine
monophosphate (GMP), guanosine di phosphate (GDP) and GTP.
Adenine
Sugar
Phosphate
AMP
Pyrimidine nucleotides: These nucleotides contain pyrimidine nitrogenous bases like
thymine, cytosine and uracil. They are cytidine monophosphate (CMP), cytidine di
phosphate (CDP) and CTP. Likewise TMP, TDP, TTP;UMP, UDP, UTP.
Cytosine
058
Sugar
Phosphate
CMP
CHAPTER - 6 | Nucleotides and Nucleic Acids
3. Write functions of nucleosides and nucleotides.
A. Nucleotides and nucleosides have several important biological functions.
1. Nucleotides are involved in signal traduction.
2. Nucleotides are required for the formation of nucleic acids.
3. Nucleotides are high energy compounds.
4. Nucleotides are components of some water soluble vitamin coenzymes.
5. Nucleotides serve as second messengers. Many hormones mediate their action
through second messengers.
6. Some nucleotides function as donors of sugars, nitrogenous compounds and phosphates.
7. Nucleosides function as carriers or donors of groups.
8. Nucleoside analogs are used as anti cancer agents.
9. Some nucleotides function as alarmones. They alaram cell when something goes
wrong in the cell.
4. What are unusal nucleosides? Give examples.
A. Unusual nucleosides: Are those nucleosides in which nitrogenous base and sugar are
unusal. Ribothymidine and pseudouridine are examples of unusual nucleosides.
Ribothymidine consist of thymine and ribose. It is present in ribonucleic acids (RNA)
which is not usually found. Pseudouridine is a unusual nucleoside of uracil. In this
nucleoside carbon –carbon bonding occurs between uracil and ribose instead of usually
carbon –nitrogen bond.
5. Write about Synthetic analogs of purines, pyrimidines and
nucleosides
A. Some synthetic purine and pyrimidine analogs are used as anticancer agents and antiviral
agents. Purine analogs are mercaptopurine, thioguanine, aminopurine etc. Pyrimidine
analog is 5- fluro uracil.
Nucleoside analogs are used as anticancer agents, antiviral agents and mutagens.
Deazauridine, 6-aza uridine, ara-A, ara-C and fluro deoxyuridine are nucleoside analogs
used as anticancer agents. Azidothymidine (AZT), dideoxy cytidine and iododeoxyuridine
are used as anti viral agents. Bromodeoxy uridine is used as mutagen.
6. W rite about Pharmacologically important purines
A. Caffeine of coffee, theophylline of tea and theobromine of tea are some purines of
pharmacological importance. Caffeine and theophylline act as CNS stimulants. Inhalers
used by asthma patients contains theophylline. It releives nasal and bronchial congestion
of these patients.
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BIOCHEMISTRY - Questions and Answers
7. Describe nucleotides of biochemical (Physiological) importance
A. Cells present in various organs of human body and other mammals contain several free
nucleotides. These free nucleotides are involved in many biochemical or biological process
that are given below.
i. Adenine nucleotides and their physiological importance
1. Adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine
monophosphate (AMP) are most important adenine nucleotides.
2. ATP, ADP, and AMP are high energy compounds.
3. ATP is popularly called as 'energy currency' of cell. Energy exchange in biochemical
reactions occurs through ATP.
4. ADP is required for the formation of ATP in electron transport chain and in energy
yielding reactions.
5. cAMP, a cyclic nucleotide of adenine is known as second messenger. Many
hormones action occurs through cAMP.
6. Many coenzymes of water soluble vitamins contain adenine nucleotides. For
example NAD+, FAD, NADP, coenzyme A and cobamide coenzymes.
7. PAPS (Phospho adenosine phosphosulfate) serve as donor of sulfate in biosynthetic
reactions.
8. ATP is required for replication and protein biosynthesis.
9. Some adenosine nucleotides are involved in blood pressure and platelet function.
10. Diadenosine nucleotides are neurotransmitters.
11. Oligoadenylate mediates action of interferon.
12. Poly adenylate serve as tail of mRNA
ii. Guanine nucleotides and their physiological importance
1. Like ATP, ADP; Guanosine triphosphate (GTP) and guanosine diphosphate (GDP) also
exist in cells.
2. GTP and GDP are high energy compounds.
3. Cyclic GMP or cGMP mediates actions of several hormones.
4. GTP and GDP are components of G-proteins which are involved in signal transduction
of several physiological processes like taste, odor, vision, metabolic regulation etc.
5. GTP is required for replication and protein biosynthesis.
6. Guanine nucleotides are required for catalytic function of ribonucleic acids or
ribozymes.
7. Mucopolysacharide formation requires guanine nucleotides.
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CHAPTER - 6 | Nucleotides and Nucleic Acids
iii. Cytosine nucleotides of physiological importance
1. CTP (Cytidine triphosphate), CDP (Cytidine diphosphate) and CMP (Cytidine
monophsphate), are high energy compounds.
2. Cyclic CMP or cCMP also occurs in cells.
3. CDP, CMP serve as donor of nitrogenous compounds during biosynthesis.
4. CMP-NANA serve as donor of NANA in the biosynthesis of gangliosides.
5. CDP- choline serve as donor of choline in phospholipids biosynthesis.
iv. Uracil nucleotides of physiological importance
1. UTP (Uridine triphosphate), UDP (Uridine diphosphate) and UMP (Uridine
monophosphate) are high energy compounds.
2. UDP- Glucuronic acid serve as donor of glucuronic acid in the synthesis of
mucopolysacharides, bilirubin diglucuronide and detoxification reactions.
3. UDP is carner of sugar and aminosugars needed for synthesis of glycogen, gangliosides,
glycoproteins etc.
v. Thymine nucleotides of physiological importance
1. TTP (thymidine triphosphate), TDP (thymidine diphosphate) and TMP (thymidine
monophosphate) are high energy compounds.
2. TTP and d TTP are used for the synthesis of nucleic acids.
vi. Hypoxanthine and xanthine nucleotides of physiological importance
Hypoxanthine and xanthine are two purine bases not found in nucleic acids. But their
nucleotides have important role in metabolism.
1. I DP (inosine diphosphate), IMP (inosine monophosphate) are nucleotides of
hypoxanthine. They are high energy compounds.
2. IMP is intermediate in purine nucleotide biosynthesis.
3. XMP (xanthosine monophosphate) is an intermediate in purine nucleotide
biosynthesis.
8. What are nucleic acids? Classify. Write their composition.
A. Nucleic acids are acidic substances present in nucleus. Two types of nucleic acids are found
in cells. They are deoxy ribonucleic acid (DNA) and ribonucleic acid (RNA). Pentose sugar
in DNA is deoxy ribose where as in RNA it is ribose. Both DNA and RNA are polymers of
nucleotides and often referred as polynucleotides. Due to deoxy ribose nucleotides present
in DNA are known as deoxy ribonucleotides. They are designated as dADP, dATP;
dGDP;dGTP, dTTP, dTDP, dCTP, dCDP etc. RNA contains ribonucleotides they are
designated as ATP, ADP;GTP, GDP;CTP, CDP;UTP, UDP etc.
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BIOCHEMISTRY - Questions and Answers
NUCLEIC ACIDS
9. Describe structure and function of DNA.
A. DNA structure
1. It consist of two poly nucleotide chains.
2. These polynucleotide chains coil along long axis in the form double helix.
3. Each polynucleotide is made up of four types of nucleotides.
4. Individual nucleotides are joined by phosphodiester bonds.
5. Four types of nucleotides are present in two chains. They are adenylicacid, guanylic
acid, cytidylic acid and thymidylic acid.
6. Each poly nucleotide chain or strand has direction or polarity and 5'and3'ends.
These ends may be in either free form or phosphorylated form.
7. Sugar and phosphate forms back bone of two strands.
8. The two strands are complementary to each other.
9. Base composition of a strand is complementary to opposite strand. If thymine is found
in one strand adenine appears in opposite strand and vice versa. Like wise if guanine
appears in one strand cytosine is found in opposite strand and vice versa.
10. Further bases of opposite strands are involved in pairing. It is popularly known as base
pairing rule. Adenine of one strand pairs with thymine of opposite strand through two
hydrogen bonds. Guanine of a strand pairs with cytosine of opposite strand through
three hydrogen bonds.
11. Due to the presence of three hydrogen bonds GC pair is stronger than AT pair.
12. This base pairing makes copying mechanism simple and easier.
13. Complementary nature of two strands and base pairing rule are most outstanding
features of Watson-Crick model.
51
31
Hydrogen Bonds
A : Adenine
A====T
G : Guanine
C : Cytosine
T : Thymine
C====G
31
51
14. The base pairs are stacked. The pitch of the helix is 34 A◦and contain ten base pairs. The width
of the helix is 20 A◦.
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CHAPTER - 6 | Nucleotides and Nucleic Acids
15. Due to the presence of hydrogen bonds through out molecule DNA is highly stable.
16. Major and minor grooves are present on the double helix.
17. Watson- Crick model DNA is known as B-DNA.
Functions
1. DNA is genetic material of living organisms. It contains all the information needed for the
development of entire organism or individual.
2. DNA is transferred from parent to offspring or generation to generation.
3. DNA contains information required for formation of individuals proteins.
4. Information is present in DNA in the form of genes.
5. Amount of DNA present in the cell of an organism depends on complexity of organisms.
6. Human cells contain more DNA than bacterial cells or viruses.
7. DNA amount in given cell is independent of nutritional or metabolic state of the organism.
8. DNA flows from generation to generation in any given species.
9. DNA determines physical fitness of an organism or susceptibility to disease.
10. How many types of RNA are there? Name them. Write structure
and function of each one.
A. There are three types of RNAs in cells. They are present in prokaryotes as well as
eukaryotes.
They are
1) Messenger RNA or mRNA
2) Transfer RNA or tRNA
3) Ribosomal RNA or rRNA.
Messenger RNA
1. Majority of mRNA molecules are linear polymers.
2. They contain about 1000-10, 000 nucleotides.
3. They have 3' or 5' free or phosphorylated ends
4. Life span of m RNA
molecules varies from few
Hair pin
Loop Poly A Tail
minutes to days.
5. Some RNAs have secondary
cap
structure.
6. Intra strand base pairing
1
5
31
m
GTP
among complementary
bases leads to folding of
linear molecules into hair pin like secondary structure.
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BIOCHEMISTRY - Questions and Answers
7. In some m RNA at 5'and 3' ends special nucleotides or sequences occurs.
8. Poly A tail is present in some mRNAs at 3' end.
9. At 5' end some mRNA are capped. Methylated GTP is cap.
10. At 5' and an AG rich shine – Dalgarno sequence is present in some mRANs.
Functions:
1. mRNA carries genetic information from nucleus to cytoplasm.
2. Generally one mRNA contains information for formation one protein.
3. The sequence of mRNA is complementary to strand from which it is copied.
4. In mRNA genetic information is present in the form of genetic code.
5. Occassionaly one mRNA contains information for the formation of more than one protein.
Transfer RNA (tRNA)
It is smallest of all RNAs and contains up to 1000 nucleotides. It contains several unusual bases
like pseudouridine, dihydro uracil, mythylated adenine and guanine and isopentenyl adenine etc.
Due to intra strand base pairing between complementary bases tRNA molecules exist in
characteristic secondary structure shape. Secondary structure of tRNAs is in the form of clover
leaf.
Structural features of clover leaf
1. It contains an aminoacid arm at 3' end CCA is characteristic sequence of aminoacid arm.
2. An arm containing unusual pseudouridine and ribothymidine. Hence it is known as TφC arm
in which pseudouridine is indicated with psi (φ) symbol.
3. An anticodon arm containing IGC sequence. Generally this arm recognizes codon on mRNA.
4. Dihydrouridine (UH2) arm or DHU arm that contains dihydrouracil.
5. A guanine containing 5'end.
6. An extra arm in some tRNAs exist between TφC arm and anti codon arm.
A31
C
51
DHU
Arm
C
T
D
H
U
φ
C
Extra Arm
Anticodon Arm
064
T φ C Arm
CHAPTER - 6 | Nucleotides and Nucleic Acids
Functions
1. It serve as adaptor molecule in protein biosynthesis. It carries aminoacids to site of protein
synthesis.
2. For every aminoacid one specific tRNA molecule exist.
3. Stability of eukaryotic and prokaryotic tRNA varies.
Ribosomal RNA (rRNA)
It is found in combination with proteins in ribosomes. It contains about 100-600 nucleotides.
Prokaryotic and eukaryotic ribosomes contain several RNA that differ in sedimentation
coefficient. Due to intra strand base pairing between complementary bases secondary structures
are found in rRNA molecules. They are known as domains. 16S rRNA with 1500 nucleotides has
four major domains.
Functions
1. It is involved in initiation of protein synthesis.
2. It is required for the formation of ribosomes.
11. Write very briefly about organization of eukaryotic DNA.
A. In the nucleus of eukaryotes DNA is present as nucleo protein chromatin which is
combination of DNA and basic proteins histones. In eukaryotes chromatin is present as
chromosomes. Each eukaryotic cell contains 23 pairs of chromosomes and one DNA
molecule is present in each chromosome. Chromatin has beaded structure.
Beads
Linker DNA
Chromosome
Histone
Bead is a nucleosome in which DNA is coiled around basic protein histone octamer. The
nucleosomes are joined by linker DNA as well as histones.
12. Explain the following
A. DNA denaturation B. Mitochondrial DNA
A. DNA denaturation
Exposure to heat leads to separation of strands. On cooling strands coils. Formation of
DNA molecules due to base pairing between two strands is known as annealing. It is very
useful in genetic engineering particularly in techniques based on hybridization.
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BIOCHEMISTRY - Questions and Answers
Heat
Cool
Strands
DNA
Separation
DNA
B. Mitochondrial DNA
It is DNA present in mitochondria of eukaryotic cell. It is different from DNA present in
nucleus. Mitochondrial DNA base composition differs from nuclear DNA. Usually
mitochondrial DNA is circular double stranded molecule.
13. Write differences between DNA and RNA
A. Differences between DNA and RNA
DNA
RNA
1. Double stranded molecule.
1. Single stranded molecule
2. Found in combination with proteins.
2. Except rRNA other RNAs exist as free
molecules
3. Pentose sugar is deoxyribose.
3. Pentose sugar is ribose.
4. Sum of the purine bases is equal to
4. Some of purine bases is not equal to sum of
sum of pyrimidine bases.
pyrimidine bases.
A+G=C+T
A+G ≠ C+T
5. Pyrimidine base uracil is absent.
5. Thymine a pyrimidine base is not usually
found.
6. Only one form of DNA predominantly
6. More than three types of RNAs occurs.
occurs.
7. Resistant to alkaline hydrolysis.
7. Easily hydrolyzed by alkali.
8. Modified bases are usually absent.
8. Unusual and modified bases are found.
9. Lacks catalytic activity.
9. Some RNAs act as enzymes or posses
catalytic activity.
14. Define ribosome. Classify them. Write their composition.
Draw diagrams.
A. They are complexes of proteins and nucleic acids. They are large molecules compared to
proteins and nucleic acids. They are classified based on sedimentation coefficients (S).
Even ribonucleic acid components of ribosomes are identified based on sedimentation
coefficients. . There are two types of ribosomes. They are70S ribosome of prokaryote and
80S ribosome of eukaryotes.
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CHAPTER - 6 | Nucleotides and Nucleic Acids
50S
34 Proteins + 5S and 23S RNA
30S
21 Proteins + 16 S RNA
70S
The 70S ribosome contains a large 50S sub unit and small 30S subunit. The 50S sub unit
consist of 34 proteins and two RNAs or 23S and 5S RNA. The 30S subunit contains 21
proteins and one 16S RNA. The 80S ribosome contains a large 60S subunit and small 40S
subunit.
15. Define a plasmid. Give example. How they are useful?
A. Plasmids are circular DNA molecules present in antibiotic
resistance bacteria. pBR322 of an intestinal bacteria E. Coli is
an example.
pBR322
Plasmids contain genes for inactivation of antibiotics. They are
used as vectors in genetic engineering.
Other model questions are
16. Name nucleoside and nucleotides of adenine and cytosine. Write their
importance.
17. Define nucleoside. List purine and pyrimidine nucleosides.
18. Give examples for cyclic nucleotides. Write their importance.
19. Write a note on tRNA.
20. List purine nucleotides and their functions.
21. Give examples for pyrimidine nucleotides. Mention their functions.
22. Write a note on mRNA.
23. Write about synthetic purine and pyrimidine analogs of clinical
importance.
24. Define chromatin and nucleosome. Explain how they are related?
25. Write the functions of nucleic acids.
26. Purine and pyrimidine bases.
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CHAPTER - 7 | Biological Oxidation
Chapter
7
Biological Oxidation
1. Define biological oxidation. Write its importance. Explain the role of
oxygen in this process.
A. Biological oxidation is related to utilization of respiratory oxygen (O2) in living organisms.
Biological oxidation is a major way of regenerating coenzymes which are reduced in
metabolic pathways. It is final aspect of all energy yielding compounds in the body
Oxygen act as final electron acceptor in the respiratory chain and get reduced to water.
Oxygen is directly associated with biological oxidation reactions in the body. Formation of
several new compounds and removal of toxins are dependent oxygen. . Oxygen is a source
for the formation of reactive oxygen species (ROS) and super oxide etc. However oxygen is
extremely toxic to cells at high concentration. This property of oxygen is exploited in cancer
therapy by combining with radiation.
2. What are high energy compounds? Give examples. Mention their
other roles.
A. High energy compounds are those compounds that yield large amount of free energy on
hydrolysis. The energy yield is expressed as standard free energy change. ∆G01is the
symbol used for standard free energy change. Usually compounds that yield more than 7. 3
kcal / mol of energy are considered as high energy compounds. Nucleoside mono, di and
triphosphates, phosphocreatine, thiol esters, enol phosphates and acylphosphates are few
such high energy compounds.
1. Nucleoside phosphates :ATP, ADP and AMP are adenine based nucleoside phosphates.
ATP is involved in the energy transfer in living systems. It is known as energy currency
of cell. Energy released in exergonic reactions is used to form ATP and in endergonic
reactions energy released on ATP hydrolysis is consumed.
Exergonic
ADP + Pi
ATP
Reaction
Endergonic
ATP
ADP+Pi
reaction
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CHAPTER - 7 | Biological Oxidation
So ATP is link between energy yielding and energy consuming reactions. In addition
energy released on ATP hydrolysis is used for muscle contraction, cell motility,
transport of ions across membrane etc. GTP, GDP, CTP, CDP, TTP, TDP ; UTP and
UDP are nucleoside di and triphosphates that serve as high energy compounds in living
systems.
2. Acyl phosphates : They are formed from two types of acids. For example 1, 3–bis
phosphoglycerate is the combination of glyceric acid and phosphoric acid. So it is mixed
anhydride. On hydrolysis it yields about 12 kcal /mol energy and 3- phosphoglycerate is
product.
OH O
|
||
P -O-CH2 –CH—C - O - (P)
P - Phosphate
1, 3-Bis phosphoglycerate
3. Enol phosphates: They are esters of enols with phosphoric acid. Phospho enol pyruvate
(PEP) is an example for enol phosphate. On hydrolysis it yields about 15 kcal / mol
energy and pyruvate is product.
O– P
|
CH2= C – COOH
PEP
4. Thio esters: They are esters of thiol with acid. Acetyl –CoA is an example for thiol ester.
On hydrolysis 7. 5 kcal /mol energy is released and acetic acid is product.
O
||
CH3 – C ~ S-CoA
5. Phosphocreatine: It is a guanidinium group containing high energy compound present
in skeletal muscle. On hydrolysis it yields 10 kcal / mol energy and creatinine is
product.
NH
H
||
P – N – C – N – CH2 – COOH
|
CH3
Creatine phosphate
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BIOCHEMISTRY - Questions and Answers
3. Write a note on cytochromes of electron transport chain.
A. They are heme proteins persent in electron transport chain. However cytochromes also
exist outside of electron transport chain. Cytochromes of electron transport chain are cyt b,
cyt c1, and cyt c. Cytochromes b and c, are integral membrane proteins where as cyt c is
peripheral membrane protein. Cyt b and cyt c1 are components cytochrome reductase of
electron transport chain. During electron transfer in respiratory chain iron of cytochromes
undergo oxidation reduction s. The iron oscillates between Fe2+and Fe3+ states
Cytochrome oxidase
It is final component of electron transport chain. It consist of two cytochromes. They are
cytochrome a or Cyt a and Cyto chrome a3 or Cyt a3. Each cytochrome contain two metal
ions. They are iron and copper. These metal ions participates in oxidation and reduction
reactions. Iron oscillates between Fe2+ and Fe3+ and copper oscillates between Cu+and Cu2+.
Cu+ is is reduced form and Cu2+ is oxidized form. In the respiratory chain cytochrome oxidase
catalyzes transfer of electrons from Cyt c to molecular oxygen.
4. Write very briefly about cytochrome P450 and ubiquinone
A. Cytochrome P450 is cytochrome outside electron transport chain. Cyt P450 is also a heme
protein. It is component of mixed function oxidases or mono oxygenases or hydroxylases.
Cyt P450 directly interacts with oxygen. Iron of Cyt P450 participates in oxidation and
reduction reactions.
Two types of Cyt P450 dependent hydroxylases are identified. They are 1. Microsomal Cyt
P450 hydroxylase. 2. Mitochondrial Cyt P450 hydroxylase.
Ubiquinone
It is also known as coenzyme Q or CoQ. It is non protein component of electron transport
chain. It is mobile carrier of electron transport chain. It collects hydrogens or electrons
from NADH and FADH 2 and transfers to cytochromes. It participates in oxidation and
reductions reactions via semiquinone
5. Write about Iron sulfur proteins.
A. They are proteins containing iron sulfur clusters. They are present in respiratory chain.
Some occur outside respiratory chain also. In these proteins iron is complexed with organic
as well as inorganic sulfur. Cysteine residues of proteins contributes organic sulfur. The
iron of these proteins is referred as non heme iron (NHI). Iron and sulfur are involved in
oxidation reduction reactions. Iron oscillates between Fe2+ (ferrous) and Fe3+ (ferric) state.
Ferrous is reduced state and ferric is oxidized state of iron. In electron transport chain or
respiratory chain iron sulfur (Fe:S) proteins transfer electrons from NADH –CoQ
reductase and succinate- CoQ reductase to ubiquinone.
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CHAPTER - 7 | Biological Oxidation
6. Define redox reactions and redox potantial. Write the symbol for
redox potential. What is its importance in respiratory chain?
A. Redox reactions: Oxidation and reduction reactions are known as redox reactions. The
oxidant and reductant of redox reactions are known as redox pair.
Redox potential:Electromotive force (e. m. f) of a redox pair is known as redox potential.
It is indicated with symbol Eo1. Redox potential of a redox pair indicates ability of redox pair
to either gain or loose electrons.
Further redox potential has critical role in arrangement of components of respiratory
chain. Location of specific component of respiratory chain depends on its redox potential.
Starting components of respiratory chain have negative redox potential where as terminal
components of respiratory chain have positive redox potential. Further ATP synthesis in
respiratory chain also depends on redox potential difference between redox pair of
respiratory chain. Approximately 0. 15volts potential difference is required for one ATP
formation.
7. Describe Respiratory chain (RC) or Electron transport Chain (ETC)
A. Electron transport chain consist of several components. These components are involved in
electron transfer and they are arranged in sequence. They carry electrons from NADH to
final electron acceptor oxygen. Most of the components of electron transport chain are
proteins. However some non protein components also act as carriers of electrons. In the
respiratory chain electrons flow from NADH to Co Q and then to cytochromes. From there
they move to molecular oxygen. From FAD electrons flow to CoQ.
The position of particular component in the respiratory chain is determined by redox
potential of that component. Usually initial components of have negative redox potential
and terminal components have positive redox potential. Hence due to redox potential
difference electrons flows from negative to positive components in the respiratory chain.
Electrons from substrate like malate and glutamate flow to NAD. This electron transfer is
catalyzed by NAD linked dehydrogenases. However electrons from substrates like
pyruvate and α-ketoglutarate flow to NAD via FAD. Coenzyme Q collects electrons from
FAD linked dehydrogenases like acyl –CoA dehydrogenases, succinate dehydrogenase etc.
NAD
FMN
CoQ
Cyt b
Cytc1
Cyt c
Cyt a
Cyt a3
O2.
Recent research indicated presence of complexes in electron transport chain rather than
individual components. Respiratory chain consist of four complexes and three mobile
carriers. They are complexes I to V and mobile carriers are NAD, CoQ, cyt c and molecular
oxygen. Complex I. is NADH- dehydrogenase or NADH –CoQ reductase. This complex also
contains iron sulfur centers and FMN. So electrons collected by NAD from substrates is
transferred to CoQ by this complex I through FMN and iron sulfur centre. This results in
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BIOCHEMISTRY - Questions and Answers
oxidation of NAD. Complex II is succinate –CoQ reductase. It transfers electrons from
substrates like succinate through FAD and iron sulfur centres to Co Q. . Now from CoQ
electrons are transferred to cyt c via cyt b, cyt c1 and iron sulfur centre by complex III which
is also known as cytochrome reductase. This lead s to oxidation of CoQ. Complex IV is
cytochrome oxidase. . It transfers electrons from cyt c to final electron acceptor molecular
oxygen. As a result cyt c is oxidized and oxygen is reduced to water.
Succinate
Complex ІІ
NAD+
Complex І
CoQ
Complex ІІІ
Cyt C
Complex ІV
H2O
Oxidative phosphorylation
Synthesis of ATP using energy released when electrons flow in the respiratory chain from NAD+
to oxygen is oxidative phosphorylation. It consist of two processes an oxidation and
phosphorylation. These two processes are coupled. It is different from substrate level
phosphorylation with respect to location, mechanism, susceptibility to inhibitors etc. Substrate
level phosphorylation is not combination of two processes and hence it does not involves coupling.
Further it is insensitive to inhibitors. It occurs in metabolic pathways and located outside of
mitochondria and does not involve electron transfer. In constrast ATP synthesis in respiratory
chain is associated with three complexes of respiratory chain which is present in mitochondria.
Complex І or NADH – CoQ reductase, complex ІІІ or CoQ –cyt c reductase and com plex ІV or
cytochrome oxidase of respiratory chain are involved in ATP synthesis. Flow of electrons through
these complexes causes synthesis of ATP.
ATP Synthase
When electrons flow in respiratory chain from NAD to
oxygen ATP is generated. ATP synthase or F0F1 ATPase
IN
MEM
OUT
present in inner mitochondrial membrane catalyzes
formation of ATP from ADP and Pi. It is an integral
membrane protein. This enzyme consist of two subunits. F0
subunit and F1. It has knob like structure. Head of the knob
F0
F1
SIDE
ATP Synthase
SIDE
BRANE
is F1subunit. F0 is the base of the knob which is embedded in
membrane. F1subunit has catalytic activity. F0 subunit is proton channel. When electrons move
from outside to inside of the membrane through F0 subunit F1subunit catalyzes ATP formation
from ADP and Pi using energy released. Anti biotic oligomycin blocks ATP synthase catalyzed
ATP synthesis.
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CHAPTER - 7 | Biological Oxidation
Mechanism of oxidative phosphorylation
Chemiosmotic model: This model is proposed to explain mechanism of oxidative phosphorylation
in respiratory chain by P. Mitchell. Novel element in this model is proton translocation from
matrix to out side of mitochondrial membrane as electrons flow in respiratory chain. Due to this a
proton gradient is generated across inner mitochondrial membrane. This in turn leads to
development of potential difference also across mitochondrial membrane. Thus electron flow in
respiratory chain leads to development of electrochemical gradient across inner mitochondrial
membrane. Return of protons into matrix through proton channel of ATP synthase leads to ATP
synthesis. Protons are driven into matrix from outside by electro chemical gradient.
(H+) Protons
out side
Membrane
Electron
Transport Chain
ATP
Synthase
in side
(H+) Protons
ADP+Pi
ATP
Energy that is released when electrons pass through F0 subunit of ATP synthase is used by
F1subunit for the synthesis of ATP. This proton extrusion occurs at the three complexes of
respiratory chain. About 3 to 4 protons are extruded at each complex. This gives rise a
PHdifference of about 0. 05 units across inner mitochondrial membrane. This is equal to 0. 15 volts
potential difference at each complex which is sufficient for ATP formation. Since there are three
complexes in respiratory chain electrons flow from NAD to oxygen generates 3ATP molecules.
Latest research indicates synthesis of 2. 5 ATP only
P:O Ratio: It is ratio of number of ATP formed in the electron transport chain per atom of oxygen
consumed. When electrons flow from NAD to O2 3 ATP are formed per atom of oxygen consumed
hence P, O ratio is 3. Like wise P, O ratio is 2 when electrons flow from FAD to O2 (oxygen).
According to new research P, O ratio is 2. 5 and 1. 5 respectively
Inhibitors of Respiratory chain
Respiratory chain is inhibited by two types of inhibitors. They are a) Inhibitors of oxidative
phosphorylation. b) Uncouplers.
a) Inhibitors of oxidative phosphorylation: Inhibitors specific to each complex (site) of oxidative
phosphorylation are found. Amytal, a sedative; rotenone a fish poison and piericidin an
antibiotic inhibits oxidative phosphorylation at complex І or site 1. Antimysin A an antibiotic
and BAL an antidote for arsenic poisoning inhibit oxidative phosphorylation at complex ІІІ or
site 2. Cyanide, carbon monoxide, hydrogen sulfide and azide inhibit oxidative
phosphorylation at complex ІV or site 3.
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BIOCHEMISTRY - Questions and Answers
b) Uncouplers: These compounds uncouples or dissociates oxidation from phosphorylation in the
respiratory chain. Due to this ATP synthesis is not possible even through oxidation is possible.
Some uncouplers are 2, 4 dinitro phenol, dinitro cresol, salicylanilides, pentachrophenol and
CCCP.
8. Write a note on substrate level phosphorylation
1. It is synthesis of ATP without involvement of respiratory chain
2. A high energy compound is generated in a pathway from which ATP is synthesized.
3. For example in glycolysis 1, 3 - bis phosphoglycerate is generated initially. Then
phosphoglycerate kinase catalyzes ATP formation by converting 1, 3 – phosphoglycerate to
3-phosphoglycerate.
4. Likewise formation of ATP by pyruvate kinase of same pathway.
9. What are oxygenases ? Classify. Give examples for each.
A. Oxygenases are those enzymes which catalyzes incorporation of oxygen directly into
substrate molecules.
Two types of oxygenases are known. They are
a) Dioxygenases
b) Mono oxygenases
a) Dioxygenases : Are those enzymes that incorporate two atoms of oxygen molecules
into substrate.
Tryptophan dioxygenase and homogentisate dioxygenase are examples.
b) Mono oxygenases: Are those enzymes that incorporate only one atom of oxygen
molecule into substrate. Another atom of oxygen is reduced to water.
Phenylalanine hydroxylase, trytophan hydroxylase and cytochrome P450 hydroxylases
are examples for monooxygenases.
10. Write briefly about
A) Hydrogen peroxide
B) Superoxide
A. Hydrogenperoxide
Hydrogen peroxide is formed from reactions of riboflavin dependent aerobic
dehydrogenases and oxidases. It may be also formed from reduction of oxygen to water and
from superoxide dismutase. Hydrogen peroxide is toxic to cells so it must be eliminated.
Hydroperoxidases are type of enzymes involved in removal of hydrogen peroxide. They are
catalase and peroxidase. In erythrocytes glutathione peroxidase eliminates hydrogen
peroxide.
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CHAPTER - 7 | Biological Oxidation
Peroxidase
Catalase
2H2O
2H2O + O2
H2O2 + H2
2H2O
In macrophages hydrogen peroxide is produced as part of their normal function. Much
of the generated hydrogen peroxide is used to produce free radicals like hypochlorite
which kills bacteria that were engulfed.
B. Superoxide
Superoxide formation from oxygen occurs on addition of one electron. Superoxide is toxic to
cells. It generates free radicals which are extremely toxic to cells. So it is eliminated by
superoxide dismutase.
Superoxide dismutase
-
O2 + e
–
2
Superoxide O
–
2O2 + 2H
+
O2 + H2 O2
However in macrophages superoxide is produced from oxygen by adding electrons.
NADPH oxidase adds electrons. The superoxide formed in turn generates free radicals
hypochlorite and hydroxyl radicals which kills bacteria. Hence superoxide has role in
phagocytosis of macrophages.
Other model questions are
11. Define oxidative phosphorylation and substrate level
phosphorylation. Write their differences.
12. Write the principle of chemiosmotic hypothesis.
13. What are uncouplers? Give examples.
14. Write the components of electron transport chain in the order.
Show sites of ATP synthesis.
15. Write about inhibitors of oxidative phosphorylation.
16. Write about a) ATP synthase b) Free radicals.
17. Subsrate level phosphorylation
18. Write about P:O ratio.
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BIOCHEMISTRY - Questions and Answers
Chapter
8
Carbohydrate Metabolism
1. Name food carbohydrates. How they are digested and absorbed ? Add a
note on diseases associated with these processes.
A. Food carbohydrates :
Carbohydrates present in food are polysaccharides, disaccharides and very small amounts of
monosaccharides. Polysaccharides are present in plant and animal diets. Cereals like rice,
wheat, vegetables and roots like potato, tapioca contain starch, dextrin and inulin. Glycogen is
present mainly in animal meat. Milk, cane sugar and malt contain disaccharides like lactose,
sucrose and maltose. Bakery products, honey, sweets and fruits may contain
monosaccharides.
Carbohydrate digestion: Is a process that hydrolyzes food polysaccharides to their constituent
monosaccharides.
In the mouth : Carbohydrate digestion is initiated in mouth by salivary amylase present in
saliva. Though its action is limited it converts polysaccharides like starch, glycogen and
dextrin to maltose and oligosaccharides by hydrolyzing α 1, 4 glycosidic bonds. It has optimum
PH of 7. 0 and requires chloride for optimum activity.
Amylase
Starch or glycogen or dextrin
Maltose + Oligo saccharides.
In the stomach:Due to absence of carbohydrate breaking enzymes in gastric juice no digestion
of carbohydrate occurs in the stomach.
In the duodenum: Pancreatic amylase is major carbohydrate digesting enzyme in duodenum.
It hydrolysis alpha (α), 1, 4 glycosidic bonds of polysaccharides and converts them to maltose,
maltotriose, limit dextrin and oligosaccharides. α- dextrin is another name for limit dextrin, It
contains α 1, 6 glycosidic bonds. Amylose part of starch is converted to maltose, maltotriose
and oligosaccharides.
Amylase
Amylose
Maltose + Maltotriose + Oligosaccharides.
Amylopectin is converted to limit dextrin, oligosaccharides, maltose and maltotriose.
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CHAPTER - 8 | Carbohydrate Metabolism
Amylase
Amylopectin
Limit dextrin + Maltose + Maltotriose.
Pancreatic amylase has optimum PH 7-8 and chloride is its activator.
In the small intestine: Succus entericus which is secretion of small intestinal cells contains
enzymes like disaccharidases, α-dextrinase or isomaltase that hydrolyzes disaccharides and
limit dextrin to constituent monosaccharides. Isomaltase catalyzes hydrolysis of α 1, 6
glycosidic bonds of limit dextrin and converts to oligosaccharide and glucose.
α-dextrinase
Limit dextrin
Oligosaccharide + Glucose
Maltase catalyzes hydrolysais α (1, 4) glycosidic bonds from one end of oligosaccharide and
releases glucose.
Maltase
Oligosaccharide
Glucose + Oligosaccharide shorter by one glucose unit.
The action of maltase on oligosaccharide continues until a disaccharide is formed (maltose).
Oligosaccharide shorter by glucose
Disaccharide
Finally disaccharide containing two glucose units is also hydrolyzed to glucose.
Maltase
Maltose
Glucose+ Glucose.
Sucrase catalyzes hydrolysis of sucrose to glucose and fructose.
Sucrase
Sucrose
Glucose + Fructose.
Lactase hydrolyzes lactose of diet to glucose and galactose.
Lactase
Lactose
Glucose+Galactose.
Thus dietary (food) polysaccharides are converted to their constituent monosaccharides.
Carbohydrates absorption: The products of carbohydrate digestion are absorbed by
a. Passive diffusion and
b. Facilitated transport or secondary active transport.
Passive diffusion :Mannose and xylose are absorbed by simple diffusion. Jejunum is site of
absorption. Absorbed monsaccharides reaches liver through portal circulation.
Facilitated transport or Secondary active transportt: Glucose, galactose and fructose are
absorbed in jejunum by facilitated transport. These absorbed monosaccharides reaches liver
through portal venous system. A carrier protein is involved in the absorption. It is called as
translocase and present in enterocyte membrane. It transports glucose along with sodium.
Hence it is known as symporter. Energy needed is supplied by movement of sodium.
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BIOCHEMISTRY - Questions and Answers
LUMEN
ENTEROCYTE
MEMBRANE
Glucose
Enterocyte
Cytosol
Sodium
In the initial stage it is present on external surface or luminal side of enterocyte. There are two
binding sites on this carrier protein one glucose binding site and another for sodium binding.
When these sites are occupied by glucose and sodium it moves to cytosolic side and releases
sodium and glucose into cytosol. Glucose and galactose diffuses into blood from cytosol.
However sodium is extruded by pump mechanism which is dependent on ATP. The carrier
molecule returns to its original place to transport another glucose molecule.
Disorders of carbohydrate digestion and absorption
They are due to either defective enzymes or defective transporters. Some of them are given
below.
Lactose intolerance:It is due to deficiency of lactase. Hence patients of this disease fail to
utilize lactose present in diet. Acumulation of lactose in the intestine leads to diarrhoea and
abdominal pain, flatulence etc. due to fermentation of lactose by intestinal bacteria.
Isomaltase and sucrase deficiency: It occurs in childhood. Isomaltase and sucrase are
deficient.
Disacchariduria:It is characterized by excretion of disaccharides in urine. It is due to
deficiency of disaccharidases.
Malabsorption syndromes of monosaccharides: They are due to defective transporter. Due to
defective transporter absorption of monosaccharides is impaired.
2. Define glycolysis. Describe reactions of this process. Add a note on
its energetics.
A. Glycolysis is the degradation of glucose to pyruvate or lactate by a sequence of enzyme
catalyzed reactions.
Glycolysis takes place in the cytosol of most of cell types. In the skeletal muscle end
product of glycolysis is lactate. It is known as anaerobic glycolysis.
Reactions of glycolysis:
1. Glucose enters glycolysis by phosphorylation catalyzed by hexokinase. It is ATP
dependent irreversible reaction. Glucose is phosphorylated on 6thcarbon. Magnesium
ion (Mg2+) is required for this reaction and glucose -6-phosphate is product. In liver
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CHAPTER - 8 | Carbohydrate Metabolism
glucokinase is present. However it phosphorylates glucose only after a meal when blood
glucose level is more.
2. Isomerization of glucose -6-phosphate of first reaction to fructose -6-phosphate is
second reaction and this reaction is freely reversible.
Phosphoglucoisomerase catalyzes this reaction.
Glucose
+ ATP
Hexokinase
Glucose-6-phosphate + ADP
Mg2+ (1)
Phosphoglucose
Glucose-6-phosphate
Fructose-6-phosphate
Isomerase (2)
3. Another ATP dependent phosphorylation of fructose -6- phosphate occurs in the third
reaction which is catalyzed by phosphofructo kinase. Fructose -1, 6-bis phosphate is
product and like first reaction this is also an irreversible reaction requiring ATP and
magnesium ion. Up to this stage of glycolysis two high energy bonds are utilized.
4. Aldolase A splits fructose-1, 6-bis phosphate to two triose phosphates namely
glyceraldalhyde-3- phosphate and dihydroxy acetone phosphate.
Phosphofructokinase
Aldolase A
Fructose-6-phosphate +ATP
Fructose1, 6 bisphosphate
2+
Mg
(3)
(4)
ADP
Glyceraldehyde-3-phosphate + Dihydroxy acetone phosphate
5. A reversible isomerization converts dihydroxy acetone phosphate to glyceraldehyde 3- phosphate which is catalyzed by triose phosphate isomerase.
Triose phosphate isomerase
Glyceraldehyde -3-phosphate
dihydroxy acetone phosphate
(5)
Thus one glucose molecule is converted to two three carbon glyceraldehyde-3phosphate molecules.
6. An NAD+ dependent glyceraldehyde -3- phosphate dehydrogenase catalyzes oxidation
and phosphorylation of glyceraldehyde-3-phosphate to an high energy 1, 3-bis
phosphoglycerate. It is a reversible reaction and inorganic phosphate is required.
Glyceraldehyde3-phosphate dehydrogenase
Glyceraldehyde-3-phosphate+ NAD+
1, 3Pi
(6)
+
bis phosphoglycerate + NADH+H .
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BIOCHEMISTRY - Questions and Answers
7. The high energy 1, 3-bis phosphoglycerate serve as source of energy for formation of
ATP from ADP in this irreversible reaction catalyzed by magnesium dependent
phosphoglycerate kinase. 3-phosphoglycerate is product of this reaction. Substrate
level phosphorylation occurres.
8. A mutase shifts phosphate of 3- phosphoglycerate to the 2nd positition in this reversible
reaction.
Phosphoglyceratekinase
1, 3-bis phosphoglycerate+ADP
3-Phosphoglycerate
Mg2+ (7)
ATP
Phosphoglycerate mutase
2-phosphoglycerate.
(8)
9. A high energy compound is generated in this reaction from 2- phosphoglycerate by
enolase. Phospho enol pyruvate is product of this reversible reaction. Manganese or
magnesium ions are required. Another substrate level phosphorylation occurres here
10.
Synthesis of ATP from ADP once again occurs in this reaction of glycolysis. The
reaction is irreversible and requires magnesium. Phosphoglyceratekinase catalyzes
this reaction and pyruvate is product of this reaction. If glycolysis ends with pyruvate
then it is called as aerobic glycolysis.
2-phosphoglycerate
11.
Enolase
Pyruvatekinase
phosphoenol pyruvate
Pyruvate +ATP
2+
2+
Mg (9)
(10) Mg
ADP
In the skeletal muscle lactate is formed from pyruvate on reduction catalyzed by
lactate dehydrogenase. NADH produced in reaction (6) serve as source of hydrogen. In
the erythrocytes also lactate is formed from pyruvate. It is a reversible reaction.
Lactate
Pyruvate+NADH+H+
Lactate + NAD+
Dehydrogenase (11)
Energetics: ATP formation in glycolysis of skeletal muscle is given below.
Anaerobic glycolysis
1. Number of ATP formed by phosphoglycerate kinase
2
2. Number of ATP formed by Pyuvate kinase.
2
3. ATP consumed by hexokinase and phosphofructokinase
-2
Net 2
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CHAPTER - 8 | Carbohydrate Metabolism
So net formation of glycolysis of skeletal muscle = 2. Generally glycolysis that yields
lactate is considered as anaerobic glycolysis. There fore anaerobic glycolysis that occurs
in skeletal muscle and erythrocytes generates 2ATP molecules per molecule of glucose.
Aerobic glycolysis
ATP formation in glycolysis of hepatocyte is given below. It is considered as aerobic
glycolysis. NADH generated in 6threaction of glycolysis is oxidized in respiratory
chain because it is not used for formation of lactate from pyruvate.
1. Number of ATP s generated by phosphoglyceratekinase
2
2. Number of ATPs generated by pyruvate kinase.
2
3. Number of ATP s generated by respiratory chain oxidation
6
4. ATP s consumed by hexokinase and phosphofructokinase
-2
Net 8
Thus the aerobic glycolysis generates 8 ATP molecules. According to new research only
7 ATPs are generated.
3. Write the significance of glycolysis.
A. Glycolysis meets energy requirements of all kinds of cells. Anaerobic glycolysis mainly
supplies energy to rapidly contracting skeletal muscle. Dietary fructose and a lactose are
also metabolized by this path way. Glycolysis also supplies precursors for other pathways.
For example pyruvate is used for alanine formation and dihydroxyacetone is used for
triglyceride formation. In erythrocytes deficiency of pyruvate kinase causes hemolytic
anaemia.
4. Write the fate of pyruvate OR Write about pyruvate dehydrogenase
complex.
A. Under aerobic conditions pyruvate is transported into mitochondria by a transporter
present in mitochondrial membrane. In the mitochondria it is oxidatively decarboxylaed to
acetyl –CoA by a multi enzyme complex pyruvate dehydrogenase (PDG) complex. Three
enzymes are present in this complex. First enzyme is pyruvate dehydrogenase and
contains TPP as prosthetic group. It is designated as E1 –TPP. Second enzyme dihydrolipoyl
transacylase and containstwo sulfhydryl groups
SH
contributed by lipoicacid. It is written as E2- lipoamide
S
and E2-Lipoamide
SH
S
The first one is reduced form and latter one is oxidized form. The third enzyme is
dihydrolipoyl dehydrogenase and contains FAD as cofactor. It is written as E3 –FAD. The
first enzyme decarboxylates pyruvate and remaining hydroxyethylidine moiety is bound
to TPP.
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BIOCHEMISTRY - Questions and Answers
(1)
Pyruvate+E1-TPP
E1-TPP- hydroxyethylidine+CO2.
The second enzyme transfers hydroxyethylidine to one of sulfur of oxidized lipoamide and
releasing E1-TPP. It also shifts acetyl group to coenzyme A to form acetyl –CoA and
reduced lipoamide.
E1-TPP
S
S-Acetyl
E1-TPP-hydroxyethylidine+E2-lipoamide
E2-lipoamide
S
(2)
SH
S-Acetyl
E2-lipoamide
SH
+CoA
SH
Acetyl-CoA+E2 –lipoamide
(2)
SH.
The third enzyme regenerates oxidized lipoamide by transferring hydrogen to NAD via
FAD.
SH
S
E3 –FAD +E2 –Lipoamide
E2-lipoamide
SH
E3 – FADH2 + NAD+
(3)
+E3 –FADH2
S
E3-FAD+NADH+H+
(
3)
Fate of NADH: It is oxidized in respiratory chain. Three ATP (2. 5)s are generated.
5. Describe Citric acid cycle.
A. It is a cyclic arrangement of reactions which convert acetyl-CoA to carbon dioxide.
Oxidation of acetyl-CoA is accompanied by energy output. It is also known as Krebs cycle or
tricarboxylic acid (TCA) cycle. Enzymes of this cycle are present in mitochondrial matrix.
Reactions of TCA cycle:
1. Citricacid cycle reactions begins with formation of tricarboxylic acid citrate from acetyl
–CoA and oxaloacetate catalyzed by citrate synthase a condensing enzyme. It is an
irreversible reaction.
Citrate Synthase
Oxaloacetate + Acetyl -CoA
Citrate + CoASH.
(1)
2. In the second reaction citrate is isomerized to isocitrate via cis aconitate and involves
loss and addition of water. This reaction is catalyzed by aconitase an iron containing
protein. It is a reversible reaction.
Aconitase
Citrate
Aconitase
Cis-aconitate
Isocitrate
2
2
H2O
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H2O
CHAPTER - 8 | Carbohydrate Metabolism
3. Dehydrogenation and decarboxylation of isocitrate occurs in the third reaction of the
cycle. It is catalyzed by NAD+ dependent isocitrate dehydrogenase. Isocitrtate is
converted to α-ketoglutarate via oxalosuccinate. It is a reversible reaction.
Isocitrate Dehydrogenase
Isocitrate +NAD+
oxalosuccinate
(3)
(3)
+
NADH+H
α-ketoglutarate+Co2
4. Like pyruvate in this reaction α-ketoglutarate undergoes oxidative decarboxylation
catalyzed by α-ketoglutarate dehydrogenase complex. It requires lipoic acid, TPP,
CoA, FAD and NAD. It is an irreversible reaction. Succinyl-CoA an high energy
compound and NADH are products of this reaction.
α-ketoglutarate
Dehydrogenase
α-ketoglutarate +NAD+
Succinyl-CoA + NADH +H+ Co2
(4)
5. High energy compound GTP is formed in this reaction. It is catalyzed by succinate
thiokinase. It also requires magnesium ions and it is an irreversible reaction. Succinate
is product. It is another example for substrate level phosphorylation.
6. Dehydrogenation of succinate to fumarate occurs in this reaction. It is catalyzed by
FAD dependent succinate dehydrogenase. It is reversible.
Succinate
Succinate
Thiokinase
dehydrogenase
Succinyl-CoA +GDP
Succinate
Fumarate +FADH2
(5)
(6)
GTP
FAD
7. Addion of water to fumarate by fumarase occurs in this reaction. Malate is the product
and it is a reversible reaction.
8. Finally oxaloacetate is regenerated from malate by NAD dependent malate
dehydrogenase. It is a reversible reaction.
Fumarase
Malate Dehydrogenase
Fumarate +H2O
Malate
oxaloacetate +NADH+H+
(7)
(8)
+
NAD
The reactions of cycle starts again using oxaloacetate and it continues as long as
acetyl-CoA molecules are available.
Energetics: Amount of ATP generated by citric acid cycle per molecule of acetylCoA oxidation is given below.
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BIOCHEMISTRY - Questions and Answers
1. Respiratory chain oxidation of NADH generated isocitrate
dehydrogenase
3
2. Respiratory chain oxidation of FADH generated by succinate
dehydrogenase
2
3. Respiratory chain oxidation of NADH generated by α-ketoglutarate
dehydrogenase
3
4. Respiratory chain oxidation of NADH generated by Malate
dehydrogenase.
3
5. GTP generated by succinate thiokinase is equal to one ATP.
1
Total 12
There fore 12 ATPs are produced in citric acid cycle when one acetyl- CoA is oxidized.
Only 10 ATPs are produced as per latest research.
Regulation: Citrate synthase, isocitrate dehydrogenase and α-ketoglutarate
dehydrogenase are regulatory enzymes. They are subjected to allosteric regulation.
ATP and NADH are allosteric (regulators) or inhibitors and ADP is allosteric activator.
So Citric acid cycle rate depends on cellular levels of ATP and NADH.
Significance: It is final common metabolic pathway for oxidation of carbohydrates,
lipids and proteins. Intermediates of TCA cycle are used for anabolic reactions. Fatty
acids, cholesterol, aminoacids and porphyrins are compounds formed from citric acid
cycle intermediates.
7. Describe glycogen metabolism.
A. It consists of
A. Glycogenesis
B. Glycogenolysis
Glycogensis: It is the formation of glycogen from glucose. It occurs in almost all cells but
liver and skeletal muscle are major organs.
Reactions :
1. Glucose -6- phosphate of glycolysis conversion to glucose -1-phosphate is the first
reaction of glycogenesis. It is a reversible reaction and catalyzed by
phosphoglucomutase.
2. Formation of active sugar UDP- glucose occurs in second reaction. It is an irreversible
reaction It is catalyzed by UDPG – pyrophosphorylase and uses UTP as energy source.
The inorganic pyrophosphate (PPi) released is converted to two molecules of inorganic
phosphate (2Pi) by pyrophosphatase.
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CHAPTER - 8 | Carbohydrate Metabolism
phospho gluco
UDPG- Pyro
mutase
Phosphorylase
Glucosse-6-phosphate
Glucose-1-phosphate
UDP- glucose (UDPG)
(1)
(2)
UTP ppi
3. UDP-glucose or active glucose serve as donor of glucose units. A glycogen primer
accepts glucose from UDP- glucose which is catalyzed by glycogen synthase and
involves glycosidic linkage of type 1, 4. The action of glycogen synthase continues until
glycogen primer is elongated by 6-11glucose residues.
Glycogen
Synthase
Primer glycogen +UDP-glucose
UDPG UDP
primer glycogen
(n+1)residues
(3)
(3)
UDP
Elongated primer glycogen.
4. Now new branch is created in primer glycogen by transferring oligosaccharide
containing six glucose residues from newly formed fragment to adjacent chain of
primer glycogen. This reaction is catalyzed by branching enzyme. It involves hydrolysis
and formation of glycosidic linkages of α 1, 4 type and α 1, 6 type respectively.
Branching Enzyme
Elongated primer glycogen
primer glycogen with new branch.
(4)
5. Further elongation of new branch by glycogen synthase and further branching by
branching enzyme leads to formation of glycogen molecule.
Glycogen Synthase
Primer glycogen with new branch
Elongation of new branch of glycogen.
(5)
Further
Glycogen with elongated new branch
Glycogen.
Branching
elongation
Significance: Glycogen formation occurs immediately after meal. Glycogen formed in
liver and skeletal muscle function as stored form of energy.
Glycogenolysis: It is degradation of glycogen to glucose or lactate. It occurs in liver and
skeletal muscle.
Reactions: 1. Degradation of glycogen is initiated by enzyme phosphorylase. It
hydrolyzes 1, 4 glycosidic bonds from an end of a branch of glycogen and releases glucose
as glucose -1- phosphate. Its action continues until four glucose residues remain on either
side of branch point. Action of phosphorylase converts glycogen to limit dextrin.
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BIOCHEMISTRY - Questions and Answers
Phosphorylase
Glycogen
Glucose -1- phosphate +glycogen shorter by one glucose residue
(1)
Pi
Glycogen with one glucose less
Glucose -1-phosphate+ glycogen
(1)
With 4 glucose residues on either side of branch point.
2. In this reaction a trisaccharide chain containing 3glucose units of partly hydrolyzed
branch is transferred to adjacent branch to expose 1, 6 glycosidic bond at branch point.
It is catalyzed by glucan transferase. It involves breaking as well as formation of
glycosidic bonds.
3. A debranching enzyme hydrolyzes 1, 6 glycosidic bond. This results information of
glycogen with one branch less.
4. Now further action of phosphorylase continues on another branch and this is followed
by glucan transferase and debranching enzyme. Thus the combined action of these
enzymes results in degradation of glycogen to glucose 1-phosphate.
Glucan transferase
Glycogen with glucose 4 units on either side of branch point
Debranching
Enzyme
Glycogen with exposed
glycogen with one branch short
Branch
3
Glucose- 1- phosphate
2
4
5. Glucose-1- phosphate is converted to glucose -6-phosphate by phosphoglucomutase in
this reaction.
6. Free glucose is released from glucose -6-phosphate in liver by the action of glucose -6phosphatase. However in muscle glucose -6- phosphate enters glycolysis and get
converted to lactate.
Phosphogluco
glucose-6
Mutase
phosphatase
Glucose-1- phosphate
glucose -6- phosphate
glucose+Pi
(6)
(6) liver
glycolysis
Glucose-6-phosphate
lactate.
Muscle
(6)
Significance : Glycogenolysis in liver meets body glucose requirements in between
meals and starvation. In the skeletal muscle glycogenolysis meets energy requirement
in between meals.
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CHAPTER - 8 | Carbohydrate Metabolism
Glycogen storage diseases
Glycogen metabolism is defective in several diseases due to deficiency of enzymes of either
glycogen formation or break down. Some are given below.
a. von Gierk's disease or type І glycogen storage disease : It is due to deficiency of glucose-6phosphatase. This results in accumulation of glycogen in liver, kidney etc. Hence enlargement
of liver occurs. Symptoms are hypoglycemia, hyper uricemia, hyper lipidemia and ketosis.
b. Pompe's disease or TypeІІ glycogen storage disease:It is due to defective glycogen break down
in lysosomes. Lysosomes contain α-glucosidase which usually hydrolyzes glycogen in normal
people. Lack of this enzyme leads to accumulation of glycogen in lysosomes of all types of cells
occurs. It is a fatal condition. In heart accumulation leads to cardiomegaly. Children born with
this defect may die in second year of life due to cardiorespiratory failure.
c. Coris disease or TypeІІІ glycogen storage disease :In this disease glycogenolys is blocked due to
deficiency of debranching enzyme and limit dextrin accumulates in liver. Hence this condition
is also known as limit dextrinosis.
d. Anderson's disease or Type ІV glycogen storage disease:This condition is characterized by
accumulation of amylopectin an intermediate of glycogenesis in liver. It is due to deficiency of
branching enzyme. In other organs like heart and spleen also accumulation of amylopectin
occurs. It is a serious disease. Since amylopectin accumulation occurs in organs this condition
is also called amylopectionosis.
e. McArdle's syndrome or TypeV glycogen storage disease: In this disorder glycogen accumulates
in skeletal muscle of affected persons due to lack of phosphorylase. Lactic acid production is
not increased in muscle after exercise indicating block in glycogenolysis. Muscle cramps and
diminished tolerance to exercise are symptoms.
f. Her's disease or TypeVІ glycogen storage disease: This condition is characterized by
accumulation of glycogen in liver due to deficiency of phosphorylase of glycogenolysis.
8. Describe hexose monophosphate (HMP) shunt pathway.
A. Enzymes of HMP shunt pathway are present in cytosol of liver, adipose tissue, blood cells
mainly red blood cells and neutrophils of white blood cells, adrenal cortex, testis, ovaries,
lactating mammary gland, thyroid etc. In the skelatal muscle this pathway is less active. It
is also known as direct oxidative pathway and pentose phosphate pathway.
Reactions:
1. Glucose-6-phosphate of glycolysis is starting compound of this pathway. Dehydrogenation
of this molecule by NADP dependent glucose-6-phosphate dehydrogenase is initial
reaction of this pathway. 6-phosphogluconolactone and NADPH are products. Magnesium
or calcium ions are also needed for this reaction.
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BIOCHEMISTRY - Questions and Answers
2. Lactonase hydrolyzes lactone to 6-phosphogluconate in presence of magnesium,
manganase or calcium ions.
Glucose-6-phosphate
Dehydrogenase
lactonase
Glucose-6- phosphate
6-phosphoglucorolactone
(1)
(2)
+
NADP+ NADPH+H
Phosphogluconate.
6-
3. Another NADP dependent dehydrogenation converts 6 phosphogluconate to pentose
phosphate. It occurs in two steps. In the first step phosphogluconate is dehydrogenated at
3carbon to 3-keto-6- phosphogluconate and NADPH is formed. In the second step
spontaneous decarboxylation of 3-keto -6-phosphogluconate yields ribulose phosphate.
phospho gluconate
Dehydrogenase
+
6-phosphogluconate+NADP
3-keto-6-phosphogluconate
(3)
NADPH+H+
Ribulose- 5- phosphate.
(3)
Co2
4. Isomerization of ribulose-5-phosphate to ribose-5-phosphate occurs in this reaction.
5. Another molecule of ribulose-5-phosphate is epimerized to xylulose 5-phosphate in this
reacton.
pentose phosphate
pentosephosphate
Isomerase
Epimerase
Ribose-5- phosphatre
Ribulose-5- phosphate
xylulose-5-phosphate.
(4)
(5)
Rearrangements between two pentose phosphates in subsequent reactions generates
intermediates of glycolysis.
6. A two carbon fragment glycaldehyde of xylulose-5-phosphate transfer to ribose-5phosphate yields 7 carbon sedoheptulose-7-phosphate and glyceraldehydes -3- phosphate.
This reaction is catalyzed by TPP and magnesium dependent transketolase enzyme.
Trans ketolase
Xylulose-5-phosophate+ Ribose-5- phosphate
Sedoheptulose-7- phosphate
TPP (6)
+Glyceraldehyde -3-phosphate.
7. In this reaction sedoheptulose -7-phosphate is converted to 4 carbon erythrose-4phosphate and fructose -6- phosphate. It involves transfer of 3 carbon dihydroxy acetone
phosphate moiety of seven carbon sugar to another 3 carbon sugar glyceraldehydes -3phosphate. The reaction is catalyzed by transaldolase.
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CHAPTER - 8 | Carbohydrate Metabolism
Trans aldolase
Sedoheptulose-7-phosphate+glyceraldehyde -3- phosphate
Erythrose -4phosphate+ Fructose -6-phpsphate.
(7)
8. This is another trans ketolase catalyzed reaction. Erythrose -4- phosphate is converted to
fructose -6-phosphate by transferring 2 carbons fragment from xylulose -5-phosphate by
transketolase. The remaining three carbon fragment of xylulose -5-phosphate is released
as glyceraldehyde -3- phosphate. TPP and magnesium ions are required.
Transketolase
Erythrose -4-phosphate +xylulose -5 phosphate
Glyceraldehyde-3-phos
TPP Mg2+
(8)
Phate + Fructose -6-phosphate.
Thus pentose phosphates generated are converted to fructose -6-phosphate and
glyceraldehyde-3-phosphate which enters glycolysis for further utilization.
Significancte:
1. NADPH produced is used for the biosynthesis of fatty acids, cholesterol. deoxyribonucleotides, bile acids, glutamate, hormones and detoxification by cytochrome P450 hydroxylase.
2. In erythrocytes NADPH is used for removal of hydrogen peroxide by glutathione and
conversion of methemoglobin to normal hemoglobin.
3. In neutrophils NADPH is used for superoxide biosynthesis.
4. Pentose phosphates are used for nucleic acid and nucleotide biosynthesis.
5. This pathway convertes glucose to directly Co2 and hence it is called as direct oxidative
pathway of glucose.
6. Pentose of nucleic acid breakdown are used for energy production after they are converted
to intermediates of glycolysis by this pathway.
7. Xylulose of uronicacid pathway is either converted to glucose or intermediates of this
pathway.
8. Glucose -6-phosphate dehydrogenase deficiency:It is sex linked inherited disease of HMP
shunt pathway. In these individuals a tenfold less active glucose -6-phosphate
dehydrogenase is produced in erythrocytes. They appear normal until they are exposed to
certain drugs. In presence of antimalarial drug primaquine, sulfonamide antibiotics and
painkiller aspirin the less active enzyme becomes inactive. As a result NADPH production
is blocked and susceptibility of erythrocytes to hemolysis increases. Therefore affected
individuals develop hemolytic anaemia on exposure to those drugs. Fava beans also cause
this disease. However favism is the name given to this disease.
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BIOCHEMISTRY - Questions and Answers
9. Define gluconeogenesis. Write reactions involved in the formation of
glucose from lactate.
A. Gluconeogenesis is the synthesis of glucose from non carbohydrates like pyruvate of amino
acids, lactate, glycerol etc.
Formation of glucose from lactate
It involves participation enzymes of glycolysis, citric acid cycle, key enzymes of
gluconeogenesis and cytosolic malate dehydrogenase. The enzymes are present in
mitochondria and cytosol.
Key enzymes of gluconeogenesis:These enzymes makes reversal of glycolysis. They by
pass irreversible reactions of glycolysis which prevent reversal of glycolysis. They are 1.
Pyruvate carboxylase. 2. Phosphophenol pyruvate carboxykinase. 3. Fructose -1, 6-bis
phosphatase 4 Glucose -6-phosphatase.
Site:Gluconeogenesis mainly occurs in liver.
Reactions of gluconeogenesis:
1. Lactate dehydrogenase converts lactate to pyruvate using NAD as hydrogen acceptor
pyruvate so formed enters mitochondria through specific transporter present in inner
mitochondrial membrane.
2. In the mitochondria pyruvate carboxylase converts pyruvate to oxaloacetate. It is a ATP
and biotin dependent carboxylation.
Lactate
Pyruvate
Dehydrogenase
Carboxylase
Lactate +NAD+
pyruvate
Oxaloacetate+ADP+Pi
(1)
(2)
NADH+H+
ATP Co2 Biotin
3. Oxaloacetate formed in mitochondria must enter cytosol where enzymes of glycolysis are
present. But oxaloacetate is impermeable to mitochondrial membrane. Malate
dehydrogenase of TCA cycle converts oxaloacetate to malate which is permeable to
mitochondrial membrane.
4. In the cytosol oxaloacetate is regenerated from malate by cytosolic malate dehydrogenase.
Malate
Malate
Dehydrogenase
Dehydrogenase
Oxaloacetate+NADH+H+
NAD++ Malate
(3)
(4)
Oxaloacetate+NADH+H+.
5. Phosphoenol pyruvate an intermediate of glycolysis is formed from oxaloacetate in this
reaction catalyzed by phosphoenol pyruvate carboxykinase (PEPCK). It requires GTP as
energy source.
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CHAPTER - 8 | Carbohydrate Metabolism
6. Enolase of glycolysis converts phosphoenol pyruvate to 2-phosphoglycerate.
PEPCK
Oxaloacetate+GTP
Enolase
Phosphoenol pyruvate
(5)
2-phosphoglycerate.
(6)
GDP
7. Another enzyme of glycolysis phosphoglycerate mutase converts 2-phosphoglycerate to 3phosphoglycerate.
8. Phosphoglyceratekinase of glycolysis converts 3- phosphoglycerate to 1, 3bisphosphoglycerate.
Phosphoglycerate
phosphoglycerate
Mutase
kinase
2-phosphoglycerate
3-phosphoglycerate
1, 3-bis Phosphoglycerate.
(7)
(8)
ATP ADP
9. Glyceraldehyde -3-phosphate dehydrogenase of glycolysis forms glyceraldehyde-3phosphate from 1, 3-bisphosphoglycerate.
10. Triose phosphate isomerase of glycolysis converts a molecule of glyceraldehyde-3phosphate to dihydroxy aectone phosphate.
Glyceraldehyde-3-phosphate
Dehydrogenase
(10)
1, 3-bisphosphoglycerate+NADH+H+
Glyceraldehyde-3-phosphate
9
Pi
Isome
+
Dihydrooxyacetone phosphate.
NAD
rase
11. Reversible action of aldolase of glycolysis generates fructose-1, 6-bisphosphate from
glyceraldehydes -3-phosphate and dihydroxy acetone phosphate.
Aldolase
Glyceraldehyde -3-phosphate +Dihydroxyacetone phosphate
Bis phosphate.
Fructose -1, 6(11)
12. Fructose-1, 6-bisphosphatase of gluconeogenesis generates fructose-6-phosphate from
fructose-1, 6-bis phosphate.
13. Glucose-6-phosphate is formed from fructose-6-phosphate by action of phosphohexose
isomerase of glycolysis.
Fructose-1, 6-bis
phosphohexose
Phosphatase
Isomerase
Fructose-1, 6-bisphosphate
Fructose-6-phosphate
Glucose
(12)
(13)
-6-phosphate.
Pi
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BIOCHEMISTRY - Questions and Answers
14.
Glucose-6- phosphatase last key enzyme of gluconeogenesis generates glucose from.
Glucose-6- phosphate by hydrolyzing phosphate.
Glucose-6-phosphatase
Glucose -6-phosphate
Glucose + Pi
(14)
H2o
Significance:During fasting and starvation gluconeogenesis meets body glucose
requirement. Gluconeogenesis is the only source of glucose to organs like brain, skeletal
Muscle, , erythrocytes etc. If gluconeogenesis is blocked brain dysfunction occurs.
Gluconeogenesis clears metabolic waste product like lactate. Excess aminoacids of dietary
origin are converted to glucose by gluconeogenesis.
10. Write reactions of Uronic acid pathway.
A. Reactions:
1. UDP-glucose of glycogenesis serve as starting compound of this pathway. To give UDPglucuronic acid UDP-glucose undergoes 4 electron transfer reaction. It is catalyzed by
UDP- glucose dehydrogenase and 2NADH are generated.
2. Glucuronic acid is formed from UDP glucuronic acid on hydrolysis catalyzed by
hydrolase. UDP-glucuronic acid serve as active or donor of glucuronic acid.
UDPG
Dehydrogenase
UDP-glucose+2NAD+
UDP- glucuronicacid
(1)
2NADH+2H+
Hydrolase
Glucuro
(2)UDP nic acid
3. An NADPH dependent reduction of glucuronic acid to gulonic acid by gulonate
dehydrogenase occurs in this reaction.
4. Oxidation of L-gulonate to 3-keto –L-gluconate by NAD+ dependent dehydrogenase is
four th reaction.
Dehydro
Genase
Glucuronicacid + NADPH+H+
NAD+
L-gulonate
(3)
3-keto-L(4)
NADP+
Gluconate +NADH+H+.
5. A decarboxylase converts 3-keto –L-gulonate to L-xylulose by removing a carbon of
3- Keto –L- gulonate as carbon dioxide.
6. An NADPH dependent reduction of L-xylulose to xylitol by dehydrogenase is sixth
reaction.
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CHAPTER - 8 | Carbohydrate Metabolism
3-Keto- L- gulonate
Decarboxylase
Dehydrogenase
L-xylulose
Xylitol+NADP+
(5)
(6)
CO2
NADPH +H+
7. A ketopentose D-Xylulose is formed from xylitol in this reaction catalyzed by
dehydrogenase. It involves removal of hydrogen by NADP+ from Xylitol.
8. Finally xylulose -5- phosphate an intermediate of HMP shunt is formed by
phosphorylation of xylulose.
Dehydrogenase
Xylitol +NADP+
Xylulose kinase
D-Xylulose
Xylulose-5-phosphate
(7)
(8)
+ ADP
+
NADPH+H
ATP
11. Write the significance of uronic acid pathway.
A. Glucuronic acid is used for synthesis of mucopolysacharides, detoxification, conjugation
with bilirubin, steroid hormone etc.
In plants and mammals other than man Vit. C is synthesized from gulonate by this
pathway.
This pathway utilizes glucuronic acid of endogenous origin for energy production.
Dietary xylitol is utilized by this pathway.
12. Write about essential pentosuria
A. This inherited disease is characterized by excretion L-xylulose in urine. It is due to
deficiency of enzyme xylitol dehydrogenase. Due to lack of this enzyme L-xylulose can not
be converted to xylitol and it accumulates in blood and get excreted in urine.
Drugs like barbiturate and paracetamol increases utilization of glucose by this pathway.
13. Trace route of fructose conversion to glucose and pyruvate.
A. In liver fructose is converted to either glucose or intermediates of glycolysis. However in
skeletal muscle and adipose tissue fructose is converted to intermediates of glycolysis.
Reactions:
1. Fructose metabolism begins with phosphorylation. In liver fructokinase phosphorylates
fructose to fructose -1-phosphate. In skeletal muscle and adipose tissue hexo kinase
phosphorylates fructose to fructose -6-phosphate. This enters glycolysis. Magnesium ions
are required.
Hexokinase
Glycolysis
Fructose-6-phosphate
Fructokinase
Fructose+ATP
(1)
Fructose -1- phosp
(1) Phosphate+ADP.
ADP
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BIOCHEMISTRY - Questions and Answers
2. Aldolase B present in liver splits fructose -1-phosphate to glyceraldehyde and dihydroxy
acetone phosphate.
3. Aldehyde kinase in liver converts glyceraldehyde to glyceraldehyde -3-phosphate. ATP
is phosphate donor.
Aldolase
Fructose-1-phosphate
Dihydroxyacetone phosphate+glyceraldehyde.
(2)
Glyceraldehyde+ATP Aldehydekinase Glyceraldehyde -3-phosphate.
(3)
4. Now glucose is formed from glyceraldehyde -3-phosphate and dihydroxy acetone
phosphate by reversal of glycolysis and gluconeogenesis. Alternatively they can be used for
energy production by remaining reactions of glycolysis.
Pyruvate
Glyceraldehyde-3-phosphate +dihydroxy acetone phosphate.
(4)
Gluconeogenesis
Glucose
14. Write about inherited disease of fructose metabolism.
A. Inherited diseases of fructose metabolism are
1. Essential fructosuria : It is due to deficiency of fructo kinase. Hence fructose utilization
is blocked. Fructosuria and fructosemia develops on consumption of fructose
containing diets.
2. Hereditary fructose intolerance:It is due to deficiency of aldolase B. People affected with
this condition appear normal until they are exposed to fructose containing diets.
Consumption of fructose causes vomiting and diarrhoea in these individuals. Hence
they dislike sweets. Hypoglycemia, fructosemia, fructosuria develops on consumption
of fructose. Other symptions are jaundice, liver enlargement, kidney disease and
growth retardation.
15. Describe Galactose metabolism.
A. Galactose is converted to glucose in liver. Further galactose is required for synthesis of
lactose, galactolipids and mucopolysacharides.
Reactions:
1. Initial reaction of galactose utilization is phosphorylation catalyzed by galactokinase.
ATP and magnesium ions are required and galactose-1-phosphate is product.
2. Transfer of galactose to UDP-glucose replacing glucose occurs in this reaction. Glucose
is released as glucose-1-phosphate. Reaction is catalyzed by galactose-1-phosphate
uridyl transferase.
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CHAPTER - 8 | Carbohydrate Metabolism
Galactose-1-phosphate
Uridyl transferase
Galactose+ATP
Galactose-1-phosphate
UDP
(1)
(2)
ADP
UDP-Glucose
-Galactose +Glucose-1-Phosphate.
Galactokinase
3. UDP-Galactose serve as donor galactose for synthesis of lactose, galactose containing
lipids etc. Alternatively it is converted to UDP –glucose by UDP galactose epimerase. It
occurs in two steps and involves participation of NAD+. First UDP-galactose undergoes
dehydrogenation to 4-keto –UDP galactose and NADH is produced. In the second step
reduction of 4-keto –UDP-galactose by NADH results in formation of UDP-glucose and
NAD+.
Epimerase
UDP –galactose +NAD
+
Epimerase
4-keto-UDP-galactose+NADH+H
+
(3)
(3)
UDP-Glucose+NAD+.
4. From UDP-glucose, glucose is liberated as glucose -1-phosphate after in corporation
into glycogen followed by phosphorylase action.
(4)
UDP-glucose
(4)
glycogen
Glucose -1- phosphate
Glucose.
Galactosemia :This inherited disease is due to deficiency of galactose-1-phosphate
uridyl transferase. So galactose utilization in affected persons is blocked.
Accumulation of galactose leads to galactosemia and galactosuria. Vomiting and
diarrhoea occurs on consumption of milk. Cataract of eye due to accumulation of
galactitol a reduced product of galactose, mental retardation, jaundice and liver failure
are other symptoms. Continued intake of galactose may lead to death. By withdrawing
galactose containing products in diet death can be prevented. Adult galactosemics
tolerate milk due to development of other routes of galactose utilization.
16. What is normal blood glucose level? How it is regulated?
A. Normal blood glucose level: The normal blood glucose level is 60-90 mg% in post absorptive
conditions. After a meal blood glucose level raises to 110-130 mg%. It is known as post
prandial blood glucose level. In fasting blood glucose level falls to 50-60mg%. In normal
people this level is brought back to normal level. How ever the blood glucose level in
normals is determined by
a. Rate of entry of glucose into blood from various routes.
b. Rate of removal of glucose from blood by various pathways.
Blood glucose sources: Dietary carbohydrates are digested and products glucose, fructose,
galactose reach liver. In the liver galactose and fructose are also converted to glucose.
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BIOCHEMISTRY - Questions and Answers
Dietary carbohydrates keep blood glucose level with in limits up to 3hours after food in
take. Liver glycogenolysis meets blood glucose requirements up to 10 hour after food in
take.
Liver gluconeogenesis meets blood glucose requirements up to 36 hours after food in take
and beyond that period if food is not taken.
Blood glucose removal: Pathway of carbohydrate metabolism uses glucose in various ways.
a. Glycolysis use glucose for energy b. Glycogenesis use glucose for glycogen formation. c.
HMP shunt use glucose for NADPH and pentose production. d. Uronic acid pathway use
glucose for uronic acid production. e. Glucose is used for fat formation.
A finely regulated homeostatic mechanism maintains stable blood glucose level in which
liver, extra hepatic tissues and various hormones are involved. They maintain stable
glucose level either by affecting glucose sources or glucose removal.
Role of liver:Liver plays crucial role in maintenance of stable blood glucose level. Live rcells
(hepatocytes)are freely permeable to glucose. Movement of glucose across hepatocyte
membrane is not influenced by insulin. When blood glucose level rises liver brings down to
normal level by converting excess glucose into glycogen, fat and pentoses. Similarly when
blood glucose level is below normal liver rises blood glucose level to normal by forming
glucose from glycogen (glycogenolysis) and non carbohydrate sources (Gluconeogenesis).
Extra hepatic tissues involved in blood glucose regulation or homeostasis are skeletal
muscle, kidney and erythrocytes. These extra hepatic tissues are not freely permeable to
glucose.
Skeletal muscle:When the blood glucose level rises skeletal muscle lowers by converting
glucose to glycogen. If the blood glucose level falls below normal it indirectly contributes to
blood glucose by supplying lactate. During starvation muscle aminoacids particularly
alanine is used for glucose formation.
Kidney:When blood glucose level falls below normal kidney contributes to blood glucose
through gluconeogenesis. If the blood glucose level is above normal kidney brings down to
normal by eliminating glucose in urine.
Erythrocytes:When the blood glucose level is high they remove glucose through HMP
pathway, 2-3-bis phosphoglycerate cycle and glycolysis. If glucose level falls it contributes
to blood glucose by supplying lactate.
Hormones:Many hormones are involved in maintenance of stable blood glucose level.
Based on their action on blood glucose level they are divided in to two types. They are 1.
Hypoglycemic hormones and 2. Hyper glycemic hormones.
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CHAPTER - 8 | Carbohydrate Metabolism
Hypoglycemic hormone
As the name implies this hormone lower blood glucose level. Insulin is the only known
hormone of this category.
Insulin :Insulin is secreted by beta cells of islets of Langerhans in response to increased
blood glucose level or hyperglycemia. Insulin plays crucial role in the regulation of blood
glucose level. It lowers blood glucose level by
a. Increasing up take of glucose by peripheral tissues like skeletal muscle and adipose
tissue. In the muscle excess glucose is converted to glycogen and in adipose tissue fat is
synthesized.
b. Increasing utilization of glucose by various pathways. Insulin increases rate of
glycolysis, HMP shunt, fatty acid synthesis, pyruvate dehydrogenase complex and
glycogenesis. At same time it decreases rate of glycogenolysis and gluconeogenesis.
Activitives of enzymes of glycolysis, HMP shunt, glycogenesis, fatty acid synthesis is
increased by insulin. Activities of enzymes of gluconeogenesis and glycogeneolysis are
decreased by insulin.
Hyper glycemic hormones
As the name implies these hormones raises blood glucose level. Glucagon, epinephrine
(norepinephrine), glucocorticoids, anterior pituitary hormones and thyroid hormones are
hyperglycemic hormones.
Glucagon:It is another hormone produced by pancreas. Alpha cells of islets of Langerhans
secretes this hormone in response to hypoglycemia. It is an antagonist of insulin. It
increases blood glucose level by a. Promoting gluconeogenesis in liver. b. Inhibiting
glycogenesis.
Epinephrinc (Nor epinephrine) : Adrenal medulla secretes these hormones in response
to hypoglycemia. It increases blood glucose level by a. Increasing gluconeogenesis in liver.
b. Inhibiting glycogenesis. c. Stimulating glycogenolysis.
Glucocorticoids: Adrenal cortex secretes glucocorticoids into blood stream. They
increase blood glucose level by a. Reducing glucose utilization by peripheral tissues. b.
Enhancing gluconeogenesis by inducing formation of enzymes of gluconeogenesis.
Anterior pituitary hormones:Anterior pituitary gland increases blood glucose level by
secreting two hormones. They are growth hormone and adereno corticotrophic hormone
(ACTH).
Growth hormone:Secretion of growth hormone occurs as response to hypoglycemia. It
increases blood glucose level by i. Inhibiting uptake of glucose by peripheral tissues. ii.
Promoting fat mobilization. iii. Liver gluconeogenesis.
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BIOCHEMISTRY - Questions and Answers
ACTH: It increases blood glucose level by producing glucocorticoids and acting on glycogen
metabolism.
Thyroid hormone:Thyroxine increases blood glucose level by i. Affecting glucose
utilization by peripheral tissues. ii. Affecting glucose absorption in intestine.
17. Write about 2. 3-bis phosphoglycerate cycle. or Rapoport-Leubering
cycle
A. This cycle is active in erythrocytes. It deals with formation and degradation of 2, 3-bis
phosphoglycerate.
Formation of 2, 3 –bis phosphoglycerate (2, 3 –BPG): Phosphoglycerate mutase catalyzes
formation of 2, 3 –BPG from 1, 3-bis phosphoglycerate of glycolysis.
Glycolysis
1, 3 –bis phosphoglycerate
2, 3-bis phosphoglycerate.
Degradation of 2, 3 –BPG: 2, 3-BPG is degraded to 3-phosphoglycerate by a
phosphatase. Further fate of 3-phosphoglycerate occurs in glycolysis.
2, 3-bis phosphoglycerate
3-phosphoglycerate
Glycolysis.
Significance:In erythrocytes 2, 3-BPG helps in unloading of oxygen by hemoglobin.
18. Explain glucose- lactate cycle OR Cori cycle
A. In this cycle lactate that is produced in rapidly contracting skeletal muscle enters blood
stream because it is a dead end of glycolysis. Through blood stream it reaches liver where it
is converted to glucose through gluconeogenesis. Glucose so formed enters blood stream
and reaches skeletal muscle for utilization. Thus the liver supplies glucose to skeletal
muscle which in turn supplies lactate to liver. These reactions constitutes coricycle or
glucose –lactate cycle.
Skeletal muscle
Glucose
Blood
Lactate
Liver
Lactate
Gluconeogenesis
Glycolysis
Glucose.
Blood
19. Define Diabetes mellitus. Classify. Write about each class. Mention
clinical and biochemical symptoms.
A. Diabetes mellitus is disease due to lack of action of insulin and characterized by elevated
blood glucose level and glucose in urine. There are two types of diabetes mellitus.
I. Type І Diabetes mellitus or insulin dependent diabetes mellitus (IDDM) or Juvenile onset
diabetes mellitus :As the name implies it appears in young people. The age of affected
people is always below 30 years. It accounts about 20% of diabetic cases. Usually
individuals of this disease are thin or lean and appears as under nourished. It is due to
absence of insulin. Hence patients of this disease are treated with insulin injections.
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CHAPTER - 8 | Carbohydrate Metabolism
ii. Type ІІ diabetes mellitus or non insulin dependent diabetes mellitus (NIDDM)or Adults
on set diabetes mellitus :As the name implies it appears in adults. The age of affected
people is always above 30years. It accounts about 80% of diabetes cases. Usually
individuals of this disease are obese. It is due to lack of insulin action i. e. insulin is
present but due to lack of insulin receptors its action is lost. Hence patients of this
disease cannot be treated with insulin injections.
Biochemical and clinical symptoms: In acute diabetic patients following biochemical
and clinical symptoms are seen. a. Hypoglycemia b. Glycosuria c. Polyuria d. Increased
hunger (Polydipsia) e. Increased thrist (polyphasia) f. Ketosis in type І diabetes g. Weight
loss h. Delayed wound healing. i. Keto acidosis. j. Coma and death.
20. Describe Glucose Tolerance Test (GTT)
A. The ability of body to oxidize a load of glucose given is referred as glucose tolerance. This
test is used to distinguish normal people from people with increased or decreased tolerance
that occurs in diseases like diabetes mellitus, hormonal disorders etc.
Procedure: After over night or 12 hour of fasting GTT is done. Fasting blood and urine
samples are collected. The subject is asked to drink 200 ml water which contain test dose of
glucose. A standard dose of 50gm of glucose or 0. 75 -1. 5 gm per Kg body weight is usually
dissolved in 200ml water. The time is noted and for every 30 minutes blood and urine
samples are collected for two and half hours. Glucose in the blood samples and urine
samples is determined. Usually blood glucose level is measured quantitatively and
qualitative Bendicts test is used for urine sugar analysis. The blood glucose values are
plotted against time.
300
Severe Diabetes
200
Blood
Glucose
mg%
150
Mild Diabetes
100
Normal
50
0
30
60
90
Time
120
150
Normal glucose tolerance (response):The fasting blood glucose level is with in range of 6090mg%. The blood glucose level reaches a peak with in 30 to 60 minutes after consuming
glucose test dose. The peak value is 110-130mg%. The initial rise is due to absorption of
glucose. However increased blood glucose level returns to normal at the end of 2 hours due
to increased glucose utilization. None of the urine samples contain glucose because the
blood glucose level is below renal threshold for glucose which is 175mg%.
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BIOCHEMISTRY - Questions and Answers
Decreased glucose tolerance :Diabetes mellitus is mainly responsible for the diminished
glucose tolerance. Fasting blood glucose values are above 120mg % and depends on
severity of disease. After test dose of glucose, blood glucose level rises sharply and extent of
increase is more than that seen in normal people. The most striking is high blood glucose
level even after 2 hours. In mild diabetes minimum one urine sample contains glucose. All
urine samples contain glucose in the case of severe diabetes. Decreased glucose tolerance
also occurs in Cushing's syndrome, thyrotoxicosis, hyper activity of pituitary gland and in
liver disease.
Increased glucose tolerance: Increased tolerance is seen in addison's disease, myxedema,
cretinism, hypopituitarism etc. In cases with impaired glucose absorption also increased
tolerance occurs. Sprue, celiac disease, and idiopathic steatorrhea are some intestinal
disorders associated with increased tolerance. Usually a flat glucose tolerance curve is
obtained.
21. Write about glycosurias.
A. Glycosurias are conditions associated with excretion of glucose and or sugars in urine.
Most common disease is diabetes mellitus. Renal diabetes is another disease of glycosuria
where excretion of sugar or glucose in urine is due to defective reabsorption of glucose in
renal tubules. Other glycosurias are
1. Lactosuria : It is characterized by excretion of lactose in urine. It occurs in pregnancy.
2. Galactosuria: It is associated with excretion of Galactose in urine. It occurs in
galactosemia.
3. Fructosuria: It occurs in hereditary fructose intolerance and associated with excretion
of fructose in urine.
4. Pentosuria: It occurs in essential pentosuria and characterized by excretion of pentose
xylulose in urine.
22. Test for reducing substances in urine.
A. Benedict's test is used for detection of reducing substances in urine. It involves boiling of
urine with Benedict's qualitative reagent. Depending on amount of reducing sugar in urine
variety of colors are produced. Reducing monosaccharides and disaccharides are detected
in urine by performing this test. Under alkaline conditions reducing sugar decomposes to
enediols which reduces cupric to red cuprous oxide.
Other model questions are
23. Write about enzymes involved in carbohydrate digestion.
24. Write about diseases of carbohydrate digestion and absorption.
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CHAPTER - 8 | Carbohydrate Metabolism
25. Write a note on
a) Lactose intolerance b) Disaccharidases
26. Write irreversible reactions of glycolysis.
27. Write the significance of citric acid cycle.
28. Define glycogenesis. Write the reactions of this process.
29. Explain how glycogen is degraded?
30. Write a note on glycogen storage diseases.
31. Write the significance of HMP shunt pathway.
32. Write about glucose -6-phosphate dehydrogenase deficiency.
33. Define key enzymes of gluconeogenesis. Write reactions they
catalyze.
34. Write significance of gluconeogenesis.
35. Write biochemical symptoms and enzyme defect in the following
a) Galactosemia b) Hereditary fructose intolerance c) Essential pentosuria
36. Write sources and routes of of blood glucose.
37. Write the role of liver in blood glucose level maintenance.
38. What is the action of insulin on blood glucose level?
39. Write the extra hepatic tissues role in blood glucose regulation.
40. Explain how hyperglycemic hormones raises blood glucose level.
41. Define glucose tolerance. What conditions it is altered?
42. Expand IDDM and NIDDM. Write briefly about any one of them.
43. Write normal response of glucose tolerance test.
44. McArdle syndrome
45. von Gierk's disease
46. Direct oxidative pathway of glucose.
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CHAPTER - 9 | Lipid Metabolism
Chapter
9
Lipid Metabolism
1. Name lipids present in diet. Explain how they are solubilized, digested
and absorbed?
A. Lipids present in the diet are triglycerides, phospholipids, Cholesterol and its esters, fatty
acids, glycolipids, carotenes and sterols other than cholesterol. They are present in plant
and animal food stuffs. They are vegetable oils, or cooking oils of plant origin and eggs,
meat, cheese, milk, butter and fat of animal origin.
Solubilization of lipids: Since lipids are insoluble in aqueous environment of lumen
digestion of lipids requires their initial solubilization. Bile salts present in bile are
responsible for solubilization of dietary lipids. Bile salts form emulsion with dietary lipids.
They increases surface of lipid at water inter phase for the action of enzymes.
Digestion of lipids
Lipid digestion : Hydrolysis of triglycerides, phospholipids and cholesterol esters to
glycerol, free fatty acids, mono acylglycerol and cholesterol is known as digestion of lipids.
In mouth :Due to lack of favourble conditions no digestion of lipid occurs in the mouth.
In the stomach :Mechanical emulsification of food lipids allows action of gastric lipase to
some extent. However acidic environment limit action of enzymes on lipids.
In the small intestine :Major part of lipid digestion occurs in small intestine by pancreatic
enzymes. Pancreatic juice contains lipase, cholesterol esterase and phospholipase. Lipase
as such not able to interact with emulsion particle containing dietary lipids. Colipase
which is also present in pancreatic juice and bile salts aids lipid digestion by lipase.
Pancreatic lipase hydrolyzes ester bonds of 1, 3 carbons of triglycerides. 2monoacylglycerol and free fatty acids are formed.
lipase
Triglyceride
2-monoacylglycerol + Free fattyacids.
colipase
Majority of 2-monoacylglycerol about 72% comes out of emulsion particle and forms mixed
micelles. The remaining about 28% is converted to 1-mono acylglycerol by an isomerase.
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CHAPTER - 9 | Lipid Metabolism
2-monoacylglycerol
mixed micelles
72%
Isomerase
2-monoacylglycerol
1-monoacylglycerol.
28%
Lipase act on 1-monoacylglycerol and hydrolyzes about 22% of monoacylglycerol to
glycerol and free fatty acids. The remaining 6%monoacylglycerol is absorbed as such.
Lipase
Absorbed
1-monoacylglycerol
6%
glycerol + Free fatty acids
22%
Cholesterol esters are hydrolyzed by cholesterol esterase to cholesterol and free fatty acids.
Phospholipase A2 hydrolyzes phospholipids ester bond on 2 carbon to form
lysophospholipid and free fatty acids.
cholesterol
Cholesterol ester
cholesterol + Free fatty acid
esterase
Phospholipase A2
Phospholipids
lyso phospholipids + Free fatty acid
Lysophospholipase act on lysophospholipid and forms glycero phocholine and fatty acid.
Lysophospholipase
Lyso phospholipids
Glycerophosphocholine +Free fatty acid
Lipid absorption
In the proximal part of jejunum 2-monoacyglycerol, free cholesterol, fatty acids,
lysophospholipids interact with bile salt micelles and forms mixed micelles. To the brush
border membrane mixed micelles carry these products of digestion where they are
absorbed through specific transporter present in enterocyte membrane.
Bile
Lysophospholipids + monoacylglycerol + cholesterol+ Free fatty acids
Salts Micelles
Specific
Mixed micelles
Brush border
Cytosol of enterocyte
Membrane
Transporter
2. Write the fate of absorbed lipids in intestine. Explain how they enter
circulation?
A. In the enterocyte of intestine mono acylglycerol, free fatty acids are converted to
triglycerides. Lysophospholipids are converted to phospholipids by esterification.
Cholesterol esters are formed from cholesterol and free fatty acids.
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BIOCHEMISTRY - Questions and Answers
Monoacylglycerol + Fatty acids
Triglycerides
Lysophospholipid +Fatty acid
Phospholipid
Cholesterol + Fatty acids
Cholesterol ester
In the enterocyte triglycerides, phospholipids and cholesterol ester formed combines with
proteins to form lipoproteins chylomicrons. These chylomicrons are released into lymph of
intestinal lymphatics. Due to absorption of dietary lipids intestinal lymph appears milky
and is called as chyle. Finally chylomicrons enters systematic circulation through thoracic
duct.
apo
Triglycerides, phospholipids, cholesterol ester
Chylomicrons
Lipoproteins
Intestinal lymphatics
Thoracic duct
Blood.
For the triglycerides and cholesterol ester synthesis only long chain fatty acid are used.
So absorbed short and medium chain fatty acids with glycerol directly enters portal venous
blood.
3. Write about diseases associated with lipid absorption or
digestion.
A. Chyluria :People affected with this disease excretes milky urine due to abnormal
connection between intestinal lymphatics and urinary tract. Since only long chain fatty
acids are used for resynthesis of lipids it is corrected by replacing diet with short and
medium chain containing fatty acids. Chylous fistule is another name for this disease.
Chylothorax : It is characterized by accumulation of milky pleural fluid in the pleural
space of lungs due to abnormal connection between lungs pleural space and intestinal
lymphatics. The condition is corrected by supplying diet containing short and medium
chain fatty acids.
Pancreatitis : In this condition bile flow is obstructed. As a result digestion of lipid is
affected.
Cholestasis : In this condition bile flow is blocked. Since bile is required for fat digestion,
in cholestasis lipid digestion is affected.
4. Describe fatty acid oxidation.
A. Fatty acids are oxidized in the mitochondria of several types of cells. Liver cells,
adipocytes, cardiac myocytes, renal cells, Pulmonary cells, muscle cells, and to some extent
in neuronal cells of brain. Fatty acid oxidation involves.
i. Initial activation in cytosol or outer mitochondrial membrane.
ii. Translocation of activated fatty acids into mitochondria
iii. Beta (ß) –oxidation in mitochondria.
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CHAPTER - 9 | Lipid Metabolism
i. Fatty acid activation :It involves conversion of fatty acid into corresponding CoA form.
Acyl-CoA synthetases converts fatty acids to acyl-CoAs using ATP, CoASH and
magnesium ions. They are also called as thiokinases. They are present in outer
mitochondrial membrane. ATP is converted to AMP and pyrophosphate (PPi).
Pyrophosphatase converts pyrophosphate to phosphate.
Acyl-CoA
Pyro
Synthetase
phosphatase
Fatty acid +ATP
Acyl-CoA + PPi
2Pi
2
Mg +
AMP
ii. Transport of acyl-CoAs into mitochondria : Acyl - CoAs are impermeable to inner
mitochondrial membrane. Carnitine translocates activated fatty acids from out side to
matrix of mitochondria. It begins with transfer of acyl-CoA to carnitine catalyzed by
carnitine acyl transferase-І (CAT-І) present in outer mitochondrial membrane. Acyl
carnitine is product of this reaction. Carnitine- acylcarnitine transloc present in inner
mitochondrial membrane translocates acyl-carnitine into matrix of mitochondria. In
the matrix of mitochondria carnitine-acyl transferase –ІІ (CAT-ІІ) transfers acyl
residue to CoA from acyl carnitine and free carnitine is released. To complete the
translocation process translocase pumps back carnitine to out side of mitochondria.
Carnitine – acyl trans
ferase-І (CAT-І)
Acyl-CoA+carnitine
Acyl-carnitine + CoA
Outer mitochondrial
Membrane
Trans locase
Acyl-carnitine
Acyl - carnitine in matrix
Inner mitochondrial
membrane
carnitine acyltransferase-ІІ
(CAT-ІІ)
Acyl-carnitine+CoA
Acyl-CoA+ carnitine.
Matrix of mitochondria
Translocase
Carnitine
carnitine out side of mitochondria.
Carnitine- acylcarnitine
iii. Beta oxidation of fatty acids : As the name implies fatty acid oxidation involves
sequential removal of two carbon fragments from carboxy terminus by cleaving fatty
acid at beta carbon. An acyl-CoA shorter by two carbon atoms and an acetyl-CoA are
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BIOCHEMISTRY - Questions and Answers
products. Reactions of beta oxidation continues until acyl-CoA is completely converted
to acetyl-CoA.
Acyl-CoA
Beta oxidation
Acetyl –CoA + Acyl-CoA shorter by two carbons
Beta Oxidation
Acyl-CoA shorter by four carbons
Acetyl-CoA.
Reactions :
1. First reaction of beta oxidation is dehydrogenation of acyl-CoA by FAD dependent acyl
CoA dehydrogenase. Acyl –CoA is converted to enoyl-CoA and FAD H2 isformed.
2. Hydratase catalyzes addition of water across double bond in the second reaction. ß
–hydroxyacyl-CoA is product.
Acyl-CoA
Dehydrogenase
Acyl-CoA+FAD
Hydratase
Enoyl – CoA
ß-Hydroxy acyl-CoA.
(1)
(2)
FADH2
H2o
3. An NAD+ dependent dehydrogenation occurs in this reaction. Beta hydroxy acyl-CoA
dehydrogenase catalyzes this reaction and ß-ketoacyl-CoA is product.
4. Clevage of ß – ketoacyl-CoA at beta carbon by ß – ketothiolase (thiolase) in this
reaction yields acyl-CoA that is shorter by two carbons and acetyl-CoA.
ß-hydroxyacyl-CoA
Dehydrogenase
ß-hydroxyacyl-CoA+NAD+
ß-ketoacyl-CoA+ NADH+H+
(3)
ß-ketothiolase
ß-ketoacyl-CoA+CoA
Acetyl-CoA+Acyl-CoA shorter by two carbons.
(4)
Acyl-CoA shorter by two carbons enters beta oxidation reactions and thus the cycle
continues until acetyl-CoA is produced from the acyl- CoA.
Energetics of beta oxidation: Energy production by beta oxidation process taking
palmitic acid as an example. Since palmitic acid is 16 carbon saturated fatty acid it
under goes beta oxidation process seven times and produces 8 Acetyl-CoA s, 7FADH2
and 7 NADH. Acetyl –CoA is completely oxidized in citric acid cycle. FADH2 and NADH
are oxidized by respiratory chain.
Amount of ATP generated when palmitic acid is oxidized by beta oxidation.
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CHAPTER - 9 | Lipid Metabolism
1. ATP generated by oxidation of 8 Acetyl-CoA s in citric acid cycle.
08x12 = 96
2. ATP generated by oxidation of 7 FADH2 in respiratory chain
07x02 = 14
3. ATP generated by oxidation of 7NADH in respiratory chain
07x03 = 21
131
ATP consumed for activation of fatty acid
-2
Net =
129
Therefore complete oxidation of palmitic acid produces 129 ATP molecules. As per
latest concepts only 106 ATPs are generated.
5. Define Alpha oxidation. Write reactions of this process. Name disease
associated.
A. As the name implies Alpha oxidation involves oxidation of fatty acid by sequential removal
of one carbon units from carboxy terminus after cleaving fatty acid at alpha carbon atom. It
occurs in peroxisomes. It does not generate energy and requires no CoA intermediates.
Reactions:
1. A monoxygenase brings about hydroxylation of alpha carbon of fatty acid in first
reaction. Hydroxy fatty acid is produced.
2. Dehydrogenation and oxidative decarboxylation converts hydroxy fatty acids shorter
by one carbon atom.
Refsum's disease : It is due to block in α- oxidation of phytanic acid. So phytanic acid
accumulates in blood and liver. Main symptoms are peripheral neuropathy,
abonormalities in skin and bone. Symptoms disappear on consuming phytanic acid free
diet.
6. Write about omega oxidation.
A. As the name implies Omega (ω) oxidation involves oxidation of fatty acid by oxidizing
omega (ω) carbon to carboxylic group. It occurs in smooth endoplasmic reticulum.
Reactions :
1. A cyt P450 dependent mixed function oxidase first catalyzes hydroxylation of carbon of
fatty acid. ω– hydroxy fatty acid is product.
2. Further oxidation at ω-carbon generates di carboxylic acid which under goes beta
oxidation.
7. Name ketone bodies. Describe metabolism of ketone bodies.
A. Ketone bodies are acetone, acetoacetic acid and beta hydroxybutyric acid. Ketone body
metabolism consist of a. Ketogenesis and b. Ketolysis.
Ketogenesis : Synthesis of ketone bodies is known as ketogenesis. It occurs in liver.
Acetyl CoA is precursor.
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BIOCHEMISTRY - Questions and Answers
Reactions :
1. Ketogenesis begins with condensation of two acetyl –Co A molecules catalyzed by
thiolase. Acetoacetyl-CoA is product.
Thiolase
2 Acetyl –CoA
Acetoacetyl –CoA+ CoA.
(1)
2. Acetoacetate forms from acetoacetyl- CoA by two ways.
a. In this route acetoacetyl –CoA condenses with another molecule of acetyl –CoA to
form beta hydroxy beta methyl glutaryl-CoA (HMG-CoA) catalyzed by HMG –CoA
synthase. A lyase catalyzes splitting of HMG –Co A to acetoacetate and acetyl CoA.
HMG - CoA
Synthase
Acetoacetyl-CoA+Acetyl- CoA
beta hydroxybeta methyl glutaryl -CoA+
2a
CoA.
HMG CoA
lyase
HMG-CoA
acetoacetate+ Acetyl-CoA.
2a
b. In another route decarboxylation of acetoacetyl- CoA by deacylase yields aceto
acetate
Deacylase
Acetoacetyl-CoA
Acetoacetate+ CoA.
2b
3. ß- hydroxybutyrate is formed from acetoacetate. NADH dependent dehydrogenase
catalyzes this reaction.
Dehydrogenase
Aceto acetate +NADH +H+
beta hydroxybutyrate +NAD+
(3)
4. Spontaneous decarboxylation of acetoacetate yields acetone.
Spontaneous
Acetoacetate
Acetone+CO2.
(4)
Significance : Under certain conditions citric acid cycle is unable to produce energy
from entire acetyl-CoA generated either from beta oxidation or pyruvate. This
excess acetyl –CoA are converted to ketone bodies in liver. . Liver distributes ketone
bodies thus generated among various organs. So ketogenesis allow distribution of
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CHAPTER - 9 | Lipid Metabolism
excess fuel (Acetyl –CoA) among organs. These ketone bodies produced in liver
reaches various organs through systemic circulation. They are taken by peripheral
tissues for utilization.
Ketolysis : Ketolysis is the degradation of ketone bodies. It occurs in cardiac
muscle, brain, kidney and to some extent by skeletal muscle.
Utilization of acetoacetate
Reactions :
1. Activation of aceto acetate is first reaction of its utilization. Acetoacetyl-CoA synthase
an ATP and magnesium dependent enzyme converts aceto acetate to corresponding
CoA. AMP and PPi are products. PPi is further hydrolyzed by pyrophospahtase.
2. Thiophorase or acetoacetate- succinyl-CoA transferase transfer CoA from succinyl
–CoA to acetoacetate. This is another mode of activation.
Acetoacetyl-CoA
Synthase
Acetoacetate+ ATP +CoA
Acetoacetyl –CoA + AMP+PPi
Pyro phosphatase
(1)
PPi
2Pi.
(1)
Thiophorase
Acetoacetate+succinyl- CoA
Acetoacetyl-CoA+ succinate.
(2)
A thiolase cleaves acetoacetyl –CoA to two molecules of acetyl-CoA. These acetylCoA s are utilized by citric acid cycle.
Thiolase
AcetoacetylCoA+
2 acetyl –CoA
CoA
(3)
TCA cycle
Energy
Utilization of beta hydroxy butyrate
Beta hydroxy butyrate is utilized by two ways.
Reactions:
1. A dehydrogenase converts beta hydroxy butyrate to aceto acetate which is used for
energy production as detailed above.
2. In a minor route a synthetase activates beta hydroxybutyrate to beta hydroxyl butyryl
–CoA. Dehyrogenation of beta hydroxybutyryl-CoA yields acetoacetyl-CoA.
dehydrogenase
ß- hydroxybutyrate +NAD
+
NADH+H+Aceto acetate
(1)
TCA
Cycle.
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BIOCHEMISTRY - Questions and Answers
Synthetase
Dehydrogenase
ß- hydroxybutyryl -CoA
Acetoacetyl CoA.
(2)
(2)
CoA
The acetoacetyl-CoA is converted acetyl-CoA as described above.
ß-hydroxybutyrate
Utilization of Acetone
Utilization of acetone by peripheral tissues is slow. Usually it is removed in urine or as Co2
through lungs.
Significance : Some tissues like cardiac tissue and kidney prefers ketone bodies for energy
production than glucose. Ketone body utilization for energy production is more significant
in prolonged starvation.
Regulation of ketone body metabolism :
1. Ketogenesis largely depends on mobilization of free fatty acids.
2. CAT-І is mainly involved in controlling ketone body formation. In fed conditions CAT –І
activity is more so more acetyl –CoA is formed from beta oxidation. Hence ketogenesis
is more in starvation.
Medical importance :
1. Normal blood ketone body level is 1mg%. Under normal conditions ketone body
formation is balanced by their utilization. So ketone body level in blood remains
constant.
2. Ketosis: If ketogenesis is more than ketolysis accumulation of ketone bodies in blood
occurs. It is known as ketonemia. Excess ketone bodies are excreted in urine. It is
known as ketonuria. Ketonemia and ketonuria gives rise to ketosis. Symptoms are
headache, vomiting and coma. Kotosis occurs in i. Prolonged starvation. ii. Diabetes
mellitus iii. von Geirke's disease iv. Fever. v. Severe muscular exexcise.
3. Ketoacidosis: It occurs in uncontrolled diabetes mellitus and in prolonged starvation
due to depletion of blood bicarbonate. To maintain normal blood pH ketone bodies are
usually neutralized by bicarbonate buffer. But ketone bodies are produced in excess in
uncontrolled diabetes mellitus. So more bicarbonate is needed for neutralization and
blood bicarbonate depletion occurs. This leads to decreased blood PH i. e. Acidosis. The
condition is called as keto acidsis because acidosis is due to more ketone bodies.
8. Describe fatty acid biosynthesis De novo.
A. Site :Cytosol of liver, adipose tissue, lung, mammary gland, brain and kidney contain
enzyme system for fatty acid biosynthesis.
Precursor : Acetyl-CoA of pyruvate oxidation, NADPH of HMP shunt and cytosolic malic
enzyme are precursors of fatty acid synthesis.
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CHAPTER - 9 | Lipid Metabolism
Transport of acetyl-CoA : Acetyl-CoA which is precursor of fatty acid biosynthesis is formed
in mitochondria. But fatty acid synthesis occurs in cytosol and acetyl-CoA is impermeable
to mitochondrial membrane. Since mitochondrial membrane is permeable to citrate, acetyl
–CoA enters cytosol in the form of citrate.
1. As a part of citric acid cycle, citrate is formed form acetyl-CoA. A tricarboxylate
transporter present in mitochondrial membrane transports citrate out of
mitochondria.
2. In the cytosol acetyl- CoA is regenerated by ATP : Citrate lyase from citrate.
Citrate
ATP:Citrate
Synthase
lyase
Acetyl—CoA+ oxaloacetate
Citrate+CoA
Acetyl-CoA +
ATP
(2)
Oxaloacetate + ADP + Pi
3. Oxaloacetate is converted to malate by cytosolic malate dehydrogenase.
4. A cytosolic malic enzyme converts malate to pyruvate in presence of NADP+.
Malate
Dehydro
Malic
genase
enzyme
Oxaloacetate
Pyruvate + CO2 +NADPH + H+.
Malate
(3)
+
(4)
+
NADH+H NAD
NADP
+
The transport of acetyl-CoA is accompanied by formation of NADPH in cytosol. This
NADPH and acetyl –CoA are used for fatty acid synthesis.
Fatty acid synthase complex
In the cytosol fatty acid synthase complex synthesizes fatty acids by using acetyl –CoA and
NADPH. This multi enzyme complex is dimer consisting two monomers or subunits. Each
monomer has two sulfhydryl (SH)groups, activities of seven enzymes and an acyl carrier
protein (ACP). Phosphopantothein of ACP contributes one-SH at one end and another –SH
is contributed by cysteine residue of one of seven enzymes. The two monomers are
arranged in head to fail manner. Cysteine –SH of one monomer is in close proximity with
phosphopantothein –SH of another monomer. Individual monomers are inactive only
dimer is active. Functional unit consist of one half one monomer and complementary half
of another monomer. Hence two fatty acids are produced at a time.
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BIOCHEMISTRY - Questions and Answers
Monomer-1
Monomer-2
Cys
Pan
|
|
SH
SH
SH
SH
|
|
Pan
Cys
Fatty acid synthase complex.
Reactions : Multi enzyme complex uses only one acetyl-coA as such rest of acetyl – CoA s
are used in the form of malonyl-CoA.
Formation of malonyl-CoA : Acetyl- CoA is converted to malonyl-CoA by carboxylation
which depends on biotin and energy. Acetyl-CoA carboxylase is the enzyme that catalyzes
this reaction.
Acetyl-CoA
Carboxylase
Acetyl-CoA + ATP+ CO2
Malonyl-CoA+ ADP+ Pi
Biotin
Fatty acid synthase reactions :The availability of acetyl –CoA, NADPH initiates fatty acid
synthase dependent reactions of de novo fatty acid synthesis.
1. First reaction of fatty acid synthase complex is transfer of acetyl –CoA to Cysteine-SH
of fatty acid synthase complex which is catalyzed acetyl trans acylase.
Acetyl trans
Acylase
Acetyl –CoA + Fatty acid synthase complex
Acetyl enzyme complex + CoA
(1)
2. A molecule of malonyl –CoA is transferred to pan-SH of other monomer of fatty acid
synthase complex. Malonyl trans acylase catalyzes this reaction.
Malonyl trans
Acylase
Acetyl enzyme complex + malonyl –CoA
Acetyl –malonyl enzyme +CoA.
(2)
3. A condensing enzyme ß- ketoacyl synthase catalyzes condensation of acetyl and
malonyl groups. A keto acyl enzyme complex is formed. Cysteine –SH group of the
monomer becomes free
ketoacyl Synthase
Acetyl –molonyl enzyme complex
ß-ketoacyl enzyme complex.
(3)
4. An NADPH dependent ketoacyl reductase reduces ketoacyl group to hydroxyacyl
group. ß-hydroxyacyl enzyme is formed.
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CHAPTER - 9 | Lipid Metabolism
ketoacyl reductase
ß-ketoacyl enzyme complex +NADPH +H+
ß- hydroxyl acyl enzyme +NADP.
(4)
5. In this reaction a water molecule is removed from ß-hydroxyacyl enzyme by hydratase.
Enoyl enzyme is formed.
6. Another NADPH dependent reaction of enoyl reductase generates butyryl enzyme. The
four carbon butyryl moiety is on the phosphopantotheine-SH of enzyme complex.
Hydratase
Enoyl Reductase
ß- hydroxyacyl enzyme
Enoyl enzyme
Butyryl enzyme +NADP+.
(5)
(6)
H2O
NADPH+H+
7. All of the above multi enzyme reactions are repeated six times incorporating malonyl
–CoA each time to generate palmitoyl moiety.
8. Finally palmitoyl moiety is released from multi enzyme complex by last enzyme of
complex thioesterase.
Thioesterase
Butyryl enzyme
palmitoyl enzyme
(7)
palmitic acid + fatty acid synthase complex.
(8)
Regulation of fatty acid synthesis : De novo biosynthesis of fatty acids is subjected to
both allosteric and hormonal regulation. Acetyl CoA carboxylase is regulatory enzyme.
Allosteric control : Acetyl –CoA carboxylase exist in two forms an active form and
inactive form. Polymer of acetyl –CoA carboxylase is a active form. Monomer is inactive
form. Citrate is a allosteric activator and long chain acyl-CoA is allosteric inhibitor.
Further activity of acetyl-CoA carboxylase is inversely related to plasma free fatty acid
level. Hence in starvation and diabetes due to increased fatty acid level synthesis of
fatty acids is inhibited.
Hormonal regulation :Glucagon inhibits fatty acid synthesis where as insulin promotes
fatty acid synthesis. These hormones act by cAMP mediated phosphorylation of acetylCoA carboxylase.
9. Explain how triglyceride are synthesized?
A. Synthesis of triglycerides : Triglycerides are synthesized in liver, adipose tissue, and
intestine. In liver and adipose tissue dihydroxyacetone phosphate of glycolysis is used for
triglyceride biosynthesis. However liver is able to synthesize triglycerides from glycerol
also. In intestine triglycerides are formed from monoacylglycerol pathway.
Synthesis of triglycerides from glycerol and dihydroxy acetone phosphate :
1. In liver glycerol and dihydroxy acetone phosphate are converted to glycerol 3- phosphate.
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BIOCHEMISTRY - Questions and Answers
The former reaction is catalyzed by kinase and latter is catalyzed by NADH dependent
dehydrogenase.
Glycerokinase
Glycerol
Dehydrogenase
Glycerol-3-phosphate
Dihydroxy acetone phosphate
(1)
ATP
(1)
NAD+
ADP
NADH+H+
2. Now incorporation of fatty acids into glycerol-3- phosphate occurs. Activated long chain
fatty acids of both saturated and unsaturated are used. Lysophosphatidate is product. Acyl
transferase catalyzes this reaction.
3. Another fatty acid incorporation leads to phosphotidate formation.
Acyl
Transferase
Glycerol-3- phosphate
Acyl
Transferase
Lysophosphatidate
(2)
Acyl-CoA CoA
phosphatidate.
(3)
Acyl-CoA CoA
4. Removal of phosphate from phosphatidate by phosphatase yields 1, 2-diglyceride.
5. In the intestine monoacyl glycerol is converted to 1, 2 –diglyceride by incorporation of fatty
acid.
Phosphatase
Acyl Transferase
Phosphatidate
1, 2 – diglyceride
Monoacyl glycerol.
(4)
(5)
pi
CoA
Acyl-CoA
6. Transfer of another acyl-CoA to 1, 2 –diglyceride by transferase produces triglyceride.
Acyltrans
ferase
1, 2- diglyceride
Triglyceride+CoA.
(6)
Acyl-CoA
Significance:Triglyceride biosynthesis is linked to fatty acid biosynthesis. In well fed state
triglyceride biosynthesis is more. In starvation and diabetes triglyceride biosynthesis is
less
10. Explain triglyceride degradation or lipolysis. Add a note on
hormonal action on lipolysis.
A. Hormone sensitive lipase present in adipose tissue hydrolyzes triglycerides to free fatty
acids and di or monoglycerides. Di or monoglyceride lipase hydrolyzes monoglycerides or
diglycerides to glycerol and fatty acids.
Hormone sensitive
Triglycerides
free fatty acids +mono or diglycerides
Lipase
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CHAPTER - 9 | Lipid Metabolism
Mono or diglyceride
Mono or diglycerides
Glycerol+ free fatty acids.
Lipase.
Triglyceride breakdown is more in energy deficient and stress conditions. In starvation
and diabetes also triglyceride breakdown is more.
Action of hormones on lipolysis
Hormones like insulin, glucagon, epinephrine, nor epinephrine, glucocorticoids, growth
hormone etc. affects triglyceride breakdown. Glucagon, epinephrine and glucocorticoids
stimulates lipolysis. Insulin antagonizes lipolysis. As the name implies these hormones
affect activity of hormone sensitive lipase. It exist in two forms an active form and an
inactive form. Lipolytic hormones keeps this enzyme in active form by promoting cAMP
dependent phosphorylation. In contrast, insulin suppresses lipolysis by inhibiting
cAMP dependent phosphorylation.
Phosphorylation
Hormone sensitive lipase (inactive)
hormone sensitive lipase (active)
(+)
Epinephrine, glucagon etc
phosphorylation
(-)
of hormone sensitive
insulin
lipase
(+) activation (-) inhibition
11. Define fatty liver. What are the causes ?
A. Abnormal accumulation of lipid or fat in the liver is known as fatty liver.
Usually lipid content of liver does not exceed 5% but in fatty liver the lipid content raises to
25-30 %. Several factors cause abnormal accumulation of lipid. They are
1. Increased free fatty level in plasma :Mobilization of fat causes increased plasma free
fatty acid level. These excess fatty acids are taken up by liver and converted to
triglycerides. However proteins required for the formation of lipoprotein VLDL occurs
at normal rate. This leads to accumulation of lipid in liver. Raised plasma free fatty acid
level occurs in i. Diabetes ii. High fat diet. iii. Starvation. iv. Malnutrition. Hence fatty
liver occurs in all these conditions
2. Due to block in lipoprotein production :If lipoprotein production particulerly VLDL is
blocked due to lack of substances required for its formation fatty liver occurs even at
normal rate of triglyceride synthesis. Because for triglyceride movement from liver to
peripheral tissues VLDL is needed. However supply of deficient substance prevents fat
accumulation.
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BIOCHEMISTRY - Questions and Answers
12. Define lipotrophic factors. Give examples.
A. Lipotrophicfactors : Are those substances or compounds that prevent accumulation of fat
in liver. They are choline, methionine, betaine, vit. E, pyridoxine, poly unsaturated fatty
acids (PUFA) and pantothenicacid. They cure fatty livers.
13. Describe cholesterol biosynthesis.
A. Biosynthesis of cholesterol : Cholesterol biosynthesis occurs in cytosol and microsomes of
most of the cells of the body. Some of the organs of cholesterol biosynthesis are liver,
adrenal cortex, testis, ovaries, brain, placenta, skin and blood vessels.
Precursors :Acetyl-CoA of pyruvate, aminoacids and fatty acids and NADPH of HMP shunt
are precursors for cholesterol formation.
Reactions :
1. Condensation of two acetyl –CoA molecules catalyzed by ß- keto thiolase is the first
reaction of cholesterol biosynthesis.
2. Aceto Acetyl-CoA formed in the initial reaction condenses with another molecule of
acetyl-CoA catalyzed by HMG-CoA synthase. In this reaction HMG- CoA serve as
source of isoprenoid units of cholesterol biosynthesis.
ß-keto
HMG-CoA
Thiolase
synthase
2Acetyl –CoA
Acetoacetyl – CoA
HMG-CoA + CoA
(1)
(2)
CoA
Acetyl-CoA
3. Reduction of HMG-CoA by an NADPH dependent HMG-CoA reductase is the third
reaction. Mevalonate is product.
4. An ATP dependent phosphorylation of mevalonate by mevalonate phosphotransferase
occurs in this reaction. Mevalonate-5- phosphate is product.
HMG-CoA
HMG-CoA
Mevalonate
Reductase
Mevalonate phosphotransferase
(3)
2NADPH+2H
+
Mevalonate-52NADP
+
ATP
phosphate+ADP
5. Another phosphorylation catalyzed by a kinase is fifth reaction.
Mevalonate-5-pyrophosphate is product.
kinase
Mevalonate-5- phosphate + ATP
Mevalonate –5-pyrophosphate+ ADP
(5)
6. An ATP dependent decarboxylation catalyzed by decarboxylase converts mevalonate5- pyrophosphate to isopentenyl pyrophosphate (IPP).
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CHAPTER - 9 | Lipid Metabolism
Decarboxylase
Mevalonate-5- pyrophosphate +ATP
Isopentenyl pyrophosphate +ADP +
(6)
Pi + Co2
7. Isomerization of isopentenyl pyrophosphate to dimethyl allyl pyrophosphate (DMAP)
by an isomerase occurs in this reaction.
Isomerase
Isopentenyl pyrophosphate
Dimethyl allyl pyrophosphate.
(7)
8. The remaining reactions of cholesterol biosynthesis is carried out by two isoprenoid
isomers. A condensation reaction between two isomers catalyzed by prenyl transferase
generates geranyl pyrophosphate.
Prenyl Transferase
Dimethyl allyl pyrophosphate + isopentenyl pyrophosphate
(8)
Geranyl pyrophosphate + PPi.
9. Gerenyl pyrophosphate condenses with one molecule of isopentenyl pyrophosphate
catalyzed by farnesyl pyrophosphate synthase. Farnesyl pyrophosphate is product.
Farnesyl pyrophosphate
synthase
Gerenyl pyrophosphate + isopentenyl pyrophosphate
Farnesyl
(9)
pyrophosphate + PPi.
10. Two molecules of farnesyl pyrophosphate undergo condensation in this reaction.
Squalene synthase catalyzes this reaction. Squalene is product.
11. NADPH dependent squalence monoxygenase catalyzes oxidation of squalene to
squalene -2-, 3-epoxide.
Squalene
Synthase
Farnesyl pyrophosphate
Squalene
mono Oxygenase
Squalene
Squalene-2, 3(10)
(11)
PPi
NADPH+H+O2
epoxide+NADP++H2O.
12. Formation of lanosterol by cyclization of squalene-2, 3 –epoxide occurs in this
reaction. It is catalyzed by squalene oxidocyclase.
Squalene oxido
Squalene-2, 3- epoxide
cyclase
Lanosterol.
(12)
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BIOCHEMISTRY - Questions and Answers
13. Removal of three methyl groups and double bond shifting leads desmosterol
formation from lanosterol.
14. Finally cholesterol is formed from desmosterol
Lanosterol
Desmosterol
(13)
cholesterol.
(14)
15. In the skin cholesterol is formed from lanosterol via 7-dehydro cholesterol.
Lanosterol
7- dehydrocholesterol
(15)
cholesterol.
(15)
14. Write about cholesterol transport in the body.
A. It consist of a. Transport of dietary and hepatic cholesterol b. Extra hepatic tissue
cholesterol transport.
a. Transport of dietary and hepatic cholesterol :Dietary cholesterol is transported to liver
after incorporation into chylomicrons. Cholesterol formed in the intestine is
transported to liver in the same way. In the liver cholesterol is released from
chylomicrons. In the liver dietary cholesterol and cholesterol synthesized are
incorporated into VLDL and LDL and they are secreted into plasma. However LDL
contains highest proportion of cholesterol. Through receptor mediated endocytosis
LDL are taken up by extra hepatic tissues where cholesterol is released. The released
cholesterol may be stored or used for cell membrane. LDL cholesterol is known as bad
cholesterol because its accumulation leads to atherosclerosis.
b. Extra hepatic tissue cholesterol transporter or reverse cholesterol transport : Extra
hepatic tissue free cholesterol is esterified to fatty acid of HDL lecithin by cholesterol
lecithin acyl transferase. As a result lecithin is converted to lysolecithin.
Cholesterol+ fatty acid of HDL lecithin.
Cholesterol ester +lysolecithin.
The cholesterol ester formed migrates into core of HDL and transported to liver. In the
liver cholesterol is eliminated as bile acids. This cholesterol transport is known as
reverse cholesterol transport. HDL cholesterol is known as good cholesterol because
transport of peripheral tissue cholesterol by HDL to liver lowers plasma cholesterol
level.
15. Write normal plasma cholesterol level. In what conditions it is
elevated? How plasma cholesterol level is lowered?
A. Normal plasma cholesterol level is about 150-200mg %. Plasma cholesterol level is
elevated in atherosclerosis, coronary artery disease, diabetes, nephrotic syndrome,
hypothyroidism, obstructive jaundice and xanthomatosis. Some drugs are used to lower
cholesterol level in blood. They are known as cholesterol lowering drugs or
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CHAPTER - 9 | Lipid Metabolism
hypocholesterolemic drugs. Lovastatin is competitive inhibitor of HMG-CoA reductase.
Compactin or mevastatin is another inhibitor used to lower cholesterol level in plasma.
Nicotinicacid, D-thyroxine, questran, clofibrate etc. are also used to lower plasma
cholesterol level.
Other model questions are
16. Name the enzymes of lipid digestion.
17. Explain the role of bile salts in lipid digestion and absorption.
18. How products of lipid digestion are absorbed and transported?
19. Define beta oxidation. Write the reactions of this process.
20. Explain the role of carnitine in fatty acid oxidation.
21. Write the energetics of palmitic acid oxidation.
22. How HMG-CoA is formed? Write its fate.
23. What is normal ketone body level in plasma? In what diseases it is
elevated?
24. Write a note on ketosis and ketoacidosis.
25. Explain how ketone bodies are formed?
26. Write about utilization of of ketone bodies.
27. Write about the structural features of fatty acid synthase complex.
28. Write the role of citrate in fatty acid synthesis.
29. Write about fatty livers.
30. Briefly write on lipotrophic factors.
31. Explain roles of HDL and LDL in cholesterol transport.
32. Write the reaction catalyzed and importance of LCAT.
33. How triglycerides are formed in adipose tissue?
34. Write the role of NADPH in lipid metabolism.
35. Write about utilization of acetyl –CoA in lipid s formation.
36. Blood cholesterol level lowering drugs.
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CHAPTER - 10 | Protein and Aminoacid Metabolism
10
Chapter
Protein and Aminoacid Metabolism
1. Name protein sources of food. Explain digestion of dietary proteins,
A. Food Proteins : Animal foods like meat, eggs, milk, fish and plant foods like cereals,
legumes, nuts, pulses, vegetables and fruits are protein sources in diet.
Digestion and absorption of proteins
Protein digestion : Hydrolysis of dietary proteins into amino acids is known as protein
digestion.
In the mouth : No protein digestion takes place in mouth due to lack of protein digesting
enzymes.
In the stomach : In acid environment of stomach dietary protein undergo de naturation.
This acid induced protein denaturation aids protein digestion. Pepsin is protein splitting
enzyme present in gastric juice. It is active in the acid environment of stomach i. e pH 1. 52. 5. It hydrolyzes peptide bonds of proteins and specific for peptide bonds in which amino
group is contributed by acidic or aromatic amino acids. Pepsin converts proteins to
peptones and proteoses.
Pepsin
Protein
Proteoses + peptones.
pH 1. 5-2. 5
In the infant stomach rennin is present. It causes coagulation of milk.
In the small intestine : Small intestine is the major site of protein digestion. Succus
entericus of small intestine and pancreatic juice contains several proteases and peptidases.
These enzymes converts peptones and proteoses to amino acids. Proteases present in
pancreatic juice are trypsin, chymotrypsin, elastase, collagenase and carboxy peptidase.
Except carboxy peptidase all other proteases are endopeptidase. They act on peptones and
proteoses and convert them to oligopeptides.
Trypsin
Proteoses
chymotrypsin
Oligopeptides, peptones
Oligopeptides.
Carboxy peptidase is an exopeptidase. It hydrolyzes peptide bonds of proteins from
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CHAPTER - 10 | Protein and Aminoacid Metabolism
carboxy terminus and release one amino acid and polypeptide shorter by one amino acid.
The action of carboxy peptidase continues until a di peptide is formed.
Carboxy
Protein
Amino acid + protein shorter by one amino acid
peptidase
carboxypeptidase
Protein shorter by one amino acid
Di peptide
+Amino acids
Aminopeptidase is an exopeptidase and hydrolyzes peptide bond of oligo peptide from
amino terminus and releases an amino acid and oligopeptide shorter by one amino acid.
The action of amino peptidase continues on oligo peptide until it is converted to dipeptide.
Aminopep
Oligopeptide
Oligopeptide shorter by amino acid + aminoacid
tidase
Amino peptidase
Oligo peptide shorter by one amino acid
Amino acids + dipeptide.
Di peptidase hydrolyzes dipeptide into amino acids.
Dipeptidase
Dipeptides
Amino acids.
2. How products of protein digestion are absorbed? Mention about
disorders associated.
A. Protein digestion products absorption : Amino acids produced from dietary proteins in the
lumen are absorbed into portal blood. Mediated transport or secondary active transport is
major mechanism of amino acid absorption. Various classes of amino acids are absorbed by
different carriers present in enterocyte membrane. There are five different carriers for five
different classes of amino acids. For neutral amino acids one carrier, methionine and
phenyl alanine another carrier, acidic amino acids third carrier, basic amino acids fourth
carrier and fifth carrier for iminoacids. All these carriers are symporters like glucose
transporter. They allow sodium transport along with aminoacids.
In some diseases aminoacid absorption and protein digestion are affected. They are
a. Celiac disease : It is due to absorption of oligo peptides of wheat protein gluten. These
peptides are produced from gluten by action of protein digesting enzymes. Further they
act as antigens and produce immune response in children. Symptoms are inflammation
and atrophy of intestinal mucosa. This results in impaired absorption in the small
intestine.
b. Non tropical sprue :It is due to absorption of gluten of oat oligo peptides and symptoms
are similar to those of celiac disease. Gluten free diet consumption relieves symptoms.
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BIOCHEMISTRY - Questions and Answers
c. Hartnup disease : In this condition aromatic amino acid carrier in the intestine is
defective. So their absorption is blocked.
d. Pancreatitis : In this condition protein digestion is affected due to block in flow of
pancreatic juice which contains enzymes of protein digestion.
3. Explain with examples various types of aminoacid deamination.
A. Amino acid deamination is removal of amino group of amino acids. It is the first step of
amino acid degradation. It occurs by several ways. They are
a. Transamination followed by oxidative deamination.
b. Oxidative deamination
c. Non oxidative deamination
A. Transamination followed by oxidative deamination :It involves initial transfer of
amino group of aminoacid to α-ketoglutarate followed by oxidative deamination of
glutamate that is formed by trans amination. Transaminases catalyzes transfer of
amino group of amino acids to α-ketoglutarate. They are present in most of the tissues.
They require pyridoxal phosphate as coenzyme. Among many transaminases alanine
transaminase and aspartate transaminase are most important. Both of them transfer
amino groups of alanine and aspartate to α-ketoglutarate.
Alanine
Transaminase
Alanine+α-ketoglutarate
Pyruvate + glutamate
P. Po4
Aspartate
Aspartate +α-ketoglutarate
Oxaloacetate + glutamate.
Transaminase
The amino group that is collected is removed from glutamate as ammonia by oxidative
deamination catalyzed by glutamate dehydrogenase.
Glutamate
Dehydrogenase
Glutamate +H2O + NADP+
α-ketoglutarate + Ammonia + NADPH +H+.
B. Oxidative deamination :This type of deamination of amino acids is catalyzed by amino
acid oxidases. They are of two types.
a. D-amino acid oxidase b. L- amino acid oxidase. L- amino acid oxidase catalyzes
oxidative deamination of all amino acids except glycine and it is FMN dependent
enzyme. D- amino acid oxidase acts on glycine and it is an FAD dependent enzyme.
These enzymes first oxidizes amino acid to an imino acid which is followed by
hydrolytic loss of ammonia. Further they produce hydrogen peroxide.
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CHAPTER - 10 | Protein and Aminoacid Metabolism
L-Amino acid
oxidase
L- Amino acid
Imino acid
FMN FMNH2
α-keto acid + Ammonia
H2O
D-Amino acid
oxidase
D- Amino acid
Imino acid
α-keto acid + Ammonia.
FAD FADH2
H2O
FMNH2 + O2
FMN + H 2 O2
FADH2 + O2
FAD +H2 O2.
Non oxidative deamination : Specific enzymes catalyzes non oxidative deamination
of aminoacids. Serine dehydratase catalyzes non oxidative deamination of serine to
pyruvate. Cysteine desulfhydrase catalyzes conversion of cyteine to pyruvate.
Threonine dehydratase catalyzes conversion of threonine to α- ketobutyrate.
Serine dehy
Serine
pyruvate + Ammonia
dratase
Cysteine
Cysteine
Pyruvate+ Ammonia+ H2S
desulfhydrase
Threonine
Threonine
α- ketobutyrate +Ammonia.
dehydratase
4. Write normal plasma ammonia level. How ammonia is transported
from brain and skeletal muscle to liver ?
A. Normal plasma ammonia level is 10-20 µg %. Ammonia produced in peripheral tissue and
brain is transported to liver in the form of amino acids alanine and glutamine. More over
ammonia in free form is toxic to central nervous system. In the skeletal muscle ammonia is
used for the formation of glutamate from α-ketoglutarate by the reversal of glutamate
dehydrogenase. Trans amination transfers aminogroup to pyruvate to form alanine.
Glutamate
α- ketoglutarate + ammonia +NADPH+ H
+
Glutamate +H2O + NADP+
dehydrogenase
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BIOCHEMISTRY - Questions and Answers
Alanine
Glutamate+ pyruvate
α-keto glutarate +Alanine
Transaminase
From the brain and other peripheral tissues ammonia produced is transported to liver in
the form of glutamine. Glutamine synthetase catalyzes this reaction.
Glutamine
Glutamate + ammonia +ATP
Glutamine +ADP+Pi.
Synthetase
5. How ammonia is removed from alanine and glutamine in liver?
A. In the liver, kidney and intestine ammonia is removed from glutamine by glutaminase.
Glutaminase
Glutamine
Glutamate+Ammonia
In the liver ammonia is removed from alanine by transamination followed by glutamate
dehydrogenase action.
Alanine + α-ketoglutarate
Pyruvate +glutamate
α-ketoglutarate +NH3.
6. Describe Urea cycle. Add a note on disorders of urea cycle.
A. Urea cycle is present in liver. Enzymes of this cycle are located in mitochondria and cytosol
of hepatocytes. It converts toxic ammonia to non toxic urea. First two reactions occurs in
mitochondria and remaining three reactions takes place in cytosol. All the intermediates of
urea cycle are amino acids with out codons. Formation of urea from ammonia requires
energy in the form of ATP. For the formation of urea molecule only one ammonia molecule
is used as such another ammonia molecule is contributed by amino group of aspartate.
Carbon dioxide or bicarbonate serve as source of carbon for urea formation. Since reactions
of urea cycle are proposed by Krebs and Henseleit it is known as Krebs- Heneseleit cycle.
Reactions :
1. Condensation of ammonia and bicarbonate at the expense of two high energy bonds to
form carbamoyl phosphate is the first reaction of urea cycle. Mitochondrial carbamoyl
phosphate synthetase–І (CAPS-І) catalyzes this reaction. N-acetyl glutamate and
magnesium are cofactors required.
Carbamoyl phosphate
Synthetase-І
(1)
Ammonia +Bicarbonate+2ATP
N-acetylglutamate
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Carbamoyl phosphate + Pi+2ADP.
CHAPTER - 10 | Protein and Aminoacid Metabolism
2. Carbamoyl phosphate condenses with ornithine in the second reaction. The reaction
is catalyzed by ornithine trans carbamoylase and citrulline is the product.
Ornithine trans
Carbamoylase
Carbamoyl phosphate +ornithine
Citrulline +Pi.
(2)
Since cytosol is the site for remaining reactions of urea cycle citrulline comes out of the
mitochondria through a transporter present in mitochondrial membrane.
3. In the cytosol condensation of citrulline and aspartate yields arginino succinate. It is
catalyzed by arginino succinate synthetase and two high energy bonds are used. ATP is
hydrolyzed to AMP and PPi.
Arginino Succinate
Synthetase
Citrulline +aspartate + ATP
Arginino Succinate +AMP+ PPi.
(3)
PPi is further hydrolyzed by pyrophosphatase.
Pyrophosphatase
PPi
2Pi.
(3)
4. Cleavage of arginino succinate by arginino succinase occurs in this reaction.
Arginino Succinase
Arginino Succinate
Arginine +Fumarate.
(4)
5. Finally ornithine is regenerated from arginine by arginase releasing urea.
Arginase
Arginine
Urea +ornithine.
(5)
Ornithine enters mitochondria for continuation of urea cycle through a transporter
present in mitochondrial membrane.
Disorders of urea cycle
Diseases due to deficiency of enzyme of urea cycle are known as urea cycle disorders. They
are inherited diseases. Ammonia toxicity occurs in this diseases because conversion of
ammonia to urea is blocked. Some clinical symptoms commonly seen in these cases are
vomiting, irritability, lethargy, mental retardation, seizures, coma and death. Some of
them are given below.
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1. Hyper ammonemia Type –І: Carbamoyl phosphate synthetase is deficient in this
condition. So ammonia accumulates because its conversion to carbamoyl phosphate is
blocked. Mental retardation is major symptom of this disorder.
2. Hyper ammonemia Type –ІІ : Ornithine trans carbamoylase is deficient in this
condition. Among urea cycle disorders it is most common. So carbamoyl phosphate
accumulates and diverted to pyrimidine nucleotide formation. As a result in urine
intermediates of pyrimidine nucleotide formation like orotic acid and uracil are
excreted.
3. Citrullinemia :Arginino succinate synthetase is absent in this condition. So citrulline
accumulates in blood due to block in its utilization and leads to citrullinemia. Excess
citrulline is excreted in urine.
4. Arginino Succinic aciduria :This condition is due to absence of arginino succinase.
Hence arginino succinic acid accumulates in blood and get excreted in urine.
5. Hyper argininemia :It is due to deficient arginase. So arginine conversion to urea and
ornithine is blocked and accumulation in blood leads to excretion in urine.
6. HHH Syndrome: It is due to defective ornithine transporter. So ornithine accumulates
and carbamoylation of lysine occurs leading to formation of homocitrulline. Hyper
ornithinemia, hyper ammonemia and homocitrullineia are seen in affected persons and
hence name HHH syndrome.
7. Define carbon skeletons of aminoacids. Classify aminoacids giving
examples based on fate of their carbon skeletons.
A. After removal of amino group of amino acid the remaining structure of a amino acid is the
carbon skeleton. Twenty different amino acids gives rise to twenty different carbon
skeletons. Based on fate of carbon skeleton amino acids are classified into
1. Glucogenic amino acids.
2. Ketogenic aminoacids.
3. Glucogenic and ketogenic amino aids.
1. Glucogenic aminoacids: Are those aminoacids whose carbon skeletons are converted to
either glucose or intermediates of TCA cycle. Final products of these aminoacids are
pyruvate, oxaloacetate, α-ketoglutarate, fumarate and succinate.
Examples :Glycine, alanine, valine, serine, threonine, aspartate, glutamate,
aspargine, glutamine, cysteine, methionine, histidine, arginine and proline.
2. Ketogenic aminoacids :Are those amino acids whose carbon skeletons are converted to
either fat like substance or intermediates of fatty acid oxidation. Final products of
these aminoacids are acetyl –CoA or acetoacetyl –CoA.
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CHAPTER - 10 | Protein and Aminoacid Metabolism
Examples : Leucine is the only ketogenic amino acid. Isoleucine, phenylalanine,
tyrosine, tryptophan and lysine are also ketogenic amino acids.
3. Glucogenic and ketogenic amino acids :Are those amino acids whose carbon skeletons
are converted to glucose or intermediates of TCA cycle or fat like substances. Final
products of these amino acids are pyruvate, oxaloacetate, succinate, fumarate, αketoglutarate, acetyl- CoA and acetoacetyl –CoA.
Examples : Lysine, Phenyl alanine, tyrosine, tryptophain and isoleucine.
8. Describe glycine Metabolism.
A, Glycine metabolism consist of
a. Glycine synthesis b. Glycine degradation.
Glycine synthesis
1. Glycine synthesis from serine :Serine trans hydroxy methylase converts serine to
glycine. Tetrahydrofolate is coenzyme.
Serine trans
Hydroxymethylase
Serine
Glycine + methylene FH4
FH4
2. Transamination of glyoxalate yields glycine. Pyridoxal phosphate is coenzyme.
Transaminase
Glyoxalate + Glutamate
Glycine + α-ketoglutarate.
P. Po4
3. Glycine –choline cycle generates glycine from choline.
4. Glycine is synthesized from threonine by serine trans hydroxyl methylase.
Glycine degradation
1. Glycine conversion to ammonia and carbon dioxide by glycine synthase is major route
of glycine degradation in mammals and birds. Liver mitochondria contains this
enzyme. Tetrahydrofolate, NAD+ and lipoicacid are cofactors. It is also known as
Glycine cleavage system. It is made up of three enzymes and H- protein. Lipoic acid is
attached to H-protein by amide linkage like pyruvate dehydrogenase complex. The
three enzymes are a. Glycine dehydrogenase b. Aminomethyl transferase c. Lipoamide
dehydrogenase.
Glycine Synthase
CO2 + NH4+NADH+ H++ methenyl-FH4.
Glycine + FH4 +NAD
Lipoicacid
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BIOCHEMISTRY - Questions and Answers
2. Glycine is degraded to pyruvate after its conversion to serine by reversal of serine
trans hydroxymethylase. Serine dehydratase catalyzes formation of pyruvate from
serine.
Serine Trans
Glycine
Serine dehydratase
Serine
pyruvate
hydroxymethylase
3. Oxidative deamination of glycine by D- aminoacid oxidase is third route of glycine
degradation.
Oxidative
D-amino acid
Glycine
decarboxylation
Glyoxalate
Formate
one carbon pool.
Oxidase
Co2
Biologically important Compounds formed form glycine
Glycine is required for the formation of many important compounds. They are 1. Heme
formation 2. Purine ring formation 3. Creatine formation. 4. Glutathione formation 5.
Hippuric acid formation. 6. Bile acid formation. 7. Collagen synthesis 8. Serine formation.
9. Glucose formation. 10. One carbon pool.
Diseases of glycine metabolism
Some inherited diseases are due to defective glycine metabolism. Deficiencies of enzymes
of Glycine metabolism affects glycine metabolism. Defective enzymes are produced by
defective genes. Some inherited diseases of glycine metabolism are
1. Glycinuria :In this rare genetic disorder glycine is excreted in urine even though
plasma glycine level is normal. Re absorption of glycine in renal tubules is defective due
to defective trans porter. Hence glycine excreted in urine.
2. Primary hyper oxaluria :In this condition excess amount of oxalate about 15-60mg/ day
is excreted in urine even though dietary oxalate is as usual. Glyoxalate utilization by
transaminase as well as its conversion to formate is blocked. Hence glyoxalate
accumulates and get oxidized to oxalate which is excreted in urine. Calcium present in
urine combines with oxalate to form calcium oxalate crystal. These crystals deposit in
the kidney and urinary tract. Therefore symptoms are bilateral urolithiasis due to
stones in both ureters, nephro calcinosis due to stones in kidney and recurrent urinary
tract infections. Affected individual die at child hood or early adult life due to renal
failure or hypertension.
3. Non ketotic hyper glycinemia :This condition is characterized by excess glycine in blood
and urine. Glycine synthase is defective in this condition. Symptoms are severe mental
retardation and death of affected occurs in infancy.
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9. Write about biological roles of glutamine and glutamate
A. a. Glutamate biological roles :
1. In amino acid metabolism glutamate has key role. It act as amino group source for
formation of nonessential amino acids. In amino acid catabolism it act as collecting
point of amino groups.
2. Glutamate is required for synthesis of glutathione, glutamine, N- acetyl glutamate
and glucose.
3. Gamma amino butyric acid (GABA):Glutamate gives rise to this compound on de
carboxylation catalyzed by pyridoxal phosphate dependent glutamate
decarboxylase. It is an inhibitory neurotransmitter present in synaptic vesicles.
4. Gamma carboxylation :Glutamate of many proteins are carboxylated at gamma
carbon. This carboxylation of glutamate at gamma carbon has role in blood clotting
and bone formation.
5. Folic acid contains several glutamate residues.
6. Glutamate is neuro trans mitter.
b. Glutamine biological roles
1. Several compounds amino group is derived from amide group of glutamine. They
are purine nucleotides, pyrimidine nucleotides, amino sugars and co enzyme NAD.
2. Glutamate is synthesized from glutamine.
3. Detoxification reactions use glutamine particularly conjugation reactions.
4. Histidine and tryptophan synthesis needs glutamine.
5. In blood glutamine is present in high concentration about 10mg.
6. Glutamine has role in acid base balance. In kidney glutamine contributes ammonia.
10.
Trace reactions of histidine catabolism.
A. Reactions of histidine break down :
1. Non oxidative de amination of histidine by histidase or histidine ammonia lyase occurs
in the first reaction of its breakdown. Urocanic acid is the product of this reaction.
2. A water molecule addition by hydratase or urocanase is the second reaction and 4imidazolone-5-propionate is the product of this reaction.
Histidase
Histidine
Urocanase
Urocanic acid
4-Imidazolone-5- propionate.
(1)
(2)
NH4
H2O
3. In this reaction imidazolone propionate is cleaved to formimino glutamate (FIGLU)
by an hydrolase.
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BIOCHEMISTRY - Questions and Answers
4. Transfer of formimino group to one carbon carrier tetrahydrofolate (FH4)by
formimino transferase occurs in this reaction. Glutamate is product.
4-imidazolone-5-propionate
Hydrolase
(3)
Formimino glutamate Transferase
(FIGLU)
Glutamate.
(4)
Formimino FH4
5. Finally α- keto glutarate is generated from glutamate by transaminase
Transaminase
Glutamate + pyruvate
α-keto glutarate +alanine.
(5)
11. Name biologically important compounds formed from histidine
A. 1. Decarboxylation of histidine yields histamine.
2. Histidine is required for the synthesis of ergothionine, carnosine, anserine etc.
3. Glucose is formed from α-keto glutarate of histidine.
12. Write about FIGLU excretion test.
A. FIGLU excretion Test :
It is a folic acid deficiency test. Since conversion of FIGLU to glutamate is dependent on
folicacid, in folic acid deficiency this reaction is blocked. This leads to accumulation and
excreation of FIGLU in urine. The test involves administration of test dose of histidine to
patient under investigation. Excretion of more of FIGLU in urine by patient indicates folic
acid deficiency.
13. Describe catabolism of cysteine. Add a note on its sulfur fate.
A. Cysteine degradation is brought about by two pathways.
I. Dioxygenase pathway. II. Transaminase pathway.
I. Reactions of dioxygenase pathway : In mammals it is the major route of cysteine
degradation.
1. Incorporation of two atoms of oxygen in presence of NAD (P) H and iron by
dioxygenase yields cysteine sulphinate.
2. Cysteine sulphinate has two metabolic fates. i. Direct desulphination of cysteine
sulfinate yields sulfite and pyruvate. ii. Transamination followed by desulfination
catalyzed by trans aminase and desulfinase respectively produce pyruvate.
Dioxygenase
Cysteine +o2 + NAD (P)H
cysteine sulfinate +NAD(P)+
(1)
Desulfinase
Cysteinyl sulfinate
Pyruvate + sulfite.
2i
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CHAPTER - 10 | Protein and Aminoacid Metabolism
Transaminase
Cysteine sulfinate + α-ketoglutarate
Sulfinyl pyruvate +glutamate.
(2 ii)
Desulfinase
Sulfinyl pyruvate
pyruvate + sulfite.
(2 ii)
II. Reactions of transaminase pathway :
1. A transaminase present in liver and kidney of mammals transfers amino group
of cysteine. This results in formation of mercapto pyruvate.
Transaminase
Cysteine +α- keto acid
α-Amino acid + mercapto pyruvate
(1)
2. The mercapto pyruvate has at least two metabolic fates.
i. A dehydrogenase converts mercapto pyruvate to mercapto lactate which is
excreted in urine
Dehydrogenase
+
Mercapto lactate + NAD +.
Mercapto pyruvate + NADH +H
(2 i)
ii. In another route sulfur of mercapto pyruvate is released as hydrogen sulfide.
(2 ii)
Mercapto pyruvate
Pyruvate + Hydrogen sulfide.
Fate of sulfur of cysteine : In liver and kidney sulfide is converted to sulfite. Sulfite
oxidase present in liver mitochondria oxidizes sulfite to sulfate. This enzyme is coupled
to cytochrome c of electron transport chain through cytochrome b5. The sulfate is
excreted in urine
Sulfite oxidase
Sulphide
Sulfite
Sulphate
(1)
Urine.
(2)
O2
Some part of sulphate is converted to active sulphate PAPS. PAPS is donor of sulphate
for formation of sulfo lipids and glycosamino glycans. PAPS is also donor of sulphate
involving conjugation reactions of steroids, drugs etc. After conjugation with sulfate
they are excreted in urine and contributes to organic or ethereal sulfate of urine.
Sulfate +ATP
Urine
Steroids
PAPS
Sulfolipids
PAPS
Drugs
PAPS
Glycosamino glycans
Urine.
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14. Write about biologically important compounds formed from cysteine.
A. 1. Cysteine is required for synthesis of several biologically important compounds. They
are
a. Glutathione
b. Coenzyme A
c. Taurine
d. Fatty acid synthase complex
e. Cystine
f. Glucose.
2. Cysteine contributes to urinary inorganic and organic sulfate.
3. Cysteine is involved in detoxification reactions.
15. Write reactions of methionine catabolism
A. Methionne is degraded to cysteine and propionyl –CoA.
Reactions of methionine breakdown :
1. In the first reaction of methionine degradation active methionine or S- adenosyl
methionine (SAM) is formed. The reaction is catalyzed by S-adenosyl methionine
synthase ATP serve as donor of adenosine moiety and energy source. SAM is a high
energy compound and serve as donor of methyl groups. PPi is hydrolyzed by
pyrophosphatase.
SAM synthase
Methionine +ATP
S- adenosyl methionine (SAM)+PPi, PPi
2Pi.
(1)
2. Methyl transferase transfers methyl groups of SAM to an acceptor to form
S-adenosyl homo cysteine (SAH).
Methyl Transferase
S-adenosyl methionine +Acceptor
S-adenosyl homo cysteine +
(2)
(SAH)
Methylated acceptor.
3. In this reaction an hydrolase converts S- adenosyl homo cyteine to homo cysteine
and adenosine.
Hydrolase
S-adenosyl homocysteine (SAH) +H2 O
Adenosine+ homo cysteine.
(3)
4. Condensation of homocysteine with serine catalyzed by pyridoxal phosphate
dependent cystathionine synthase is the fourth reaction. Cystathionine is product.
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CHAPTER - 10 | Protein and Aminoacid Metabolism
Cystathionine
Synthase
Homocysteine+ Serine
Cystathionine.
P. Po4 (4)
5. Liberation of cysteine occurs in this reaction. Another pyridoxal phosphate dependent
cystathioninase splits cystathionine to cysteine and homoserine.
Cystathioninase
Cystathionine
Cysteine +Homoserine.
P. Po4 (5)
6. Removal of amino group homoserine by deamination forms α-ketobutyrate in this
reaction.
Deaminase
Homo serine
α – ketobutyrate + ammonia.
(6)
7. α-ketobutyrate dehydrogenase multi enzyme complex brings about oxidative
decarboxylation of α- ketobutyrate. Propionyl-CoA is product. This reaction is similar
to oxidative decarboxylation of pyruvate and α-ketoglutarate.
α-ketobutyrate
Dehydrogenase
Α-ketobutyrate + CoA
Propionyl- CoA+CO2.
(7)
8. Propionyl –CoA is converted to succinyl –CoA which enters TCA cycle.
(8)
Propionyl - CoA
16.
Succinyl-CoA
TCA cycle.
Write about important compounds formed from methionine :
A. 1. Methyl groups of active methionine is used in the synthesis of several compounds and
detoxification reactions. Transfer of methyl group of active methionine by methyl
transferase to an acceptor is called as trans methylation. Examples of compounds
synthesized by trans methylation are given below.
1. Guanido acetate
Creatine.
2. Nor epinephrine
Epinephrine.
3. Acetyl serotonin
Melatonin.
4. Ethanolamine
Choline
5. DNA, RNA
Methylated DNA, RNA.
2. Methionine gives rise to cysteine.
3. Glucose is synthesized from methionine.
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4. N-formyl methionine serve as initiating amino acid of protein synthesis.
5. Polyamine formation also requires methionine.
17. Write a note on diseases of methionine break down.
A. Diseases of methionine breakdown :These diseases are due to production of defective
enzymes of methionine metabolism. They are inherited diseases and genes are
defective.
1. Homocystinuria and homo cysteinemia :This disease is characterized by high levels of
homocystine an oxidized product of homo cysteine in urine and blood. It is due to
deficiency of cystathionine synthase. So homo cysteine is not converted to
cystathionine and hence it accumulates. Symptoms are thrombosis, mental
retardation and eye lesions.
2. Cystathionineuria :It is characterized by elevated levels of cystathionine in blood and
excretion in urine. It is due to block of cystathioninase catalyzed reaction.
3. Hyper methioninemia : It is due to deficiency of S- adenosyl methionine synthase. So
methionine is not converted to S- adenosyl methionine and accumulates in blood.
18. Decribe catabolism of Phenyl alanine and Tyrosine.
A. Phenyl alanine and tyrosine are degraded to fumarate and acetoacetate. Catabolism of
phenyl alanine involves its conversion to tyrosine. Hence tyrosine break down pathway is
same as that of phenyl alanine. In other words single pathway is responsible for the break
down of both phenyl alanine and tyrosine.
Reactions :
1. First reaction of phenyl alanine breakdown is its conversion to tyrosine by phenyl
alanine hydroxylase. This reaction uses hydrogen from tetrahydro biopterin (THB)
instead of known hydrogen donors. Transfer of hydrogen leads to formation of
dihydrobiopterin (DHB). An NADPH dependent reduction converts DHB to THB.
Phenyl alanine Hydroxylase
Phenyl alanine +o2 +THB
Tyrosine +DHB+H2O.
(1)
DHB+NADPH+H+
THB+NADP+
(1)
2. Tyrosine undergoes trans amination in this reaction. Para hydroxy phenyl pyruvate
is product.
Transaminase
Tyrosine+ α-ketoglutarate
phydroxy phenyl pyruvate +glutamate.
(2)
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CHAPTER - 10 | Protein and Aminoacid Metabolism
3. A vit. C dependent dioxygenase known as para hydroxy phenyl pyruvate hydroxylase
converts para hydroxy phenyl pyruvate to homogentisic acid which involves
decarboxylation and hydroxylation.
Para hydroxy
Phenyl pyruvate
Hydroxylase
Para hydroxy phenyl pyruvate + O2
Homogentisic acid. +C02
(3)
4. Another dioxygenase called as homogentisic acid oxidase cleaves benzene ring of
homo gentisic acid to form maleyl aceto acetate.
Homogentisic Acid
oxidase
Homogentisic acid + O2
Maleyl acetoacetate.
(4)
5. Isomerization by an isomerase converts maleyl aceto acetate to fumaryl
acetoacetate. Glutathione also required.
Isomerase
Maleyl aceto acetate
Fumaryl aceto acetate
(5)
6. An hydrolase spilts fumaryl aceto acetate to fumarate and aceto acetate.
Hydrolase
Fumaryl aceto acetate +H2 O
Fumarate +aceto acetate.
(6)
19. Write about com pounds synthesized from tyrosine.
A. 1. Tyrosine is required for formation of catacholamines i. e. epinephrine and nor
epinephrine.
2. Thyroid hormones are synthesized from tyrosine.
3. Tyrosine is required for synthesis of melanin.
4. Glucose and fat or ketone bodies synthesis occurs from tyrosine.
5. Protein synthesis requires phenyl alanine and tyrosine.
20. Write a note on various metabolic diseases of phenyl alanine and
tyrosine
A. Several metabolic diseases of phenyl alanine and tyrosine are known. They are due to
defectives enzymes produced by defective genes and hence they are inherited diseases.
1. Phenylketonuria :This condition is characterized by excretion of phenyl ketone i. e.
phenyl pyruvate in urine. It is due to deficiency of phenyl alanine hydroxylase. So
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BIOCHEMISTRY - Questions and Answers
phenyl alanine is not converted to tyrosine and it accumulates in tissues. By other
catabolic routes it is converted to phenyl pyruvate, phenyl lactate and phenyl acetate.
All are excreted in urine. Symptoms are mental retardation and convulsions.
2. Tyrosinemia :In this condition tyrosine accumulates in blood due to lack of
transaminase. It undergoes other routes of catabolism and converted to p-hydroxy
phenyl acetate and N-acetyl tyrosine. They are excreted in urine along with tyrosine.
Symptoms are skin and eye lesions, mental retardation etc.
3. Neonatal tyrosinemia : In this condition tyrosine accumulates in blood and excreted in
urine. It is due to defective p-hydroxy phenyl pyruvate hydroxylase. So para hydroxy
phenyl pyruvate conversion to homo gentisate is blocked. Accumulation and excretion
para hydroxy phenyl pyruvate occurs.
4. Alkaptonuria : It is characterized by excretion of homo gentisic acid in urine. Further
urine tuns dark on standing due to polymerization on of oxidative products of homo
gentisic acid. On exposure to O2 homogentisic acid is oxidized to quinones. Due to
deficiency of enzyme homogentisic acid oxidase homogentisic acid is not converted to
maleyl aceto acetate. So accumulation of homogentisic acid in blood and excretion in
urine occurs. Symtoms are connective tissue pigmentation and arthritis.
5. Tyrosinosis : In this condition plasma tyrosine level is elevated. It may be due to
deficiency of isomerase and hydrolase. Tyrosine undergoes other routes of break down
and products are excreted in urine. Symptoms are cabbage like odor, vomiting and
diarrhoea.
21. Trace reactions of tryptophan catabolism.
A. Tryptophen is degraded to alanine and aceto acetyl –CoA.
Reactions:
1. Tryptophan dioxygenase opens indole ring of tryptophan in the initial reaction of
tryptophan catabolism. N-formyl kynurenine is product.
Tryptophan
Dioxygenase
Tryptophan +O2
N- Formyl kyn urenine
(1)
2. Hydrolysis of N- formyl kynurenine by formylase removes formyl group as formate
and kynurenine is produced.
Formylase
N-Formyl kynurenine
kynurenine +Formate.
H2 O (2)
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CHAPTER - 10 | Protein and Aminoacid Metabolism
3. An NADPH dependent kynurenine monooxygenase catalyzes hydroxylation of
kynurenine. 3- hydroxy kynurenine is product.
Kynurenine
Mono oxygenase
Kynurenine +NADPH+ H+ + O2
3- hydroxy kynurenine + NADP++H2 O
(3)
4. An hydrolase kynureninase hydrolyzes hydroxy kynurenine to alanine and 3- hydroxy
anthranilic acid. Pyridoxal phosphate is required for this reaction.
3- Hydroxy kynurenine +H2O
kynureninase
Alanine +3- hydroxy anthranilic acid.
(4)
5. A dioxygenase opens phenyl ring of hydroxy anthranilate and inserts two oxygen atoms
to yield 2- amino -3- carboxymuconic acid semialdehyde.
Dioxygenase
3-hydroxy anthrarilic acid +o2
2-Amino-3-carboxy muconic acid semi
aldehyde.
(5)
6. Decarboxylation of product of above reaction by decarboxylase yield 2-Aminomuconic
acid semialdehyde. Co2 is released.
Decarboxylase
2-Amino-3-carboxy muconic acid semialdehyde
2- Amino muconic acid semi
(6)
aldehyde +Co2.
7. Semi aldehyde is converted to 2- amino muconic acid by NAD+ dependent
dehydrogenase.
dehydrogenase
2- amino muconic acid semi aldehyde +NAD+
2- Amino muconic acid
(7)
+NADH+H+
8. NADPH dependent reduction of 2- Amino muconic acid by a reductase yield α-keto
adipic acid.
Reductase
2- Amino muconic acid +2NADPH+ 2H
α-ketoadipic acid +2 NADP+.
(8)
+
9. Oxidative decarboxylation of α-keto adipate by α- keto acid dehydrogenase generates
glutaryl-CoA. The enzyme is similar to pyruvate and α-keto glutarate dehydrogenases.
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α-ketoacid
Dehydrogenase
α-ketoadipate + CoA + NAD+
Glutaryl- CoA+ NADH+H++ Co2.
(9)
10.
An FAD dependent dehydrogenation of glutaryl –CoA by dehydrogenase yields
gluta conyl –CoA.
Dehydrogenase
Glutaryl- CoA +FAD
Glutaconyl –CoA + FADH2.
(10)
11.
Glutaconyl-CoA undergoes decarboxylation to crotonyl- CoA.
12.
An hydratase adds water to crotonyl – CoA to yield beta hydroxy butyryl CoA.
This reaction is analogous to ß- oxidation reaction.
Decarboxylase
Glutaconyl –CoA
Hydratase
Crotonyl –CoA
ß-hydroxybutyryl-CoA.
(11)
(12)
Co2
13.
H2O
An NAD dependent dehydrogenase converts beta hydroxy butyryl- CoA to aceto
acetyl –CoA.
Dehydrogenase
Beta hydroxy butyryl- CoA +NAD+
Acetoacetyl –CoA + NAD +H+.
(13)
14.
Finally acetyl-CoA is generated from acetoacetyl –CoA by thiolase. TCA cycle
oxidizes acetyl-CoA generated.
Thiolase
Acetoacetyl –CoA
2Acetyl-CoA
CoA
TCA cycle.
(14)
22. Write organs and reactions involved in creatine formation.
A. Synthesis of Creatine :It is mainly synthesized in liver, kidney and pancreas. Three amino
acids are involved in creatine synthesis. They are arginine, glycine and methionine.
Reactions :
1. Transfer of guanidine group of arginine to glycine by transamidinase is the first
reaction. It occurs in kidney. Guanido acetate is product.
2. Trans methylation involving S- adenosyl methionine (SAM)as methyl donor yields
creatine. In the liver this reaction takes place.
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CHAPTER - 10 | Protein and Aminoacid Metabolism
Transmethylase (SAH)
Transamidinase
Arginine
ornithine + Guanido acetate
Creatine.
(1)
(2)
Glycine
SAM
3. In the skeletal muscle creatine is converted to creatine phosphate by creatine
phosphokinase (CPK).
CPK
Creatine +ATP
Creatine phosphate +ADP.
(3)
23. How creatinine is formed? Write its fate.
A. Creatinine formation :In the skeletal muscle phospho creatine serve as reservoir of energy.
It is nonenzymatically converted to creatinine.
Fate of creatinine
Creatinine diffuses from muscle and excreted in urine as waste product. About 1-1. 5gm
of creatinine excreted per day.
Non enzymatic
Creatine phosphate
Creatinine
Blood
Urine.
Pi
24. Name compounds synthesized from Tryptophan.
A. Several biologically important compounds are formed from Tryptophan. They are niacin,
melatonin, serotonin, skatole and indole. Niacin is one of the water soluble vitamin. About
1 mg of niacin is synthesized from 60 mg of Tryptophan. Melatonin is hormone secreted by
pineal gland. Serotonin is neurotransmitter. In the intestine skatole and indole are formed
from Tryptophan by bacterial action. They are responsible for characteristic odor of feces.
25. Write about a)Maple syrup urine disease b) Xanthurenic aciduria
A. a) Maple syrup urine disease : It is an inherited disease of branched chain aminoacid
metabolism. It is due to lack of α-ketoacid dehydrogenase which converts alpha keto
acids of three branched chain aminoacids to corresponding Co As. This leads to
accumulation of Valine, Leucine, Isoleucine and their ketoacids in blood and these
ketoacids are reduced to hydroxy acids which are excreted in urine. So urine gives
burnt sugar smell due to hydroxy acids and hence name of the disease as maple syrup
urine disease. Mental problems and vomiting are other symptoms.
b) Xanthurenicaciduria: It occurs in vit. B6 or pyridoxine deficiency. So kynureninase is
less active and conversion of 3-hydroxy kynurenine to alanine and 3-hydroxy
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BIOCHEMISTRY - Questions and Answers
anthranilic acid is blocked. Through alternative pathways 3-hydroxy kynurenine is
converted to xanthurenic acid and get excreted in urine.
Other model questions are
26. Write about enzymes of protein digesting enzymes.
27. Write difference between oxidative and non oxidative deamination.
28. Write sources and utilization of ammonia.
29. Name enzymatic defects in
a. Argininosuccinicaciduria b. Citrullinemia
30. Define glucogenic aminoacids. Give examples.
31. What are ketogenic aminoacids? Give examples.
32. Write differencec between glucogenic and ketogenic aminoacids.
33. Name biologically important compounds formed from cysteine.
34. Write the biochemical changes in
a. Glycinuria b. Primary hyperoxaluria.
35. How SAM is formed ? Write its importance.
36. What is transmethylation? Give examples.
37. Write enzymatic defects in
a. Phenylketonuria b. Alkaptonuria
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CHAPTER - 11 | Porphyrin and Hemoglobin Metabolism
11
Chapter
Porphyrin and Hemoglobin Metabolism
1. What are porphyrins ? Write their functions and properties.
A. Porphyrins
Porphyrins are derivatives of porphin. Porphin consist of four pyrrole rings in cyclic form.
Hence it is known as tetra pyrrole. The four pyrrole rings are joined by methenyl bridges.
Four pyrrole rings have eight substituent positions. Naturally occurring porphyrins differ
in substituent positions or groups.
Functions
1. Porphyrins are components of heme proteins. Heme is a metallo porphyrin. Iron is the
metal present in heme.
2. Heme proteins are hemoglobin, myoglobin, cytochromes, cytochrome oxidase, cyto
chrome P450 hydroxylases and enzymes of tryptophan and oxygen metabolisms. They
are tryptophan dioxygenase, cyclo oxygenase, catalase, peroxidase etc.
Properties
1. Isomers of porphyrins : Porphyrins exist in isomeric forms due to various side chains
present in different substituent groups and arrangement of side chains. For example
uroporphyrin with two types of side chains acetate (A) and propionate (P) exist in four
isomers. They are type І, type ІІ, type ІІІ and type IV. In uroporphyrin type І all side
chains are arranged symmetrically and in other types A, P are arranged
asymmetrically. In nature most common are type І and type ІІІ. In type ІІІ the side
chains on third pyrrole ring are arranged in different manner.
2. Color :All porphyrins are colored molecules.
3. Porphyrinogens : They are reduced forms of porphyrins. For example
uroporphyrinogen І is
reduced form of uroporphyrin
І. All porphyrinogens are
colorless molecules.
4. Light absorption : All porphyrins absorb light at 400nm as well as in visible region.
Light absorption property is used in identification of porphyrins.
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BIOCHEMISTRY - Questions and Answers
2. How heme is synthesized ? Add a note on regulation of heme
biosynthesis.
A. Site:Except mature erythrocytes most of cells are capable of synthesizing heme. However
liver and bone narrow are considered as major organs involved in heme production. About
20% is generated by liver and 80% by bone narrow.
Reactions of heme synthesis. :Enzymes of heme biosynthesis are present in mitochondria
and cytosol. First and last three reactions occurs in mitochondria and rest of the reactions
takes place in cytosol. Glycine and succinyl –CoA are precursors for heme production.
1. Aminolevulinic acid or ALA synthase initiates heme biosynthesis and condenses
succinyl –CoA and glycine to aminolevulinic acid. It occurs in two steps.
a. First step involves formation of α-amino-ß- keto adipic acid. Pyridoxal phosphate is
required.
b. Decarboxylation of α- amino-ß- keto adipic acid yields δ-aminolevulinic acid in the
second step.
ALA synthase
Succinyl-CoA +glycine
α-Amino ß- keto adipic acid
(1a) PLP
δ(l b)
CoA
Aminolevulinic acid + co2.
2. Formation of first pyrrole porphobilinogen (PBG) is the second reaction. It is catalyzed
by ALA dehydratase. It forms pyrrole ring by eliminating a water molecule from two
molecules of ALA.
ALA Dehydratase
δ-Aminolevulinic acid +δ –Aminolevulinic acid
Porphobilinogen
(2)
(PBG)
H2o
3. Uroporphyrinogen synthase –І or porphobilinogen deaminase (PBG Dase) condenses
four PBG molecules in a step wise manner to form hydroxymethyl bilane.
PBGD ase
4porphobilinogen
Hydroxymethyl bilane +4 NH3
(3)
4. In presence of uroporphyrinogen ІІІ cosynthase hydroxymethyl bilane is converted to
uroporphyrinogen ІІІ.
Uroporphyrinogen
ІІІ cosynthase
Hydroxymethyl bilane
Uroporphyrinogen ІІІ
(4)
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CHAPTER - 11 | Porphyrin and Hemoglobin Metabolism
Alternatively hydroxy methylbilane undergoes spontaneous (non – enzymatic)
cyclization to form uroporphyrinogen І.
Non enzymatic
Hydroxymethylbilane
Uroporphyrinogen І.
5. Decarboxylation of acetate side chains of uroporphyrinogen ІІІ by uroporphyrinogen
ІІІ decarboxylase yields coprophyrinogen ІІІ.
Decarboxylase
Uroporphyrinogen ІІІ
Coproporphyrinogen ІІІ +4co2.
(5)
6. In the mitochondria protoporphyrinogen IX is formed from coproporphyrinogen ІІІ.
The reaction is catalyzed by coproporphyrinogen ІІІ oxidase. In this reaction
decarboxylation and oxidation of propionic acid side chains on І and ІІ pyrroles occurs.
Coproporphyrinogen
ІІІ oxidase
Coproporphyrinogen ІІІ
Protoporphyrinogen IX +2CO2.
(6)
7. Protoporphyrinogen oxidase catalyzes oxidation of protoporphyrinogen IX to
protoporphyrin IX in this reaction in presence of molecular oxygen.
Protoporphyri
nogen oxidase
Protoporphyrinogen IX +O2
Protoporphyrin IX.
(7)
8. Heme formation occurs in this reaction. Heme synthase catalyzes formation of heme
from protoporphyrin IX by inserting Fe2+ (Iron) in to protoporphyrin IX.
Heme synthase
2+
Protoporphyrin IX + Fe
Heme.
(8)
Regulation of heme synthesis :Heme synthesis is subjected to regulation by 1. Feed
back inhibition 2. Repression and de repression. ALA synthase is regulatory enzyme.
In the feed back inhibition heme end product of the pathway inhibits ALA synthase. In
repression heme act as corepressor and combines with aporepressor to from repressor
which inhibits expression of ALA synthase. In de repression, heme is diverted to
detoxification enzymes synthesis like cytochrome P450 system in presence of drugs like
barbiturates. So repression is relieved.
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BIOCHEMISTRY - Questions and Answers
3. Define porphyries. Classify. Give examples for each class. Write
clinical and biochemical abnormalities of each.
A. Porphyrias
They are inherited diseases of heme biosynthesis. They are characterized by accumulation
and excretion of porphyrins or their precursors in plasma and urine respectively.
Classification of porphyrias: Based on cells or organs affected porphyrias are classified
into 1. Erythropoietic porphyrias and. 2. Hepatic porphyrias.
Erythropoietic porphyrias
A. Congenital or hereditary erythropoietic porphyria : This condition is characterized by
accumulation and excretion of uro and coproporphyrinogens of Type І in erythrocytes and
urine respectively. Further urine of the diseased persones turns to red due to conversion of
uro and coproporphyrinogens І to corresponding porphyrins. Uroporphyrinogen ІІІ
cosynthase is defective in this condition. Clinical symptoms are photo sensitivity, pink
bones and teeth, hemolytic anaemia etc.
b. Erythropoietic protoporphyria :It is due to deficiency of heme synthase. So protoporphyrin
IX is excess in erythrocytes and protoporphyrin excretion in feces is increased. Photo
sensitivity is main symptom. Liver problems and anaemia may develop later.
Hepatic porphyrias
a. Acute intermittant porphyria (AIP) : Affected individuals excrete large amounts of PBG
and its precursors in urine. It is due to deficiency of uroporphyrinogen І synthase. As a
result PBG and its precursors accumulates in plasma. Urine of the patients turn to dark
due to polymerization of PBG in urine. Clinical symptoms are abdominal pain,
neuropsychiatric symptoms but no photosensitivity.
b. Variegata porphyria (VP) : Patients of this disease excrete PBG, ALA, Uro and
coproporphyrins in urine. The urine may be colored due to presence of uro and
coproporphyrins. Protoporphyrinogen oxidase is deficient and ALA synthase may be more
active in this disease. Photosensitivity is main symptom. Other symptoms vary.
c. Hereditary coproporphyria (HCP): This condition is characterized by excretion of large
amounts of coproporphyrinogen ІІІ in urine and feces due to deficiency of
coproporphyrinogen ІІІ oxidase. Photo sensitivity is main symptom. Urine may be colored.
d. Porphyria cutanea tarda (PCT) :This is due to block in conversion of uroporphyrinogen ІІІ
to coproporphyrinogen ІІІ by uroporphyrinogen decarboxylase. Hence uroporphyrinogen
ІІІ accumulates in blood and excreted in urine. Due to presence of uroporphyrinogen ІІІ
urine appears pink. Photosensitivity is major symptom.
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CHAPTER - 11 | Porphyrin and Hemoglobin Metabolism
4. Write normal plasma bilirubin level. How bilirubin is formed?
Explain its fate in liver and intestine.
A. Normal plasma bilirubin level is 1mg%.
Formation of bilirubin
It is formed from heme of heme proteins. In liver microsomes a complex enzyme system
heme oxygenase converts heme to biliverdin in presence of NADPH and o2.
Heme oxygenase
Heme+ o2 + NADPH + H+
Biliverdin + NADP + +CO+Fe2+ +H2O
The biliverdin is reduced to bilirubin by NADPH dependent biliverdin reductase
Biliverdin+NADPH+H+
Biliverdin Reductase
Bilirubin+ NADP+
Bilirubin formed in reticulo endothetial cells (REC) is released into circulation. Since free
bilirubin is insoluble in plasma albumin combines and forms complex. Further metabolism
occurs in liver. At sinusoidal surface of hepatocyte bilirubin dissociates from albumin and
enters hepato cyte which is mediated by a transporter present in membrane of hepatocyte.
Thus bilirubin enters cytosol of hepatocyte.
Albumin
Bilirubin
A
Sinusoidal Surface
Bilirubin –Albumin complex
Bilirubin
↓ albumin.
Transporter
Bilirubin
Bilirubin in hepatocyte cytosol
Now in the cytosol bilirubin is bound to ligandin and z or y protein which carries bilirubin
to microsomes where it is detoxified by conjugation with glucuronic acid.
Ligandin
Bilirubin in cytosol
Bilirubin in microsomes.
Z or y protein
First one molecule of glucuronic acid is transferred from UDP –glucuronic acid catalyzed
by UDP –glucuronyl transferase. Bilirubin monoglucuronide is product.
Transferase
Bilirubin +UDP – glucuronic acid
Bilirubin monoglucuronide (BMG)
UDP
BMG is converted to bilirubin diglucuromide (BDG) by adding one more glucuronic acid.
Transferase
BMG + UDP-Glucuronic acid
Intestinal metabolism of bilirubin
Bilirubin diglucuronide (BDG)
UDP
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BIOCHEMISTRY - Questions and Answers
Most of conjugated bilirubin is secreted in bile. In the large intestine glucuronide is
removed by hydrolases and bilirubin is released. Intestinal bacteria converts bilirubin to
urobilinogen which is reabsorbed and re excreted into bile. However a small part enters
circulation and excreted in urine by kidney. Majority of urobilinogen is eliminated in feces
and it is popularly known as stercobilinogen.
Bacterial
Bilirubin glucuronide
Bilirubin + glucuronide.
Hydrolases
Bacterial
Enzymes
Bilirubin
Circulation
Urobilinogen
kidney
Urine.
Feces
Urobilinogen
Stercobilinogen
5. Define hyper bilirubinemias. Classify. Give examples for each class.
A. Hyper bilirubinemias
Are conditions associted with increased plasma bilirubin level. They are of two types A.
Conjugated hyper bilirubinemias B. Un conjugated hyper bilirubinemias.
A. Conjugated hyper bilirubinemias : They are characterized by more of conjugated
bilirubin in blood. Examples are Dubin- Johnson syndrome or chronic idiopathic
jaundice and Rotor syndrome.
B. Un conjugated hyper bilirubinemias : In these diseases unconjugated bilirubin level
in plasma is elevated. Examples are neonatal physiological jaundice, Crigler-Najjar
syndrome and Gilbert's disease.
6. What is jaundice? How it is classified? Write about each class.
A. Jaundice is yellowish discoloration of sclera and skin due to excess bilirubin level.
Classification
Jaundice is classified into a. Prehepatic jaundice. b. Hepatic jaundice. c. Post hepatic
jaundice based on causes.
a. Pre hepatic jaundice :Hemolytic jaundice is the other name given to this condition. It is
mainly due to increased hemolysis and hence name. Excess hemolysis leads to
formation of excess bilirubin. But liver is unable to conjugate excess bilirubin. So
accumulation of unconjugated free bilirubin occurs and plasma free bilirubin level is
elevated. Increased hemolysis is seen in hemoglobinopathies, incompatable blood
transfusion, hereditary spherocytosis and in malaria. In glucose -6- phosphate
dehydrogenase deficiency cases administration of drugs like primaquine, aspirin and
sulfonamide cause excess hemolysis.
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CHAPTER - 11 | Porphyrin and Hemoglobin Metabolism
b. Hepatic jaundice: It is also known as hepato cellular jaundice. Because liver cell
damage is the main cause for this type of jaundice. Viral infections, toxins, chemicals
damage liver cells. Hepatitis virus, mushroom poisons, chloroform, carbon tetra
chloride and phosphorus damage hepatocytes. Antibiotics use and in cirrhosis also
hepatocytes are damaged. Functions of damaged hepatocytes are impaired. So
hepatocytes may not be able to conjugate or secrete bilirubin though the bilirubin
production is normal. If conjugation is impaired plasma level of unconjugated bilirubin
is elevated. If secretion of conjugated bilirubin is affected its level in plasma is
elevated. Hence in hepatic jaundice both conjugated and free bilirubin levels are
increased.
Post hepatic jaundice : It is also known as obstructive jaundice. Bile duct obstruction
causes this disease. Stones in gall bladder and cancer of head of pancreas cause bile
duct obstruction. Due to block in bile flow conjugated bilirubin secreted into bile
returns to blood. Hence conjugated bilirubin level is elevated in obstructive jaundice.
7. Write principle and types of van den Bergh reaction. How it is useful
in jaundice diagnosis.
A. van den Bergh Reaction
It is based on Ehrlich's reaction. It is used for measurement of plasma bilirubin. In this
reaction a purple red color is produced. It is due to coupling of bilirubin with diazo reagent
or diazotized sulphanilic acid.
Two types of van den Bergh reactions are known.
a. Direct van den Bergh reaction :It measures conjugated bilirubin only. In this reaction
purple color is produced when conjugated bilirubin reacts directly with diazo reagent.
b. Indirect van der Bergh Reaction :It measures un conjugated bilirubin only. In this
reaction purple color is produced when un conjugated bilirubin reacts with diazo
reagent in presence of methanol.
van den Bergh Reaction is used in differential diagnosis of jaundice. Since un conjugated
bilrubin is more in hemolytic jaundice indirect van den Bergh reaction is obtained with
this serum. In contrast direct van den Bergh Reaction is obtained with obstructive
jaundice serum. Therefore a positive indirect van den Bergh Reaction is used to confirm
hemolytic jaundice and positive direct van den Bergh reaction is used to confirm
obstructive jaundice.
8. Write normal hemoglobin level in blood. Explain its structure and
functions.
A. Hemoglobin concentration in blood is 12-15 gm%.
Structure :Hemoglobin present in adults is known as hemoglobin Aor Hb A. It is a
conjugated protein. It consist of globin as protein and heme as non protein part. It is a
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BIOCHEMISTRY - Questions and Answers
tetramer. It consist of 4 polypeptide chains of two types. They are α chains two and ß –
chains two. Each polypeptide chain is attached to one heme group. α –chain consist of 141
amino acids and ß-chain consist of 146 amino acids.
Total 574 amino acids are present in hemoglobin molecule. It has molecular weight of
about 64, 400 daltons.
In each sub unit hydrophobic amino acids are present in interior. Hydrophilic amino acids
are present on outer surface which makes it soluble in water. Heme is in hydrophobic
interior. The sub units inter act in a unique way unlike sub units like α1, ß1 and α2, ß2 inter
act extensively and like sub units i. e α1, α2 ; ß1, ß2 interacts weekly. Over all shape is
spherical.
Function : It transports oxygen from lungs to tissues. One molecule of hemoglobin carries 4
molecules of oxygen molecules. Hemoglobin also transports co2 from tissues to lungs.
Hemoglobin serve as major blood buffer.
9. Write very briefly about glycosylated hemoglobin.
A. Glycosylated hemoglobin HbA1C
This type of hemoglobin contain glucose units. They are attached to ß-chain at Nterminus. It is a non enzymatic attachment. Usually the rate of glycosylated hemoglobin
formation depends on blood glucose level. In normal people it accounts about 4-7% of total
hemoglobin. It increases up to 20% in diabetic cases.
10. Write a note on sickle cell hemoglobin.
A. Sickle cell hemoglobin HbS
1. It is most commonly occurring and severe abnormal hemoglobin.
2. It differs from normal hemoglobin in one amino acid residue of ß –chain.
3. In this hemoglobin valine replaces glutamate at 6position of ß chain.
4. This makes sickle cell hemoglobin more positive.
5. Further this change in one amino acid affects erythrocyte shape.
6. Erythrocytes containing sickle cell hemoglobin remain normal due to oxygenation
7. Deoxygenation leads to aggregation of sub units due to hydrophobic valine in beta
chain.
8. It induces characteristic sickle shape.
9. Erythrocyte with altered shape have decreased life span and hence they undergo
hemolysis.
10. Therefore sickle cell hemoglobin causes sickle cell anaemia.
11. Write briefly on a) Hemoglobinopathies b) Thalassemias
A. a) Hemoglobinopathies are group of inherited diseases associated with production of
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CHAPTER - 11 | Porphyrin and Hemoglobin Metabolism
hemoglobin with altered composition. Only composition of α and β chains of globin part
is altered and prosthetic group remains unchanged. They are due to mutations in genes
coding Alpha and beta chains. Hemoglobin with altered composition is called as
abnormal hemoglobin or hemoglobin variant. In homozygous individuals only
abnormal hemoglobin is produced where as in heterozygous individuals both normal
and abnormal hemoglobins are produced. Production of abnormal hemoglobin triggers
hemolysis. Hence moderate to severe hemolytic anemia occurs in susceptible people
like infants, children, young girls and in pregnant women. Hb S, Hb E and Hb D are
some abnormal hemoglobins seen in hemoglobinopatheis.
b) Thalassemias are group of inherited diseases in which total synthesis of one of the
globin chain is blocked. Hence they are named according to synthesis of chain blocked.
They are of two types 1) α- Thalassemia 2) β- Thalassemia
1) α- Thalassemia : In this condition α- chain synthesis is blocked. So Hb A is not
formed. The β-chain form abnormal HbH (β4). So it is also known as HbH disease.
Further formation of abnormal HbH triggers hemolysis and anemia develops in
affected individuals.
2) β- Thalassemia : In this condition β-chain synthesis is blocked. Hence synthesis of
HbA is affected. The α- chain is unable to form tetramer but forms large inclusion
(Heinz) bodies which triggers hemolysis leading to anemia in affected individuals.
The condition is called as Cooley's anemia or β-thalassemia major in homozygotes
and β-thalassemia minor in heterozygotes
Other model questions are
12. Write formation of bilirubuin.
13. Write a note on Hemoglobin.
14. Write about acute intermittent porphyria.
15. HbS.
16. Hb A1C
17. Crigler-Najjer syndrome
18. Cooley's anemia
149
CHAPTER - 12 | Nucleotide Metabolism
12
Chapter
Nucleotide Metabolism
1. Describe denovo synthesis of purine nucleotides.
A. Denovo purine nucleotide biosynthesis
Site:Liver cytosol contains all the enzymes required for purine nucleotide biosynthesis.
Purine nucleotide formation involves construction of purine ring on ribose phosphate.
Sources of purine ring : Purine ring is generated by incorporating carbon and nitrogen
atoms from various sources. Carbons 4, 5 and 7 are derived from glycine ; carbon 2 and 8
are derived from N-10 formyl FH4 and N-5, 10 methenyl FH4 respectively ; carbon dioxide
contributes to carbon 6, nitrogen 1 is from aspartate ; nitrogen 3 and 9 are from amide of
glutamine.
Aspartate
7 Glycine
N
6
C
Co2
1N
5C
2C
4C
C8
FH4
N3
FH4
N9
Glutamine
Reactions :
1. Phosphoribosyl pyrophosphate (PRPP) formation from ribose -5- phosphate is the first
reaction. PRPP synthetase catalyzes this reaction. ATP and Mg2+ are required. AMP is
formed from ATP.
PRPP
Synthetase
Ribose -5-phosphate +ATP
5- phosphoribosyl pyrophosphate + AMP.
2+ (
Mg 1)
2. Phosphoribosyl amido transferase catalyzes formation of phosphoribosyl amine by
transferring amide of glutamine.
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CHAPTER - 12 | Nucleotide Metabolism
Amido
Transferase
5-phosphoribosyl pyrophosphate +glutamine
Phosphoribosyl amine +
(2)
Glutamate +PPi.
3. 5-Phosphoribosyl glycinamide is formed in this reaction from glycine and
phosphoribosyl amine. An ATP dependent glycinamide kinosynthetase catalyzes this
reaction.
Synthetase
Phosphoribosyl amine + ATP + Glycine
+ ADP +Pi.
phosphoribosyl glycin amide
(3)
4. Formylation by trans formylase generates phosphoribosyl –N- formyl- glycinamide
from phosphoribosyl glycinamide.
Transformylase
5-Phosphoribosyl glycinamide
Amide+FH4.
Phosphoribosyl -N-formyl glycin(4)
5. Transfer of amide group of glutamine again to carbonyl oxygen of glycinamide is the
fifth reaction. This ATP dependent reaction is catalyzed by 5- phosphoribosyl –Nformyl glycinamidine synthetase. 5-phosphoribosyl –N- formyl glycinamidine is
product of this reaction.
glycinamidine synthetase
5- phosphoribosyl –N- formyl glycinamide + ATP + glutamine
(5)
5- phosphoribosyl –N- formyl glycinamidine + ADP + Pi + glutamate.
6. An ATP dependent imidazole ring of purine formation occurs in this reaction. 5phosphoribosyl amino imidazole synthetase brings this reaction.
Imidazole
Synthetase
51 –phosphoribosyl -5-.
5- phosphoribosyl –N- formyl glycinamidine +ATP
(6)
amino imidazole + ADP +Pi + H2O.
7. Carboxylation of amino imidazole ring by carboxylase is seventh reaction.
Carboxylase
1
51- phosphoribosyl –5-
5 – phosphoribosyl -5- amino imidozole + CO2
(7)
Amino imidazole-4-carboxylate.
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BIOCHEMISTRY - Questions and Answers
8. An ATP dependent transfer of amino group of aspartate to carboxyl introduced in
above reaction takes place in presence of synthetase
Synthetase
1
5 – phosphoribosyl – 5-amino imidazole -4-carboxylate + ATP +Aspartate
(8)
1
5 -phosphoribosyl -4- (N- succino carboxamide)- 5- Amino imidazole.
9. A lyase eliminates fumarate from product of above reaction to form 51- phosphoribosyl 4- carboxamide -5-amino imidazole.
Lyase
51- phosphoribosyl -4- (N-succino carboxamide)- 5- Amino imidazole
51(9)
phosphoribosyl -4- carboxamide -5 amino imidazole + Fumarate.
10. Formylation once again by another transformylase incorporates formyl group to form
51-phosphoribosyl -4-carboxamide-5- formamido imidazole.
Trans formylase
5 - phosphoribosyl -4- carboxamide -5- amino imidazole
51- phosphoribosyl
(10)
+N-10 formyl FH4.
4-carboxamide-5-Formamido imidazole + FH4.
1
11. A cyclohydrolase catalyzes ring closure by dehydration to yield first purine
nucleotide inosine mono phosphate (1MP).
Cyclohydrolase
1
5 - phosphoribosyl -4- carboxamide -5-form amido imidazole
inosine
(11)
monophosphate (IMP) + H2O.
Synthesis of AMP and GMP from IMP :Synthesis of AMP and GMP from IMP occurs by
two separate pathways.
Formation of AMP from IMP occurs as shown below
Adenylo succinate
Synthetase
IMP +aspartate+GTP
Adenylo succinate + GDP + Pi.
(1)
Adenylo succinase
Adenylo succinate
AMP + Fumarate.
(2)
Formation of GMP from IMP occurs as below shown.
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CHAPTER - 12 | Nucleotide Metabolism
dehydrogenase
IMP +NAD+ + H2O
xanthosine mono phosphate (XMP) + NADH + H+.
(1)
GMP synthetase
XMP+ ATP +glutamine
GMP + glutamate +AMP +PPi.
(2)
Formation of ADP, GDP and ATP, GTP.
AMP +ATP
ADP + ADP ; ADP
ATP; ADP
dADP
dATP
GMP +ATP
ADP + GDP ; GDP
GTP ; GDP
dGDP
d GTP.
2. Write about Pyrimidine nucleotide synthesis denovo.
A. De novo pyrimidine nucleotide biosynthesis
Site :Cytosol of liver cells. In pyrimidine nucleotide biosynthesis pyrimidine ring is formed
first then pentose phosphate is added later. Pyrimidine ring is generated from aspartate
and carbamoyl phosphate.
Reactions :
1. Carbamoyl phosphate synthetase –ІІ (CAPS –ІІ) catalyzes formation of carbamoyl
phosphate from glutamine amide nitrogen and carbon dioxide. ATP is required for this
reaction.
CAPS-ІІ
Co2 + glutamine + 2ATP
Carbomoyl phosphate + 2ADP + Pi.
(1)
2. Reaction of aspartate and carbamoyl phosphate in presence of aspartate trans
carbamoylase (ATC ase) yields carbomyl aspartate + Pi.
ATC ase
Carbomyl phosphate +Aspartate
Carbamoyl aspartate + Pi.
(2)
3. An intra molecular water elimination leads to pyrimidine ring formation from
carbamoyl aspartate. Dihydro oratase catalyzes this reaction.
Dihydroorotase
Carbamoyl aspartate
Dihydroorotate + H2o.
(3)
4. An NAD+ dependent dehydrogenase converts dihydro orotate to orotic acid.
Dehydrogenase
+
Orotic acid + NADH +H+.
Dehydro orotate +NAD
(4)
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BIOCHEMISTRY - Questions and Answers
5. Orotidine monophosphate (OMP) is formed from orotic acid in this reaction. Orotate
phosphoribosyl transferase catalyzes transfer of ribose -5- phosphate to orotate from
PRPP.
Phosphoribosyl
Transferase
Orotate +PRPP
Orotidine mono phosphate (OMP) +PPi.
(5)
Pyro phosphatase
PPi
2Pi.
6. First pyrimidine nucleotide uridine mono phosphate (UMP) is generated from OMP
by decarboxylase.
Decarboxylase
Orotidine mono phosphate
Uridine mono phosphate (UMP) + CO2.
(6)
Synthesis of CTP and dTTP from UMP
UMP +ATP
ADP +UDP
UTP + Glutamine +ATP
UDP
UDP +ATP
UTP + ADP.
CTP +glutamate + ADP + Pi.
d UDP
d UMP
d TMP
dTDP
dTTP.
3. Write about salvage pathways of purine nucleotides.
A. Salvage pathways are active in tissues which lack de novo pathways. Blood cells and brain
are dependent on these pathways. These pathways use pre formed purine bases of either
exogenous and endogenous origin. They uses even nucleosides for nucleotide formation.
Purine salvage pathways :They are involved in the conversion of purine bases, purine
nucleosides into nucleotides.
1. Hypoxanthine guanine phosphoribosyl transferase (HGPRT ase) converts
hypoxanthine and guanine to IMP and GMP respectively. PRPP act as donor of
ribose phosphate.
HGPRT ase
Hypoxanthine + PRPP
IMP +PPi ; Guanine + PRPP
GMP +PPi.
2. Adenine is converted to AMP by adenine phosphoribosyl transferase
Adenine + PRPP
AMP + PPi.
3. Adenosine and guanosine are converted to AMP and GMP by kinases.
Adenosine
Adenosine +ATP
AMP +ADP.
kinase
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CHAPTER - 12 | Nucleotide Metabolism
Guanosine
Guanosine + ATP
GMP +ADP.
kinase
4. Write a note on Pyrimidine salvage pathways.
A. Pyrimidine salvage pathways are:
1. Pyrimidine phosphoribosyl transferase (PPRT ase) salvages free pyrimidine bases
using PRPP as donor of ribose phosphate.
PPRT ase
Thymine + PRPP
PPi
TMP +PPi
Uracil +PRPP
UMP+ PPi
2Pi.
2. Pyrimidine nucleosides are salvaged by kinases.
Uridine kinase
Uridine +ATP
UMP +ADP
Deoxythymidine +ATP
deoxycytidine +ATP
dCMP +ADP.
d TMP +ADP.
5. Write about inhibitors of nucleotide biosynthesis.
A. Inhibitors of nucleotide biosynthesis are useful as anti bacterial, antiviral and anticancer
agents.
Antibacterial agents : Sulfa drugs like sulfanilamide and tri methoprim are used to treat
bacterial infections. They are folic acid analogs and block folic acid dependent reactions of
nucleotide biosynthesis in bacteria. Bacterial growth is impaired due to lack of folic acid.
Thus bacterial infection is controlled.
Anticancer agents : Several inhibitors of nucleotide biosynthesis are used as anti cancer
agents. For example folic acid analogs aminopterin and amethopterin work as anti cancer
agents by blocking folic acid dependent reactions. Azaserine and acivicin block glutamine
involving reactions. Mercapto purine and fluorouracil work by blocking purine and
pyrimidine nucleotide formation.
Anti viral agents : Acyclovir and AZT (azothymidine) are antiviral agents.
6. Wrie about a) Orotic acid uria b) Lesch-Nyhan syndrome.
A. They are diseases of nucleotide formation. Defective nucleotide formation causes
diseases.
a) Orotic acid uria :It is an inherited disease of pyrimidine nucleotide formation. It is due
to lack of orotate phosphoribosyl transferase and decarboxylase. It is characterized by
more orotic acid in blood and its excretion in urine. Clinical symptoms are growth
retardation and anaemia.
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BIOCHEMISTRY - Questions and Answers
b) Lesch – Nyhan syndrome :HGPRT ase is deficient in this condition. More of purine
nucleotides are produced. Symptoms are self mutilation, mental retardation, anemia
and hyper uricemia.
7. How purine nucleotides are degraded?
A. Site : Liver is the major site of purine nucleotide degradation.
Reactions: 1. Nucleases generate purine nucleotides from nucleic acids.
Nuclease
Nucleic acids
AMP
Nucleic acids
GMP
(1)
2. AMP is converted to inosine by two routes.
a. Deamination of AMP by adenylate deaminase followed by dephosphorylation.
Adenylate
Dephosphory
Deaminase
lation
AMP
IMP
Inosine + Pi.
2a
2a
NH3
b. Dephosphorylation followed by deamination. Nucleotidase catalyzes phosphate
removal from AMP and adenosine deaminase (ADA) catalyzes removal of amino
group of adenosine.
Adenosine
Nucleotidase
AMP
deaminase
Adenosine
2b
Inosine +NH3
2b
pi
3. A nucleotidase converts GMP to guanosine.
Nucleotidase
GMP
guanosine +Pi.
3
4. Now inosine and guanosine are converted to xanthine by two separate routes.
a. Transfer of ribose by nucleoside phosphorylase followed by oxidation. Xanthine
oxidase (XO) catalyzes latter reaction.
Nucleoside
Phosphorylase
Inosine +Pi
Ribose -1- phosphate + Hypoxanthine.
4a
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CHAPTER - 12 | Nucleotide Metabolism
Xanthine
Oxidase
Hypoxanthine + o2 + H2 O
Xanthine + H2 O2.
(4b)
b. Transfer of ribose by nucleoside phosphorylase followed by deamination.
Guanase catalyzes latter reaction.
Nucleoside
Phosphorylase
Guanosine + Pi
Ribose -1- phosphate + guanine.
(4b)
Guanase
Guanine
Xanthine + NH3.
(4b)
5. Finally uric acid which is end product of purine degradation is formed from
xanthine by action of xanthine oxidase.
Xanthine oxidase
Xanthine + H2 O + O 2
Uric acid + H2 O2.
(5)
8. Write normal plasma uric acid. Write about conditions with raised uric
acid level.
A. Normal plasma uric acid level is below 6mg%.
Gout : It is common disease of purine nucleotide degradation. Plasma uric acid level is
elevated which is characteristic sign of gout.
Symptoms : Deposition of uric acid crystals occurs in soft tissues because uric acid is less
soluble in aqueous environment of body fluids. Tophi is the name given to urate crystals
that are found in joints, cartilage of fingers and toes. Arthritic type gouty attacks occurs in
affected individuals.
Causes :
1. Over production of uric acid causes gout. Increased purine nucleotide production leads
to excessive uric acid production. It occurs in HGPRT ase deficiency, Hyper active
PRPP synthetase, leukaemia, von Gierke's disease and polycythemia.
2. Impaired removal of uric acid by kidneys causes gout. It is called as renal gout. It
occurs due to defective
uric acid transport in renal tubular cells and
glomerulonephritis.
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9. Write very briefly about SCIDD.
A: It is usually known as
severe combined immunodeficiency disease (SCIDD). It is due to
adenosine deaminase deficiency. Due to lack of this enzyme DNA synthesis is decreased
and lymphocytes do not mature. Affected persons are susceptible to infections.
Other model questions are
10. How inosine monophosphate is synthesized?
11. Write the formation of ATP and GTP from IMP.
12. Write purine ring. Label sources of its various elements.
13. Write the formation of UMP.
14. Draw structure of Pyrimidine ring. Label sources of its various
elements.
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CHAPTER - 13 | Replication, Transcription & Translation
13
Chapter
Replication, Transcription & Translation
REPLICATION
1. Define replication. Explain process of replication.
A Replication :Synthesis of new DNA is known as replication.
Process of replication :Replication begins at specific locations of DNA. It is known as ori
C. It involves DNA un winding, initiation, elongation and several protein factors. Ori C
is a sequence of 245 basepairs (bp) in E. coli Chromosome. It has binding sites for DNA
binding proteins.
1. Un winding of DNA occurs due to binding of dna A a DNA binding protein to ori C.
2. Further binding of dna B and dna C to DNA facilitates unwinding. dna B is helicase. It
causes un winding of DNA.
dna A
DNA
dna B
DNA –dna A
Unwinding of DNA.
dna C
3. Single strand binding proteins (SSBP) bind to unwound DNA and stabilizes single
strand by preventing rewinding.
SSBP
DNA with a unwinding strand
DNA with stabilized unwound strand.
4. Un winding of DNA creates super coils and prevent further unwinding. DNA gyrase
removes super coils by creating negative super coils which facilitates replication.
DNA gyrase
DNA un winding
super coils in DNA
Negative super coils in DNA
Replication Favoured.
5. Another strand of DNA is also stabilized by SSBP. A replication fork is created.
Binding of SSBP
DNA with one
to another strand
Replication fork.
Unwound strand
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BIOCHEMISTRY - Questions and Answers
6. At replication fork a RNA primer is formed by action of RNA – polymerase or primase.
When the RNA primers are elongated to about 10- 200 bases DNA polymerase ІІІ form
new DNA in 51
31direction by polymerizing deoxy nucleotide triphophates (dNTPs).
RNA poly
Replication
Fork of DNA
DNA polynease ІІІ
RNA primer at
merase
replication fork
New DNA strand
dNTPS
Newly formed DNA strand is known as leading strand.
7. DNA polymerase ІІІ is able to synthesize both DNA stands but in different ways
because it cannot polymerize nucleotides in 31
51 direction.
8. It is able to synthesize leading strand continuously. It synthesizes other strand in a
discontinuous manner. It is known as lagging strand. Fragments of this strand are
known as okazaki fragments.
9. DNA polymerase - І fills up gaps between okazaki fragment to form lagging strand.
DNA polymerase ІІІ
Replication fork with leading strand
DNA polymeraseI
okazaki fragment
dNTPS
okazaki fragment with gaps filled.
10. DNA ligase joins okazaki fragments. Lagging strand is formed.
DNA ligase
okazaki fragments with
Lagging strand gaps filled.
Gaps filled
11. RNA primers are removed by exonuclease activity of DNA polymerase –І.
REPLICATION FORK
3
1
5
Leading Strand
1
3
1
DNA
Polymerase
III
1
5
160
1
DNA
1
3
5
Okazaki Fragments
RNA Primer
5
1
3
1
CHAPTER - 13 | Replication, Transcription & Translation
2. Write about inhibitors of replication and their medical importance.
A. Inhibitors of replication :Several compounds are capable of blocking replication. They are
known as inhibitors of replication. Some of them are a. Cytosine arabinoside b.
Actinomycin D c. Acyclovir.
Cytosine arabinoside : It is a nucleoside analog of cytosine and contain modified pentose.
Arabinose is present instead of usual ribose. Introduction of this compound blocks
elongation of replication.
Actinomycin D : It inferfers with interaction of DNA bases because it contains an
hydrophobic ring. This leads to destabilization of DNA and replication is blocked.
Acyclovir : It is another nucleoside analog. It is an analog of guanosine with three carbon
sugar instead of usual ribose. It inhibits polymerase action.
Medical Importance : Inhibitors of replication are used as anti cancer, antibacterial and
antiviral agents. Cytosine arabinoside and actinomycin D are anticancer agents. Acyclovir
is antiviral agent. Bleomycin is an antibiotic. It inhibits replication by breaking bonds of
DNA.
TRANSCRIPTION
3. Define transcription. Explain process of transcription.
A. Transfer of genetic information from DNA to RNA or synthesis of RNA from DNA is
known as transcription.
Process of Transcription: Transcription involves enzymes, protein factors, initiation,
elongation and termination.
Initiation.
1. Certain regions of DNA serve as signals for the synthesis of RNA. They are known as
promoter sites.
2. RNA polymerase is enzyme involved in RNA synthesis. It contains five subunits. Two
alpha, two beta and one sigma subunits. RNA polymerase recognizes promoter site
with the help of sigma factor (subunit). It binds to template strand of DNA at promoter
site. This binding leads to unwinding of DNA and initiation of RNA formation.
Sigma subunit
DNA promoter site
RNA polymerase binds promoter
unwinding
of Template strand.
3. RNA polymerase has two binding site. One for purine nucleotides and another for any
nucleotide. Assuming that ATP binds to purine nucleotide site and pyrimidine
nucleotide UTP binds to another site first phosphodiester bond formation occurs
between ATP and UTP. Thus chain growth is initiated. After this the sigma factor is
released.
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BIOCHEMISTRY - Questions and Answers
ATP, UTP
Unwound Template strand
subunit released.
Chain growth initiated
sigma
Elongation :
1. RNA polymerase continues polymerization of ribo nucleotide triphosphates
(rNTPs)as directed by template strand. DNA unwinds ahead of new RNA chain and
newly formed RNA or nascent RNA is base paired to templates strand.
RNA polymerase
New chain of RNA growth
Formation of new RNA or nascent RNA.
rNTPS
2. Rewinding of DNA leads to dissociation of nascent RNA from template strand
during elongation.
Rewinding of DNA
Nascent RNA
Dissociation of nascent RNA from template strand.
Termination :
1. Certain regions of DNA serve as signals for termination of RNA synthesis. They are
known as termination signals.
2. Formation of hair pin loop in nascent RNA occurs when termination signal is
transcribed by RNA polymerase.
Termination
Nascent RNA
Hairpin loop formation in nascent RNA.
Signal
Specific terminator protein Rho (e) recognizes hair pin loop in nascent RNA. This
terminator protein has helicase activity also. So it separates base paired nascent
RNA from template strand of RNA. This type of termination of RNA synthesis is
known as Rho (e) dependent termination. On terminatipon of RNA synthesis RNA
polymerase leaves DNA molecule.
Rho protein
Nascent RNA with
Separation of nascent RNA
Hair pin loop
transcription
Termination of
RNA polymerase leaves DNA.
1
5
31
DNA
31
rm
y Fo
51
162
l
New
NA
ed R
51
31
RNA Polymerase
CHAPTER - 13 | Replication, Transcription & Translation
3. Write about Inhibitors of Transcription
A. Several compounds block RNA formation. They are known as inhibitors of transcription.
Some of them are antibiotics. Toxins also act by blocking transcription. Toxins are
mushroom toxin α- amanitin that causes mushroom poisoning and aflatoxin produced by
fungus of ground nut that causes liver cancer. Rifamycin and rifampicin are antibiotics
used in the treatment of tuberculosis work by inhibiting transcription. Actinomycin D is
anti cancer agent used in treatment of Wilm's tumor work by inhibiting transcription.
4. Write a note on post transcriptional modifications
A. They convert precursor RNA to functional RNAs. All three types of RNAs are produced as
precursors.
Messenger RNA or mRNA: It is synthesized as precursor RNA known as heterogenous
RNA or hn RNA. Post transcriptional modifications it undergo are
a. Capping at 51 end
b. Poly adenylational at 31 end.
c. Splicing: Eukaryotic hn RNA contains non functional sequences known as introns.
Functional sequences of hn RNA are known as exons. Splicing removes introns of hn
RNA and joins exons to generate functional m RNA. Splicing of hn RNA requires small
nuclear RNA or sn RNA and splicing proteins.
tRNA or rRNA are also generated from precursors by post transcriptional
modifications.
TRANSLATION
5. Define genes and write on genetic code
A. Genes of DNA contain code words for amino acids. Genes are segments of DNA that contain
information for protein formation or poly peptide synthesis. The code words of amino acids
are known as genetic code. These code words are transferred to m RNA and t RNA by
transcription. So m RNA and tRNA contain amino acid code words.
Characteristics of genetic code :
1. It is triplet code. Two types of code words exist. They are codon and anti codon. Both codon
and anti codon consist of sequence of three nucleotides. Codons are present on m RNA and anti
codon is present on t RNA. Codon and anticodon are complementary in nature.
2. Each aminoacid is coded by codon. For example UUU code for phenyl alanine, GGG for glycine
and CCC for proline.
3. Some codons serve as initiating codons for protein synthesis. AUG is an example for initiating
codon of protein synthesis.
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BIOCHEMISTRY - Questions and Answers
4. Some codons do not code for any amino acid and they cause termination of protein synthesis.
They are known as termination codons or nonsense codons. They are UAA, UAG or UGA.
5. The codon is universal. Codon of an amino acid is identical in all species.
6. A codon codes only one amino acid. However an amino acid may have more than one codon.
7. Genetic code is comma less.
8. Base pairing occurs between bases of codon and anticodon.
6. Define translation. Describe translation
A. Synthesis of proteins using information present in RNAs is known as translation.
Mechanism of translation involves initiation, elongation, termination, ribosomes,
initiation factors (IFs), elongation factors (EFs) and releasing factors (RFs).
Activation of aminoacids :Prior to utilization for protein formation aminoacid must be
activated. Activation of aminoacid provides energy needed for peptide bond formation.
Activation involves attachment of amino acid to t RNA. Amino acyl –t RNA synthetase is
enzyme involved in activation of amino acid. ATP is required. It is converted to AMP and
PPi. Pyrophosphatase converts PPi to in organic phosphate.
Aminoacyl
tRNA
Amino acid + t RNA +ATP
Aminoacyl- tRNA (AA-tRNA)+AMP+PPi.
Synthetase
Pyrophosphatase
PPi
2Pi.
Mechanism of Translation
Initiation:
1. It begins with binding of initiation factors (IFs) to 30S ribosomal subunit. Three initiation
factors are required. They are IF-1, IF-2 and IF -3. First IF-3 binds with 30S subunit. IF-2
combines with GTP to form IF-2-GTP complex. IF-1 and IF-2-GTP complex binds to IF-3
containing 30S ribosomal subunit.
30S ribosomal subunit +IF-3
IF-3-30S subunit IF-2 +GTP
IF-3-30S +IF-1 + IF-2-GTP
30S-IF-3 -IF-1-IF-2-GTP.
IF-2-GTP.
2. Now mRNA and aminoacyl-tRNA (AA-tRNA)can bind to 30S ribosomal subunit containing
GTP and initiation factors. The mRNA
combines 30Sribosomal subunit. The initiating
tRNA containing first amino acid AA1 -tRNA joins mRNA -30S subunit complex through
codon anti codon base pairing to form initiation complex. Release of IF-3 accompanies this
complex formation.
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CHAPTER - 13 | Replication, Transcription & Translation
AA1- tRNA
30S-IF-3 -IF-1-IF-2-GTP+mRNA
m RNA-30S-IF-3-IF-1-IF-2-GTP
AA1-tRNA-mRNA-30S-IF-1-IF-2-GTP + IF-3
Initiation complex.
The initiation complex has high affinity to wards 50S ribosomal subunit and binds to one of
50S subunit from pool. Joining of 50S subunit with initiation complex causes hydrolysis of
GTP to GDP and Pi and release of IF-1 and IF-2 and leading to formation of 70S initiation
complex. In the 70S initiating complex peptidyl site or P site of ribosome is occupied by
initiating amino acyl-tRNA (AA1-tRNA)and aminoacyl site or A site of ribosome is free.
AA1-tRNA-mRNA-30S-IF-1-IF-2-GTP+50S
AA1-tRNA (P site)-A site (Free)-70S –mRNA + GDP + Pi + IF-1 + IF-2
Initiation complex.
Elongation : By sequential addition of aminoacids new polypeptide chain is elongated. t RNA s
carry aminoacids. Elongation factors EF-Tu, EF-Ts, EF-G and GTP are required.
1. tRNA carrying second aminoacid (AA2-tRNA) to be incorporated cannot directly combine
with 70S initiation complex. It requires elongation factor EF-Tu and GTP. AA2 – tRNA
combines with EF-Tu-GTP complex and interacts with 70S initiation complex. A site of
70S ribosome is occupied by AA2 –tRNA with concomitant hydrolysis of GDP and Pi. EFTu-GDP complex dissociates from 705 ribosome.
AA2 -tRNA
AA1-tRNA (Psite)-A site -70S –mRNA + EF-Tu-GTP
AA1 – tRNA (Psite)-AA2 –tRNA (Asite)-70S-m RNA + EF-Tu-GDP + Pi.
2. Presence of two aminoacyl – tRNA s on P and A sites of 70S ribosome respectively leads to
first peptide bond formation. Peptidyl transferase activity of 50S ribosome subunit
catalyzes peptide bond formation between AA1 and AA2 Aminoacids. It is accompanied by
shifting of dipeptide (AA1-AA2) to A site of ribosome. The empty tRNA remains in P site.
Peptidyl
AA1 –tRNA (Psite)-AA2-tRNA (Asite)-70S-mRNA
Transferase
tRNA (Psite)-AA1- AA2- tRNA (Asite)-70S-mRNA.
3. The incoming tRNA carrying third amino acid (AA3 –tRNA) cannot bind to Psite. It can
bind only to Asite of 70S ribosome. So tRNA carrying dipeptide (dipeptidyl-tRNA or AA1AA2 –tRNA)is translocated to free Psite in presence of elongation factor EF-G and GTP.
First EF-G combines with GTP to form EF-G-GTP complex. This complex inter acts with
70S ribosome. This leads to release of free tRNA from Psite and shifting of dipeptidyl tRNA
from Asite to Psite. During this shifting mRNA also moves by three nucleotides and third
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BIOCHEMISTRY - Questions and Answers
codon appears in Asite. Hydrolysis of GTP to GDP and Pi provides energy for this process
and EF-G is released.
EF-G+GTP
EF-G-GTP complex.
EF-G-GTP
tRNA (Psite)-AA1 - AA2 – tRNA (Asite)-70S-m RNA
AA1-AA2 –tRNA- (Psite)-A site -70S-m RNA + EF – G + GDP + Pi + t RNA
Regeneration of EF-Tu-GTP: EF-Tu- GTP regeneration is essential for continuation of
elongation or protein synthesis. Second elongation factor EF-Ts is involved in this
process. EF-Ts reacts with EF-Tu – GDP complex replaces GDP and forms EF-Tu- EF-Ts
complex.
EF-Ts+EF –Tu- GDP
EF-Ts-EF-Tu + GDP
Now GTP reacts with EF-Ts –EF –Tu complex displaces EF-Ts to form.
EF-Tu-GTP complex. EF-Ts is released.
GTP
EF-Ts – EF-Tu
EF-Tu-GTP +EF-Ts.
EF-Tu-GTP complex availability
leads to continuation of
elongation process. The
elongation process is repeated many times adding one aminoacid each time until
termination codon is encountered in Asite.
Elongation repeated n times
AA1 – AA2 - tRNA (Psite)-Asite -70S-mRNA
AA1 – AA2 ------------- AAn tRNA (Psite)-A site -70S-mRNA
Termination :
1. Exposure of termination codon on Asite signals termination of polypeptide formation.
Releasing factors (RF)and GTP are required. There are three releasing factors RF-1,
RF-2 and RF-3. They recognizes termination codon on A site and bind to site where
tRNA binds.
2. Polypeptide chain is separated from tRNA by hydrolysis of ester bond. Ribosomal
peptidyl transferase activity of ribosomes causes this hydrolysis. Further GTP is
hydrolyzed to GDP and Pi. This results in release of mRNA and tRNA from ribosomes.
Dissociation of ribosomes to 30S and 50S subunits takes place immediately.
AA1- AA2 .............. tRNA (Psite)-Asite – 70S –mRNA + RF + GTP
AA1 – AA2 .............. AAn-tRNA (Psite)–RF- GTP (Asite)-70S-mRNA.
AA1- AA2 .............. + tRNA + 50S + 30S + mRNA + RF + GDP + Pi.
Polypeptide (Protein)
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CHAPTER - 13 | Replication, Transcription & Translation
AA1
AA1
AA2
AA2
AA1
tRNA
3
1
AA2
31
5
Translocation
1
Elongation
51
lex
n Comp
itiatio
70S In
AAn
New Polypaptide
AA1
AA2
AAn
A
mRN
+
31
+
Termination
+
tRNA
31
51
AA3
30 S
50 S
+
51
7. Write about Inhibitors of protein synthesis
A. Many antibiotics are inhibitors of translation. They work by inhibiting translation at
differents stages. Even some toxins work by blocking protein synthesis. Antibiotics are
puromycin, tetracyclins, chloramphenicol, erythromycin, streptomycin, tunicamycin and
cycloheximide. Diphtheria toxin that causes diphtheria in children inhibit translation.
8. Write a note on post translational modifications
A. Nascent proteins that comes out of protein synthesizing machinery may not be
functionally active. They are converted to active or functional proteins by post
translational modifications. Some of them are phosphorylation, glycosylation,
hydroxylation, methylation, proteolytic modifications, carboxylation and iodination.
9. Explain DNA damage and DNA repair mechanisms.
A. Since DNA is genetic material for survival of species through generations integrity of DNA
must be maintained. However DNA damage can result from action of environmental,
physical, chemical, and biological agents. Therefore species evolved mechanisms for the
removal of damaged DNA.
Excision repair : An exinuclase cuts damaged DNA and removes it. DNA polymerase І
synthesizes new DNA in that gap. Then DNA ligase integrates new DNA into native DNA.
Enzymatic photo reactivation :It is involved in the removal of thymine dimer formed by
exposure of DNA to ultra violet light. A photolyase get activated on exposure to normal
light which cleaves linkage between two thymine bases.
10. Write very briefly about diseases of DNA repair.
A. Defective DNA repair mechanisms leads to diseases.
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BIOCHEMISTRY - Questions and Answers
Xeroderma pigmentosum: It is due to defective enzyme of excision repair. Characteristic
sign is development of skin cancer in affected people on exposure to ultra violet light of
sunlight.
11. Explain salient features of Lac operon model
A. Lac operon deals with regulation of gene expression at transcriptional and translational
level. It is a model proposed to explain enzyme induction and repression related to lactose
metabolism. Hence the name lac operon. An operon consist of four types of genes on DNA
segment. The four types of genes are structural genes, promoter gene (p), regulatory gene
(i) and operator gene (o). They are located adjacent to each other. The structural genes are
composed of three genes. They are z, y and a and present side by side. These structural
genes codes for enzymes required for lactose metabolism. They code for galactosidase,
permease and trans acetylase. Regulatory gene regulates operon. It codes a regulatory
protein known as repressor.
i
p
o
Regulato
Promo
Ry gene
ter gene gene
z
y
a
operator
Structural genes
DNA segment
When repressor molecule is produced it binds to operator gene and prevents transcription
of structural genes by RNA polymerase. So enzymes of lactose metabolism are not
produced or repressed. This occurs in the absence of lactose. Therefore absence of lactose
causes repression of enzymes that utilize it. When the lactose is present it combines with
repressor molecule and form complex. This prevents binding of repressor to operator gene.
So operator gene is free. RNA polymerase binds to operator gene and transcribes genes of
lactose metabolism. As a result enzymes of lactose metabolism are produced or induced
and cells use lactose. Therefore in presence of lactose (inducer)enzyme induction occurs.
12. Define recombinant DNA. How it is obtained? Write applications of
this technology.
A. Recombinant DNA is combination of DNA from two different species or organisms.
Preparation of recombinant DNA : Restriction enzymes are used for recombinant DNA
preparation. DNAs of two species or organisms are cut with same restriction enzyme to
generate sticky ends. DNA having sticky ends combines with another DNA containing
same sticky ends. Then a DNA ligase links these two DNA of different origin leading to
formation of recombinant DNA.
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CHAPTER - 13 | Replication, Transcription & Translation
Restriction
Enzyme
Human DNA
Human DNA
with Stick ends
DNA
Ligase
Rest of DNA
Restriction
Enzyme
Recombinant DNA
Bacterial DNA
Bacterial DNA
with Sticky ends
Applications:
1. Recombinant DNA technology is popularly known as genetic engineering. It is used to
produce pharmaceuticals or drugs.
2. Hormone insulin, interferons, growth hormones, blood clotting factors etc. are produced using
recombinant DNA technology.
3. It is used for development of DNA vaccines.
4. Gene therapy is another area where principles of recombinant DNA is utilized.
13. Write about PCR.
A. Polymerase chain reaction is known as PCR. In this technique DNA replication takes place
in cyclic manner in laboratory. Bacterial DNA polymerase which is stable to heat is used for
DNA amplification. Each cycle doubles the DNA amount. The product of the first cycle
becomes the template of the next cycle. . However replication of DNA by thermophilic
bacterial Taq DNA polymerase requires primers. Two oligonucleotide primers are
synthesized by using appropriate methods. These primers bind to DNA that is to be
amplified (replicated) at specific sequences on opposite strands. In the first step of a cycle
DNA to be amplified is exposed to heat to separate strands. In the second step of the cycle
primers are ligated to separated DNA strands at appropriate place. In the third step of cycle
Taq DNA polymerase is added to double initial two strands. Thus at completion of one cycle
initial amount of DNA is doubled. Likewise the initial DNA amount is amplified to several
times. Twenty cycles of PCR can amplify particular DNA segment to 105 or more times
Applications
1. PCR amplifies DNA rapidly and selectively
2. Very small amount (50-100 bp) of DNA from single cell or sperm cell or hair follicle can be
amplified to large quantities by PCR.
3. PCR is used to detect infectious agents in the body because infections are due to presence of
(viral or bacterial) foreign DNA.
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BIOCHEMISTRY - Questions and Answers
4. PCR is used for prenatal diagnosis of genetic diseases a which are due to alterations in DNA
like sickle cell anaemia, hemophilia etc.
5. PCR is used to detect certain cancers like leukemia, thyroid cancer etc.
6. PCR is used for tissue typing which is essential for organ transplantation.
7. In the forensic work amplification of little DNA recovered from the suspect or from the crime
site by PCR allows generation of sufficient DNA for finger printing.
8. DNA recovered from archaelogical materials (sites) is amplified by PCR and used to study
evolution or civilization.
9. PCR can be used to create extinct animals like dinosaurs by amplifing DNA recovered from
fossil materials.
DNA to be
Amplified
Strand
Separation
Primers
Taq DNA
Polymerase
2nd Cycle
3rd Cycle
4 Strands
16 Strands
8 Strands
14. Write a note on restriction endonucleases.
A. 1. These enzymes cut both DNA strands at specific sites.
2. They are present in prokaryotes and absent in eukaryotes.
3. They can not cut DNA of cell of their origin.
4. They hydrolyze only foreign DNA molecules. Since they restrict the entry of foreign
DNA into host DNA by cutting foreign DNA they are referred as restriction enzymes.
5. Several restriction enzymes are isolated from bacteria and viruses.
6. They differ in structural requirement, cleavage site and produces DNA fragments
with characteristic ends.
7. They are named from their origin. For example restriction endonuclease of E. coli is
known as EcoRI.
8. Restriction endonuclease of Para influenza which is known as Hpa I
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CHAPTER - 13 | Replication, Transcription & Translation
Applications :They are useful for
a) Analysis of chromosome structure.
b) Isolation of genes.
c) DNA sequencing.
d) Recombinant DNA technology.
e) RFLP (Restricted fragment length polymorphism).
15. Write about Blotting or Hybridization techniques.
A. Southern Blot, Northern Blot and Western Blot are known as blotting or hybridization
techniques.
Southern Blot
1. This technique is used for identification of DNA fragment of interest in sample of DNA.
2. First DNA sample is cleaved by restriction endonuclease into small fragments.
3. By agarose gel electrophoresis DNA fragment of interest is separated from rest of fragments.
4. Then DNA of interest is transferred on to nitrocellulose membrane by blotting.
5. A radiolabeled cDNA probe which recognizes DNA of interest is added.
6. Finally hybrid of cDNA and DNA of interest is visualized as band on x-ray film.
Northern Blot
1. This technique is used for identification of RNA from sample
2. RNA ase is used to cut given RNA sample into small fragments
3. RNA of interest is separated from rest by performing agarose gel electrophoresis
4. Separated RNA is transferred to nitro cellulose membrane by blotting.
5. Radiolabelled cDNA probe is added which recognizes RNA of interest and form hybrid.
6. The hybrid is visualized as band on x-ray film.
Western Blot
1. It is used to identify protein of interest from mixture of proteins.
2. By agarose gel electrophoresis desired protein is separated from remaining proteins.
3. To nitrocellulose membrane separated protein is transferred by blotting.
4. Radiolabelled monoclonal antibody probe is added which recognizes and form hybrid with
desired protein.
5. The hybrid is visualized as band on x-ray film.
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BIOCHEMISTRY - Questions and Answers
Other model questions are
16. Write about okazaki fragments.
17. How DNA polymerases are used in replication ?
18 Write about transcription factors.
19. Write a note on genetic engineering.
20. Write about factors of translation.
21. Write about formation and functions of RNA primers.
22 Write about genes of lac operon model.
23. Write a note on replication fork.
24. RNA polymerases
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CHAPTER - 14 | Vitamins
14
Chapter
Vitamins
1. Define vitamins. Classify. Give examples for each class.
A. Vitamins are small organic molecules which are not synthesized in the body and hence
must be present in diet. They are classified into
a. Fat soluble vitamins and b. Water soluble vitamins.
Fat soluble vitamins are Vit. A, Vit. D, Vit. E and Vit. K. Water soluble vitamins are Vit.
C and members of Vit. B complex. B complex vitamins are thiamine or Vit. B1,
riboflavin, niacin, pyridoxine, folic acid, cyanocobalamin or Vit. B12, Biotin and
pantothenic acid.
2. Write differences between water soluble and fat soluble vitamins.
A. The water soluble and fat soluble vitamins differ in several properties. They are
a. Solubility : Fat soluble vitamins are soluble in fats or organic solvents only. Water
soluble vitamins are soluble in water.
b. Bile salts : Fat soluble vitamins dependent on bile salts for their absorption. But water
soluble vitamins does not require bile salts for absorption.
c. Storage : Liver stores all fat soluble vitamins. Except vit. B12. other water soluble
vitamins are not stored.
d. Cooking conditions : Fat soluble vitamins are stable to normal cooking conditions. But
water soluble vitamins are unstable to normal cooking conditions.
e. Excretion route : Fat soluble vitamins are excreted in feces but water soluble vitamins
are excreted in urine.
3. Write chemistry, functions, deficiency symptoms, sources and daily
requirements of Vitamin A
A. Chemistry : Three compounds retinol (an alcohol), retinal (an aldehyde) and retinoic acid
(an acid) exhibit vit. A activity. They are also known as retinoids. These three form of vit A
are derived from a 20 carbon compound which contains ß- ionine ring and isoprenoid side
chain. Double bonds are present in side chain.
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BIOCHEMISTRY - Questions and Answers
Isomerism : Due to presence of double bonds in side chain Vit. A exhibits isomerism. The two
isomeric forms are all trans retinol and 11-cis –retinol.
CH2OH
RETINOL
ß-ionine ring –isoprenoid chain with double bonds.
Vit. A
Functions : 1. Retinal is required for normal vision as well as color vision.
Visual (Walds) cycle : This cycle begins with dissociation of rhodopsin ends with regeneration of
rhodopsin. Rods present in the eye contains vit. A containing visual pigment rhodopsin. It is a
conjugated protein and composed of 11-cis retinal and protein opsin. When light or photon strikes
11-cis- retinal is converted to all trans retinal. The apo protein opsin dissociates. The conversion
of rhodopsin to all trans retinal and opsin involves several intermediates. Meta rhodopsin is one
such intermediate. It generates nerve impulse by interacting with G- proteins present in rods
plasma membrane.
Light
Rhodopsin
All trans retinal + opsin
Nerve impulse.
Photons
The all trans retinal is reduced to all trans retinol by NAD dependent reductase. A small amount
of all trans retinal may be isomerized to 11-cis –retinal by an isomerase.
Isomerase
11-cis-retinal
Reductase NADH
All trans retinal
All trans retinol.
NAD
The 11-cis retinal combines with opsin to regenerate rhodopsin.
11-cis –retinal + opsin
Rhodopsin
However for normal vision this small amount of rhodopsin is inadequate so constant supply of Vit.
A1 is required for total regeneration of rhodopsin. From circulation rod cells take up all trans
retinol and isomerize to 11-cis –retinol by isomerase
rod cells
Diet
174
Blood
All trans trans retinol
11-cis-retinol.
CHAPTER - 14 | Vitamins
In retina an NAD+ dependent alcohol dehydrogenase converts 11-cis- retinol to 11-cis –retinal.
Dehydro
Genase Retina
+
11-cis-retinal + NADH+ + H +
11-cis-retinol +NAD
With availability of 11-cis- retinal in sufficient quantities more of rhodopsin is formed in retina.
Retina
11-cis-retinal+opsin
Rhodopsin
2. vit. A is required for growth and reproduction.
3. vit. A function as steroid hormone.
4. vit. A is required for glycoprotein synthesis.
5. vit. A promotes differentiation.
6. vit. A is required for integrity of epithelial alls of mucous layers of gastrointestinal, urinary,
respiratory, skin and salivary glands etc.
7. vit. A is required for nervous tissue growth and function.
8. vit. A is required for bone and tooth formation.
Vit. A deficiency symptoms
1. Nyctalopia or night blindness : It is a major Vit. A deficiency symptom. Affected persons
are unable to see in dim light or night light. So they are equal to blind during night. Hence the
name night blindness. It is common in people taking low Vit. A. If night blindness is not
treated it progresses to xerophthalmia.
Bitot's spots in conjuctive appear in children. If xerophthalmia is not controlled it leads to
keratomalacia in which corneal epithelium is degenerated. Finally ulceration of cornea may
lead to total blind ness.
2. Reproductive disorders like degeneration of testis, malformation and resorption of foetus
occurs.
3. Mucous linings of respiratory, reproductive, salivary and lacrimal glands are keratinized.
The condition is known as hyper kerotosis.
4. Keratinization of skin also occurs (Xeroderma).
5. Nervous tissue growth is affected.
6. Bone and tooth formation becomes defective.
7. Kidneys are degenerated.
Sources : Vit. A occurs in plant and animals.
Plants : In plant foods Vit. A is present as carotenes. Red palm oil is excellent source. Leafy
vegetables, yellow colored vegetables and fruits also contain vit. A. Amarnath leaves, curry
leaves, coriander leaves, spinach, cabbage and drumsticks are good sources. Yellow pigmented
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BIOCHEMISTRY - Questions and Answers
vegetables like carrot, pumpkin, sweet potato, tomato and bottle gourd are good sources. Yellow
fruits like mangoes, papaya, pineapple, jackfruit, bananas, oranges also contain vit. A.
Animals : In animal foods vit. A is present as retinol esters. Fish liver oils like halibut liver oil,
shark liver oil and cod liver oil are excellent sources. Liver of poultry and farm animals contain
vit. A. Dairy products buttermilk and eggs also contain vit. A.
Daily allowance : Adults : 2500 International units, one international unit=0. 3µg; 750ug of
retinol or 3mg carotene.
4. Write chemistry, functions, deficiency symptoms, sources and daily
requirements of Vitamin D
A. Chemistry : Two cholesterol derivatives ergo calciferol also known as vit. D2 and
cholecalciferol also known as vit. D3 exhibits vit. D activity.
CH2
CH2
HO
HO
Chole Calciferol Vit. D3
Ergocalciferol Vit. D2
Functions :
1. 1, 25-dihydroxy cholecalciferol is physiologically active form of vit. D. It is also known as
calcitriol.
Synthesis of calcitriol : In liver cytochrome P450 dependent hydroxylase forms 25-hydroxy
cholecalciferol from cholesterol.
Liver
Cholecalciferol + NADPH+O2
25- hydroxy cholecalciferol + NADP+ + H2O.
Cyt P450
Through the circulation it reaches kidney where it is converted calcitriol by α –hydroxylase
which is also dependent on cyt P450 and NADPH
25-hydroxy cholecalciferol +NADPH +O2
1, 25-dihydroxy
Cyt P450 Kidney
Cholecalciferol + NADP+ + H2 O.
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CHAPTER - 14 | Vitamins
2. Calcitriol increases calcium and phosphate absorption in the intestine. In the intestine
calcium absorption by calcitriol is promoted by two mechanisms.
a. In the intestine calcitriol increases synthesis of calcium binding proteins (CBP)which
promotes calcium absorption. From the plasma calcitriol enters intestinal cells. In the
cytosol of enterocyte it combines with receptor to form complex.
Receptor
Plasma
Enterocyte
calcitriol –receptor complex
calcitriol
This complex migrates into nucleus and interacts with DNA. As a result expression of gene
of calcium binding protein occurs.
Nucleus
Calcitriol- receptor
Transcription
DNA- calcitriol- receptor complex
mRNA of CBP
complex.
Translation
mRNA of CBP
Lumen
calcium binding protein
Increased calcium absorption.
b. Calcitriol promotes phosphate absorption in the intestine by different mechanism which
requires glucose and sodium.
3. Calcitriol promotes synthesis of osteocalcin, a calcium binding protein of bone. Osteocalcin
is required for bone formation and mineralization of bone.
4. Handling of calcium and phosphorus by kidney is influenced by calcitriol. It decreases
excretion of calcium and phosphorus by kidney.
5. Normal muscle tone is maintained by calcitriol.
Vit. D deficiency symptoms
1. Rickets : Vit. D deficiency causes rickets in children. In rickets bone and teeth formation are
affected. Skull, chest, spine, leg and pelvic deformities are commonly seen. Parietal and
frontal bossing and craniotabes are skull deformities. Pigeon breast is deformity of chest.
Bow legs or knock kees are deformities seen in legs. Usually in low income group children
vit. Ddeficiency occurs.
2. Osteomalacia : In adults vit. D deficiency causes osteomalacia. Pregment women and women
in pardha are prone to this disease. Skeletal pain and deformities of spine, legs, pelvis, ribs etc.
are seen.
3. Osteoporosis : In old people vit. D deficiency causes osteoporosis. Porous bones and bone pains
are common symptoms.
Sources : Marine fish liver oils are excellent sources. Halibut liver oil, shark liver oil, cod
liveroil are good sources. Other fish like sardines, egg and butter contain small amounts.
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BIOCHEMISTRY - Questions and Answers
Daily allowances : Adult : 5ug of vit. D or 200 international units. Pregnant and lactating
women : 10ug or 400 international units.
5. Write chemistry, functions, deficiency symptoms, sources and daily
requirements of Vitamin E.
A. Chemistry : Tocopherols are compounds that exhibit Vit. E activity. There are four types of
to copherols. They are α-tocopherol, ß-tocopherol, g-tocopherol and δ-tocopherol. Among all
α-tocopherol is most potent. These tocopherols are derivatives of tocol which is composed of
chromane ring and phytyl side chain.
HO
a - Tocopherol
Functions :
1. Vit. E functions as biological antioxidant. It is present in high concentrations in tissues
that are exposed to high oxygen tension like lungs, eyes and erythrocytes. It act as free
radial scavenger. It is present in membranes of cells, cell organelles and in cytosol. Protects
membrane lipids particularly poly unsaturated fatty acids (PUFA) from peroxidation by
peroxy radical. Peroxyradical is formed from PUFA in the membrane. PUFA peroxy
radical initiates free redial chain reaction. Tocopherol eliminates peroxy radical and thus
protects membrane lipids.
Peroxy radical + Tocopherol
Hydroperoxide + Tocopherol free redial
Tocopherol free redial + peroxy redical
Hydroperoxide + oxidized products
of tocopherol. Hydroperoxide and oxidized product of tocopherol are eliminated.
2. Vit. E is known as fertility factor for lower animals. It is required for sperm formation
in male rats and foetal development in female rats.
3. Vit. E is involved in control of muscle tone.
4. Vit. E promotes heme proteins synthesis.
5. Dietary carotenes and Vit. A are protected from oxidation by vit. E.
Vit. E deficiency symptoms
1. Hemolytic anelmia in children is main symptoms of vit. E deficiency. Since vit. E protects
erythrocytes from oxidative damage deficiency of Vit. E increases susceptibility of
erythrocytes to hemolysis.
2. Since vit. E is required for fertility, sterility is symptom of vit. E deficiency in male rats
and foetal resorption in female rats.
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CHAPTER - 14 | Vitamins
3. Vit. E Deficiency causes muscular dystrophy in animals like rat, rabbit, lamb etc.
Sources : Vegetable oils like ground nut oil, rice bran oil, sunflower oil, cotton seed oil, mustard
oil, palm oil and cereal germ oils like wheat germ oil, corn germ oil are good sources.
Daily requirement : Adults : About 10mg of vit. E per day. In pregnancy and lactation about 1213 mg of vit E is required pe day.
6. Define Pro vitamins. Give examples. How they are converted to
vitamins.
A. They are precursor forms of vitamins. For example carotene, 7-dehydrocholesterol and
ergosterol are provitamin forms of vit. A and vit. D respectively. A dioxygenase converts ßcarotene of diatary origin to 2 molecules of Vit. A retinal.
Dioxygenase
ß-carotene
2 retinal.
O2
In presence of ultra violet (UV) light provitamins of vit. D are converted to
corresponding vitamins.
UV light
7-dehydrocholesterol
cholecalciferol.
7. Write chemistry, functions, deficiency symptoms, sources and daily requirements
of Vitamin K.
A. Chemistry : Phylloquinone is the major from of vit. K present in plants. It is also known as
vit. K1 Menaquinone is the vit. K present in animals. It is termed as vit. K2 Both are
derivatives of naphthoquinone containing different side chains. Vit. K3 is menadione. It is
syntheticanalog of vit. K.
3
Vit. K1
Functions :
1. Vit. K is required for post translational modifications of blood clotting factors like
prothrombin, proconvertin, stuart factor and christamas factors.
2. It is required for carboxylation of these blood clotting factors. Gamma carbon of glutamate is
site of carboxylation. Hence it is also known as g- (Gamma) carboxylation. A carboxylase adds
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BIOCHEMISTRY - Questions and Answers
carbon dioxide to gamma carbon of glutamate (glu) residue of clotting factors in presence of
reduced vit. K1 to form gamma carboxylated glutamate (gla). Vit. K1 is converted to epoxide.
Carboxylase
Glutamate (glu) + Reduced vit. K1 + co2 + o2
Blood clotting factor.
Of blood clotting factor
Gamma carboxylated (Gla)
+ Reduced vit. K1 epoxide + H2 o.
Reduced vit. K1 is regenerated from vit. K1 epoxide by epoxide reductase in presence of NADPH
+H+.
Reductase
+
Vit. K1 epoxide +NADPH +H
Reduced vit. K1+ NADP+.
This post translational modification is essential for blood clotting. It promotes binding of
calcium to gamma carboxyl groups of prothrombin. This leads to conversion of prothrombin to
thrombin. This vit. K dependent Carboxylation is known as vit. K cycle.
3. Another calcium binding protein osteocalcin in bone also under goes vit. K dependent gamma
carboxylation of glutamate residues. A vit. K dependent carboxylase catalyzes this reaction.
Vit. K deficiency symptoms
1. Rare in adults. However in pre Mature babies vit. K deficiency occurs. Main symptom is
haemorrhage due to increased prothrombin time.
2. Prolonged use of antibiotics may produce vit. K deficiency in adults.
Sources : Green leafy vegetables like cabbage, spinach, turnipgreens, cauliflower, green peas
and soybean peas are good sources. Poultry products like eggs, liver dairy products like cheese
and butter are also good sources.
Daily requirement : Adults : 70-140mg/day, pregnancy and lactation 150-200 mg/day.
8. Write chemistry, functions, deficiency symptoms, sources and daily
requirements of Vitamin-C
A. Chemistry : Ascorbic acid, a sugar acid is known as vit. C. It undergoes oxidation easily in
at mospheric oxygen to dehydroascorbic acid.
O2
Ascorbic acid
Dehydroascorbic acid
High temperature, acid and alkali promotes oxidation. Oxidized vit. C is functionally
less active.
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CHAPTER - 14 | Vitamins
Functions :
O
1. Vit. C is biological antioxidant. It eliminates free radicals. It is free radical
scavenger. Dietary vit. C þrevents oxidation of carotenes, vit. E and B
complex vitamins present in diet.
2. Vit. C is required for bone and teeth formation.
3. It is involved in post translational modification of collagen. It is required
for hydroxylation of proline and lysine residues of collagen.
4. It is required for the absorption of iron. It maintains iron in ferrous
form.
5. It also participates in metabolic reactions.
6. Hydroxylation of steroid hormone requires vit. C
7. Bile acid formation requires vit. C
C
|
HO – C
O
HO – C
|
C
|
HO – C – H
|
CH2OH
Ascorbic Acid
or Vit. C
8. Catecholamines formation requires vit. C
9. Tyrosine metabolism requires vit. C.
10. Carnitine biosynthesis requires vit. C.
11. In high doses it reduces severity of cold and other infections.
Vit. C deficiency symptoms
1. Scurvy is main vit. C deficiency disease. Usually it occurs in mariners who stays on sea for
prolonged time.
2. Scurvy symptoms are internal hemorrhages and prone to bone fractures and infections.
3. Since vit. C is required for collagen synthesis capillaries are prone to ruptures.
4. Delayed wound healing, swollen gums and joints, loose teeth and anemia are also seen in
scurvy affected people.
5. In infants vit. C deficiency causes infantile scurvy.
Sources : Citrus fruits like lemon, orange, pineapple and grapes are good sources. Amla also
known as Indian goose berry is excellent source. Other fruits like guava, papaya, mango,
apples and bananas also contain this vitamin in good amounts. Among vegetables tomato,
cabbage, cauliflower, potato and leafy vegetables coriander leaves, amaranth leaves and
spinach leases also contain vit. C
Daily requirement : Adult : 60-80 mg/per day.
9. Write functions and deficiency symptoms of thiamin
A. Functions :
1. Thiamin pyrophosphate (TPP)or thiamin diphosphate (TDP) is active form of thiamin.
It serve as coenzyme of oxidative decarboxylation reactions and transketolase
reactions.
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BIOCHEMISTRY - Questions and Answers
CH2 – CH2 – O – (P) – O – (P)
CH2 – N
N
S
CH3 N
Thiamin Diphosphate (TDP)
a. Pyruvate dehydrogenase and α-ketoglutarate and α-keto adipate dehydrogenase
reactions TDP function as coenzyme.
Pyruvate
Acetyl- CoA, α-ketoglutarate
TPP
a- ketoadipate
Succinyl –CoA
TPP
Glutaryl-CoA.
TPP
Transketolase
b. Xylulose -5-phosphate +ribose -5- phosphate
sedoheptulose-7- phosphate
TPP
+glyceraldehyde -3- phosphate.
Deficiency symptoms :
1. Thiamin deficiency occurs in polished rice consuming areas. Beri beri is major
deficiency symptom. Two types of beri beri are identified.
a. Wet beri beri : In this type of beri beri cardiovascular system is affected. Edema is
seen in lower limbs, trunk, face etc.
b. Dry beri beri : Only central nervous system is affected. No edema. Affected people
are unable to walk and usually bed ridden.
2. Thiamin deficiency in infants causes infantile beri beri.
3. In birds thiamin deficiency causes polyneuritis.
10. Write functions and deficiency symptoms of riboflavin.
A. Functions :
1. FMN and FAD are active forms of riboflavin. FMN is flavin mononucleotide and FAD is
flavin adenine dinucleotide. FMN and FAD function as carriers of hydrogen atoms in
oxidation and reduction reactions.
Flavin – Ribitol – O – P – O – P – Ribose – Adenine
Flavin Adenine Dinucleotide (FAD)
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CHAPTER - 14 | Vitamins
A. Aminoacid oxidase, NADH –CoQ reductase are enzymes requiring FMN.
B. Succinate dehydrogenase and acyl-CoA dehydrogenase requires FAD as
coenzyme.
Deficiency Symptoms :
1. Riboflavin deficiency causes angular stomatitis, cheliosis and glossitis in humans.
Dermatitis of certain parts like nasolabial regions, urogenital regions also seen.
2. In experimental animals riboflavin deficiency causes eye lesions and growth
impairment.
11. Write functions and deficiency symptoms of Niacin
A. Functions
1. Niacin is converted to NAD (Nicotinamide adenine dinucleotide) and NADP
(Nicotinamide adenine dinucelotide phosphate) in the body. NAD and NADP act as
carriers of hydrogen atoms in oxidation and reduction reactions.
CONH2
N
Niacin
Adenine – Ribose – O – P – O – P – O – Ribose - Nicotinamide (Niacin)
Nicotinamide Ademine Dinucleotide (NAD)
a. Some NAD dependent enzymes are malate dehydrogenase, isocitrate
dehydrogenase, glyceraldehyde-3- phosphate dehydrogenase etc.
b. NADP dependent enzymes are glucose -6- phosphate dehydrogenase, glutamine
reductase etc.
Deficiency symptoms :
1. Pellagra is deficiency symptom of niacin in man. It is common in maize eating
countries. Dermatitis, diarrhea and dementia are characteristic symptoms of pellagra.
2. Niacin deficiency in experimental animals causes black tongue.
3. In some cases stomatitis and glossitis i. e. Inflammtion of oral cavity and tongue.
12. Write functions and deficiency symptoms of pyridoxine.
A. Functions : Pyridoxal phosphate (PLP) is active form of pyridoxine. It serve as coenzyme of
enzymes involved in transamination, decarboxylation, nonoxidative deamination,
desulfration, transsulfuration etc. reactions.
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BIOCHEMISTRY - Questions and Answers
CH2OH
CHO
CH2OH
HO
H3C
N
Pyridoxine
CH2-O-(P)
HO
H3C
N
Pyridoxal Phosphate
1. Pyridoxal phosphate is coenzyme of several transaminases like alanine transaminase,
aspartate transaminase etc.
2. Pyridoxal phosphate is coenzyme of glutamate decarboxylase, dopamine decarboxylase
etc.
3. Pyridoxal phosphate is coenzyme of trans sulfuration enzymes like cystathionine
synthase, cystathionine lyase etc.
4. Pryidoxal phosphate is coenzyme of desulfuration enzyme cysteine desulfhydrase.
5. Pyridoxal phosphate is coenzyme of non oxidative deamination enzymes like serine
dehydratase.
6. Pyridoxal phosphate is part of phosphorylase of glycogenolysis.
7. Pyridoxal phosphate is coenzyme of ALA synthase, kynureninase of tryptophan
metabolism.
Deficiency symptoms :
1. Microcytic hypochromic anaemia is common symptom due to decreased heme
synthesis.
2. Epileptic from convulsions occurs in children of pyridoxine deficiency due to decreased
production of g-aminobutyric acid (GABA).
3. Growth impairment, decreased mental ability, convulsions, skin lesions etc. are seen in
experimental pyridoxine deficiency humans and other animals.
13. Write functions and deficiency symptoms of Biotin
A. Functions : Biotin is the only water soluble vitamin that function as coenzyme as such. It is
coenzyme for carboxylases that catalyze carboxylation reactions. Acetyl-CoA carboxylase
of fatty acid biosynthesis, propionyl –CoA carboxylase and pyruvate carboxylase of
gluconeogenesis are some enzymes requiring biotin as coenzyme.
Deficiency symptoms :
1. Since biotin is well distributed in foods its deficiency rarely occurs in humans. However
in breast fed infants dermatitis occurs due to low biotin content in breast milk.
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CHAPTER - 14 | Vitamins
2. In experimentally induced biotin deficiency neurological problems like depression,
anemia, muscular pain, loss of hair and dermatitis are observed.
14. Write functions and deficiency symptoms of Folic acid
A. Functions : Coenzyme form of folic acid is tetrahydrofolic acid FH4.
H2N
N
N
O
N
CH2 – N –
H
N
– C – N – Glutamate
H
OH
Folic Acid
Synthesis of FH4 : An NADPH dependent dihydrofolate reductase reduces folic acid to
tetrahydrofolic acid in two steps. Dihydrofolic acid is intermediate.
Reductase
+
Folic acid +NADP +H
Reductase
Dihydrofolic acid
Tetrahydrofolic Acid.
FH2
NADP
+
NADPH+H+
FH4 + NADP+.
1. FH4 actas carrier of one carbon units. It carries one carbon moieties like methyl, formyl,
formimino and formate groups. In degradative pathways FH4 with one carbon unit is
generated. In anabolic pathways one carbon is transferred to an acceptor.
Transferase
a. Formimino glutamate+FH4
b. Methyl FH4 +cobalamin
c. Formyl FH4
Formimino FH4 + Glutamate
Methyl cobalamin +FH4
C -2 of Purine ring
d. Methenyl FH4
carbon 8 of purine ring.
Deficiency symptoms :
1. Megaloblastic anaemia is major folic acid deficiency in man. Folic acid deficiency
affects rapidly growing cells like bone marrow cells, intestinal cells because nucleotide
formation requires folic acid.
2. Thrombocytopenia
3. Macrocytic hyperchromic anemia and leucopenia
4. Diarrhoea and weakness
5. Growth impairment, intestinal ulceration and anemia are deficiency symptoms in
experimental animals.
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BIOCHEMISTRY - Questions and Answers
15. Write chemistry, absorption, transport, functions, deficiency
symptoms, sources and daily allowance of Vitamin B12
A. Chemistry : It consist of corrin ring
CH3
with central cobalt atom. Corrin ring
is tetrapyrrole. To the central cobalt
N
N
various groups are attached. So Vit.
B12 with cyanide is known as cyano
Co
cobalamin. Vit. B12 with hydroxyl
N
group attached to central cobalt is
N
called as hydroxy cobalamin. If
R
methyl group is attached then it is
Methyl Cobalamin
called as methyl cobalamin. Deoxy
adenosine containing Vit. B12 is called
as deoxy adenosyl cobalamin. Most common is cyano cobalamin.
Absorption and Transport : Absorption of Vit. B12 requires intrinsic factor. It is secreted
by parietal cells of stomach and it is a glycoprotein.
Dietary Vit. B12 combines with intrinsic factor (IF) to form complex.
Dietary Vit. B12 + Intrinsic factor
Vit. B12 - IF complex.
Through a receptor mediated mechanism this complex is absorbed in the ileum. The
complex dissociates in ileal cells to IF and Vit. B12.
Receptor
Vit. B12 –IF
Vit. B12 –IF
Ilealcells
Vit. B12 + IF
A Vit. B12 Transport protein known as transcobalamin in ileal cells transport Vit. B12 to
various tissues of the body.
Transcobalamin
Vit. B12 in ileal cell
Functions :
Vit. B12 – Transcobalamin complex
Tissues
Vit. B12 is required as coenzyme by many enzymes. They are known as
cobamide coenzymes. Two cobamide common enzymes are methyl cobamide or methyl
cobalamin and deoxy adenosyl cobamide or deoxy adenosyl cobalamin.
1. Methyl cobalamin is coenzyme of methionine synthesis.
Methionine Synthase
Homocysteine +Methyl-cobalamin
Methionine +cobalamin.
2. Methyl malanonyl –CoA mutase requires deoxy adenosyl cobalamin as coenzyme.
Methylmalonyl –CoA
186
Succinyl –CoA.
CHAPTER - 14 | Vitamins
Deficiency Symptoms :
1. Megaloblastic anemia with neurological disturbances is main symptoms of Vit. B12
deficiency. In Vit. B12 deficiency methionine sythesis is blocked. So neurotransmitter
formation is affected. This results in neurogical problems like numbness in feet and
hands and spinal cord degeneration.
2. Since Intrinsic factor is required for Vit. B12 deficiency lack of this factor also causes vit.
B12 deficiency. However it is called as pernecious anemia. Here also three systems like
erythropoietic, gastro intestinal and neurological system are affected.
Sources : Vit. B12 is present mainly in animal sources. Organ meat like liver, kidney,
brain, dairy products eggs and fish are good sources. Milk and milk products also
contain this vitamin.
Daily requirement : 3mg per day.
16. Write functions and deficiency of Pantothenic acid
A. Functions :
1. Pantothenic acid is required for the synthesis of coenzyme A (CoA). CoA is involved in
metabolism of carbohydrates, lipids and proteins.
2. Fatty acid synthase complex contains phosph pantotheine that serve as acyl carrier.
Formation of coenzyme (CoA) and phosphopantotheine
1. Synthesis of Co A from pantothenic acid involves phosphorylation, cysteinylation,
decarboxylation, adenosylation and phosphorylation.
Phosphorylation
Pantothenic acid
Cysteinylation
phosphopantothenate
De carboxylation
Phosphopanto theinyl cysteine
Adenosylation
Phosphopantotheine
DephosphocoenzymeA
phosphopantotheine
fatty acid synthase
Phosphorylation
Dephosphocoenzyme A
coenzymeA (CoA).
Deficiency : Burning feet, fatigue, restlessness and abdominal cramps are noticed in
experimentally deficiency induced human subjects.
2. In other animals experimental pantothenic acid deficiency causes dermatitis, growth
impairment and neurological symptoms.
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BIOCHEMISTRY - Questions and Answers
17. Write a note on Anti vitamins
A. They are antagonists of vitamins. They cause vitamin deficiency. Some anti vitamins are
used as drugs. They are present in foods. Some drugs are anti vitamins.
1. Avidin present in raw egg white binds biotin and prevent its absorption. This leads to
biotin deficiency.
2. Vit. K antagonists are used as anti coagulants. Dicoumarol and warfarin are Vit. k
antagonists used as anticoagulants.
3. Folic acid antagonists are used as anti cancer drugs. They are aminopterin and
amethopterin.
4. Isoniazid is used in treatment of tuberculosis. It prevents formation of pyridoxal
phosphate. So pyridoxine deficiency is likely to occur in isoniazid treatment.
5. Thiaminase destroys thiamin of foods. Hence thiamin deficiency occurs.
Other model questions are
18. Visual cycle
19. Nyctalopia
20. Calcitriol
21. Rickets
22. Vit. K cycle.
23. Tocopherol
24. Biological antioxidants
25. Beri Beri
26. Coenzyme functions of thiamin
27. Write reactions involving coenzymes of riboflavin.
28. Write coenzyme form of niacin. Write reactions involving the
coenzyme.
29. Scurvy
30. Avidin
31. Pellagra
32. Pyridoxine
33. Folicacid
34. PLP
35. Absorption of vit. B12
36. RDA of vit. C & A
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CHAPTER - 15 | Minerals
15
Chapter
Minerals
1. Define minerals. Classify. Give examples for each class.
A. Minerals are small inorganic molecules. They are not produced in the body. So diet is main
source. Based on requirement they are classified into.
a. Bulk minerals : Are those minerals that are required in more than 100 mg per day.
They are calcium, phosphorus, sodium, potassium, chloride and magnesium.
b. Trace minerals : Are those minerals that are required in less than 100 mg per day.
They are iron, iodine, fluorine, manganese, copper, zinc, selenium, molybdenum, cobalt
and chromium.
2. Write functions of minerals.
A. Functions of minerals :
1. Minerals are component of soft tissues like liver, kidney, lung etc.
2. Minerals are components of bone structure.
3. Minerals are required for several essential functions like blood coagulation, muscle
contraction, neuro transmission, membrane potential and blood pressure.
4. Minerals are involved in acid base balance.
5. Minerals are required for actions of enzymes.
6. Minerals are structural components of hemoglobin, cytochromes, vitamins, nucleic
acids etc.
7. Minerals are involved in hormone action.
8. Minerals are involved in transport of gases.
3. Write absorption, functions, deficiency symptoms, sources and daily
requirements of Calcium
A. Absorption : Duodenum and first part of jejunum are major sites of calcium absorption.
Calcitriol promotes calcium absorption against concentration gradient by active transport
mechanism.
Factors affecting calcium absorption : Absorption of calcium in the intestine is influenced
by several factors.
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BIOCHEMISTRY - Questions and Answers
1. Calcium binding protein (CBP) : In the intestinal cells calcium binding protein is
synthesized. Calcitriol promotes synthesis of this protein. This protein is known as
calbindin and increases calcium absorption in the intestine.
2. Dietary calcium and phosphorus ratio affects calcium absorption. High phosphate
decreases calcium absorption.
3. High fibre in diet interferes with calcium absorption.
4. PH or hydrogen ion concentration : Calcium absorption requires acid or neutral PH.
Calcium absorption is less under alkaline or high PHconditions.
5. Calcium absorption depends on fat digestion and absorption. If fat digestion and
absorption are impaired then calcium absorption is decreased.
6. Calcium absorption is affected by protein content in diet. Low protein in diet
decreases calcium absorption. High protein in diet increases calcium absorption.
7. Calcium absorption is decreased in presence of salts of oxalates. Because calcium
combines with oxalates and forms insoluble calcium oxalate.
8. Calcium absorption is decreased if phytic acid is present. Cereals of diet contains
phytic acid.
Functions : Calcium is involved in several important physiological processes.
1. Calcium is required for muscle contraction.
2. Neurotransmission is dependent on calcium.
3. Bone and teeth formation requires calcium. Calcium is major constituent of bone and
teeth.
4. Blood clotting is another process depends on calcium. During blood clotting calcium is
required for conversion of prothrombin to thrombin.
5. Actions of several hormones are mediated through calcium. Hence calcium is known as
secondary messenger of hormone action.
6. Calcium is required for cell motility involving cellular activities like mitosis, migration etc.
7. It is required for action of cytosolic calcium dependent proteases calpains.
8. Many enzymes requires calcium for their action. For example enzymes of HMP shunt like
glucose -6- phosphate dehydrogenase, lactonase, phosphogluconate dehydrogenase etc.
9. Enzyme activation also dependent on calcium. For example trypsin activation occurs in
presence of calcium.
10. In some marine organisms calcium triggers bioluminescence. Jelly fish exhibits this
property.
11. Calcium is involved in membrane structure and function.
12. Membrane transport requires calcium. Membrane integraty is maintained in presence of
calcium.
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CHAPTER - 15 | Minerals
Deficiency : Deficiency of calcium causes growth impairment, osteoporosis, tetany,
hyperplasia of para thyroid glands.
Sources : Milk is rich calcium source. Green leafy vegetables, eggs, fish, vegetables and fruits
are fair sources.
Dairy requirement : Adults : 800mg/day, children : 1200 mg / day.
4. What is normal plasma calcium level? Write about regulation of
plasma calcium
A. Normal plasma calcium level is 9-11mg %. Calcium exist in three forms in plasma.
Physiologically active form or ionized form : It accounts for 50-60%of total calcium. It is only
responsible for most of physiological actions of calcium.
Protein bound : It is bound to protein like albumin. It is about 35-40% of total calcium.
Non protein molecules like citrate, phosphate and bicarbonate also found in bound form with
calcium. It accounts for about 5% of total calcium.
Stable calcium is required for majority of its actions. So calcium level must be finely regulated.
Many hormones and substances are responsible for maintenance of constant calcium level.
They control calcium level within normal limits by acting at different sites. They are intestine,
bone and kidney. Para thyroid hormone (PTH), calcitonin and calcitriol are substances and
hormones involved in regulation of plasma calcium level.
Parathyroid hormone secretion : Secretion of parathyroid hormone by parathyroid gland
depends on calcium level in plasma. Decreased plasma serum calcium level is sensed by
calcium receptors present in parathyroid cells. They inturn react with G- proteins present in
membrane of parathyroid cells. The G- proteins activate phospholipase –C to produce inositol
triphosphate (IP3)which raises intracellular calcium by acting on calcium stores and opening
calcium channels. The raised intra cellular calcium leads to binding of calcium to intracellular
calcium binding protein calmodulin to form calcium calmodulin complex This complex
increases
cAMP by inhibiting phosphodiesterase. Increased
intracellular cAMP cause
release of PTH.
Low plasma calcium level
Calcium receptors
G- Protein
G-proteins
Calcium stores
Phospholipase –C
Inositol triphosphate (IP3)
Increased
Calcium channels
Calmodulin
Intracellular calcium
calcium –calmodulin
More cAMP
PTH release.
complex.
Para thyroid actions : A receptor present in membrane surface transport PTH into target
cells. PTH target tissues are bone, kidney and intestine. It increases plasma calcium level by
acting on these organs In bone it increases movement of calcium from bone by promoting
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BIOCHEMISTRY - Questions and Answers
dissolution of bone matrix. This action of PTH is mediated by cAMP. By acting on adenylate
cylase PTH increases cAMP. In the kidney PTH increases calcium reabsorption. In the
intestine PTH
increases calcium absorption by promoting formation of calcitriol. The
calcitriol also promotes calcium absorption in the kidney. Thus the combined actions of PTH
and calcitriol raises plasma calcium level to normal. With restoration of plasma calcium level
to normal actions of PTH and calcitriol are inhibited.
PTH
bone
PTH
calcitriol
Calcitriol
kidney
Release of calcium
Raised plasma calcium level
increased calcium absorption
More reabsorption of calcium
Raised calcium level.
Raised calcium level.
Calcitonin : It is another hormone involved in calcium regulation. It is produced by thyroid
gland. It is an antagonist of para thyroid hormone. Secretion of calcitonin by c cells of thyroid
gland is stimulated by raised plasma calcium level. On release calcitonin decreases plasma
calcium by acting on bone. A receptor mediated process translocates calcitonin into osteoclasts
of bone. In the osteoclasts calcitonin decreases release of calcium from bone. It prevents bone
resorption by osteoclasts. cAMP mediates action of calcitonin. Its level increases in presence of
calcitonin.
5. Write about diseases associated with calcium level in plasma.
A. Diseases of plasma calcium leval
a. Hypocalcemia : Decrease in plasma calcium level is known as hypocalcemia. It occurs in
vit. D deficiency, hypopara thyroidism, renal insufficiency, rickets and osteomalacia.
Decrease in plasma calcium level causes tetany. Clinical symptoms of tetany are hyper
excitability of nerves of face and extremities and muscular spasms.
b. Hypercalcemia : Increased plasma calcium level is known as hypercalcemia. It occurs
in hyper parathyroidism, vit. D toxicity etc. In infants idiopathic hypercalcemia occurs.
Clinical symptoms are cardiac abnormalities, neurological disturbances.
6. Write the functions of phosphate
A. 1. Phosphate is present in humans to an extant of about 500-700gm. More of it is present
in bone and teeth.
2. In the body phosphate is present in two forms. An inorganic form found in bone and
teeth which is complexed with calcium and magnesium. Another is organic form. It is
present as organic compounds in the cells and cell membranes.
3. In the bone and teeth it is present as hydroxyl apatite.
4. It is a component of nucleotides and nucleic acids.
5. High energy compounds like ATP, GTP, CTP etc contain phosphate.
6. Phospholipids formation requires phosphate.
7. Phosphate is component of blood buffers.
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CHAPTER - 15 | Minerals
8. Formation of some water soluble vitamins coenzymes involves phosphate like TPP,
pyridoxal phosphate etc.
9. Several cellular functions as well as enzyme regulation involves addition or removal of
phosphate.
10. Degradation of several compounds involves phosphate containing intermediates.
11. Second messenger molecules cAMP, cGMP are phosphate containing substances.
12. Milk protein casein is phosphoprotein i. e. phosphate containing conjugated protein.
7. Write normal phosphate level in blood and disorders of it.
A. Blood phosphate : Normal plasma phosphate level is 2. 5- 4. 5mg%. Blood phosphate level is
higher in children. It is about 4-6mg%. Two types of blood phosphate level alterations
occurs.
Hypophosphatemia : Blood phosphate level is below normal. It occurs in hyper
parathyroidism, vit. D deficiency, Fanconi syndrome etc.
Hyperphosphatemia : Blood phosphate level is elevated. It occurs in hypo para thyroidism
and vit. D toxicity. Since kidney filters phosphate from plasma in renal failure and
nephritis hyper phosphatemia occurs.
8. Write a note on Sodium functions.
A. Functionsof sodium
1. Sodium is present in high concentration in extra cellular fluids than intracellular
fluids.
2. Sodium is constituent of buffers of blood. So it is required for proper hydrogen or pH
homeostasis.
3. It is involved in nerve impulse transmission.
4. It is required for muscle contraction.
5. It is essential for the absorption of glucose in intestine.
6. Intestinal absorption of amino acids involves sodium ion co transport.
7. It is constituent of bile salts.
8. It is required for the action ATPases or pumps.
9. It is essential for growth of tissues.
9. Write functions of Potassium
A. 1. Potassium is present in high concentration in intracellular fluid than extra cellular
fluid. Hence it is major cation inside cell.
2. It is involved in membrane potential maintenance.
3. Potassium is involved in muscle function.
4. It has role in nerve impulse transmission.
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BIOCHEMISTRY - Questions and Answers
5. It is essential for bile salt formation
6. It is required for the action of ATPase.
7. Tissue growth requires potassium
8. Storage of glycogen in liver requires potassium
9. Muscle glycogen storage involves use of potassium
10. Name functions of Chloride
A. Functions of chloride :
1. It is constituent of hydrochloric acid (HCl) produced by parietal cells of stomach.
2. It is more in extra cellular fluid than intracellular fluid.
3. It is required for proper erythrocyte function i. e chloride shift.
4. It is involved in maintenance of plasma volume.
5. It is activator of some enzymes like amylase, angiotensim converting enzyme etc.
6. It is involved in neurotransmission.
11. Mention Magnesium functions
A. Magnesium Functions :
1. Like potassium magnesium is more in intracellular fluid than extra cellular fluid.
2. It is required for bone and teeth formation.
3. It is essential for kinases catalyzed reactions in carbohydrate metabolism, protein
metabolism, nucleic acid metabolism.
4. It is also required for other enzymes like RNA polymerase, glucose-6-phosphate
dehydrogenase, enolase, transketolase etc.
5. It also act as activator of some enzymes.
12. Write absorption, transport, storage, functions, deficiency
symptoms, sources and daily requirement of Iron
A. Absorption : Iron occurs in the bound state in plant foods in the ferric (Fe3+) form. However
in stomach acid environment iron dissociates and get reduced to ferrous form (Fe2+). Vit. C
and cysteine are compounds that promote iron reduction. In the stomach an iron
combining protein gastroferrin is also involved in iron absorption. An unknown
mechanism is responsible for the absorption of gastroferrin and ferrous iron in duodenum
and jejunum.
In animal foods iron occurs in heme. Heme is absorbed as such in the mucosal cells of small
intestine. In the enterocyte iron is released from heme. In the mucosal cells a copper
containing enzyme ceruloplasmin forms ferric iron by oxidation from ferrous iron. This
ferric iron undergoes to storage and transport.
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CHAPTER - 15 | Minerals
Acidic pH
Plant foods iron
vit. C
ferric iron
Ferrous iron
Intestinal mucosal cells.
Enterocyte
Anlmal food iron
protoporphyrin +ferrous iron.
ceruloplasmin
Ferrous iron
Ferric iron
storage and transport
Regulation of iron absorption : Hepcidin a 25 aminoacid peptide synthesized by liver
regulates iron absorption. If sufficient iron is absorbed further absorption is blocked by
hepcidin. Hemojuvelin another peptide also influences iron absorption by regulating
expression of hepcidin.
Iron transport : Iron enters plasma from mucosal cells of intestine aided by ferroportin. In
the plasma an iron transport protein apotransferrin is present. It combines with iron to
form transferrin. Iron is transported to various sites by this transferrin.
plasma
Iron of intestine
Transferrin
Various site of body.
Apo transferrin
Storage of iron : Liver. spleen, bone marrow and intestine are the organs where iron is
stored. These tissue contain iron storage protein apo ferritin. It combines with ferric iron to
form ferritin and stored.
Fe3+
Apoferritin
Ferritin
storage.
Functions :
1. Iron is required for the formation of hemoglobin, myoglobin and cytochromes.
2. Hemoglobin and myoglobin transport oxygen. Oxygen is attached to iron of these
molecules.
3. Cytochromes iron is involved in electron transfer or oxidation reduction reactions.
4. Iron is required for the formation of iron sulfur proteins.
5. Iron of these proteins participate in oxidation reduction reactions.
6. Iron is constituent of several enzymes.
7. Tryptophan dioxygenase, homogentisic acid oxidase, xanthine oxidase, catalase,
cytochrome p 450 – hydroxylase enzymes etc are examples for iron containing enzymes.
8. Lactoferrin present in milk is iron containing protein.
Deficiency : Since iron is essential for production of blood anemia is main symptom of iron
deficiency. Popularly it is called as iron deficiency anemia. Skin of affected people acquires
pale yellow color. Microcytic hypochromic erythrocytes are seen in blood. Usually children,
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BIOCHEMISTRY - Questions and Answers
pregnant women, adolescent girls are susceptible to this disease. Other clinical symptoms
are fatigue, breathlessness and giddiness. It is major nutritional problem in developing
countries.
Sources : Green leafy vegetables like spinach, coriander leaves, palak etc and cereals,
legumes and jaggary are good sources. Organ meat like liver, kidney, spleen etc and farm
products like eggs and fish are fair sources.
Daily requirement : 10mg/day, females : 18mg/day. In pregnancy and lactation it is about
30mg/day.
13. Write functions and deficiency symptoms of iodine.
A. Iodine Functions :
1. It is constituent of thyroid hormones.
2. Thyroxine and triiodotyronine (T3) synthesis involves iodine utilization.
3. Thyroid hormones are essential for mental development as well as physical
development.
Deficiency symptoms :
1. Goitre, Swollen thyroid gland is major symptom of iodine deficiency. It is seen more in sub
Himalayan regions of India. It occurs in other developing countries like China.
2. In children iodine deficiency causes decreased mental abilities.
3. In adults iodine deficiency causes apathy.
4. Radiation effects susceptibility is more in iodine deficiency.
5.
Mental disturbances, hypothyroidism and iodine induced hyper thyroidism are other
symptoms of iodine deficiency.
14. Write functions and deficiency symptoms of Zinc
A. Functions :
1. Zinc is an integral part of more than 2oo enzymes. Zinc metalloenzymes are involved in
carbohydrate metabolism, aminoacid metabolism, bone metabolism, nucleic acid
metabolism, electrolyte and blood pressure maintenance, gas transport and super oxide
metabolism. Some examples are lactate and malate dehydrogenase of carbohydrate
metabolism, carboxy pepetidase of protein digestion, DNA and RNA polymerase of nucleic
acid biosynthesis, alkaline phosphatase of bone metabolism, angiotensinconverting
enzyme of electrolyte and blood pressure regulation, carbonic anhydrase of gas transport
and super oxide dismutase of superoxide metabolism.
2. Zinc is constituent of zinc finger proteins which are involved in regulation of gene
expression.
3. Gustin is a zinc containing protein involved in taste bud development.
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CHAPTER - 15 | Minerals
4. Zinc is required for stability of hormone insulin.
5. Zinc is involved in maintenance of structure of ribosomes and chromatin.
6. It is involved in immune system function.
7. It is essential for tissue growth, development and regeneration.
8. For normal reproduction zinc is needed.
9. Bone and muscle formation involves participation of zinc.
Deficiency symptoms :
1. Zinc deficiency occurs in a rare genetic disease acrodermatitis enteropathy. In this disease
zinc deficiency is due to block in absorption of zinc. Clinical symptoms are acral dermatitis and
diarrhoea.
2. Hypogonadism and growth retardation are symptoms of zinc deficiency.
3. Loss of taste occurs in zinc deficiency.
4. Dermatatis and immune dysfunction are observed in experimentally induced zinc deficiency.
15. Mention functions and deficiency symptoms of Fluorine
A. Functions :
1. Fluorine is required for bone and teeth formation. In the teeth it is present in dentin in
fluoroapatite form.
2. Fluorine protects teeth enamel from action of bacterial acids. In the oral cavity fluorine act as
inhibitor of bacterial enzymes. Bacteria use glycolysis for energy production. Since fluorine
inhibits glycolytic enzymes bacterial growth is prevented. Therefore in oral cavity fluorine
prevent solubilization of enamel of teeth by acids produced by bacteria.
3. In adults fluorine prevents osteoporosis.
4. In elder people progressive loss of hearing occurs as age advances. Fluorine may slow down
this process.
Deficiency symptoms :
1. Dental caries is the major symptom of fluorine deficiency.
2. Dental caries is characterized by cavities on tooth surface due to loss of enamel.
16. Write a note on Fluorosis and defluoridation.
A. Consumption of drinking water containing excess fluorine leads to fluorosis. Fluorosis is
seen in several states of this country. It affects people of all ages. It is major health
problem in several districts of Andhra Pradesh. There are two types of fluorosis. They are
1. Dental fluorosis and 2. Skeletal fluorosis.
Dental fluorsis : Molted teeth is major symptom. Due to loss of enamel teeth appears dull,
patches and cavities are produced on surface of teeth.
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BIOCHEMISTRY - Questions and Answers
Skeletal fluorosis : Knock knees or genu valgum is main skeletal deformity seen. Major joints
becomes stiff and painful. Neurological disturbances due to spine deformities also occurs.
Defluoridation : It is process used to remove excess fluorine present in drinking water. Several
defluoridation equipments and portable units are developed by various agencies. Most of them
use absorbents and activated charcoal to eliminate excess fluorine from water.
Other model questions are
17. Write about factors affecting calcium absorption.
18. Iodine deficiency disorders.
19. Iron absorption
20. Transport and storage of iron.
21. Calcium functions
22. Functions of iron
23. Iron deficiency
24. Fluorosis
25. Functions of fluorine
26. Serum calcium homeostasis
27. Trace elements
28. Zinc
29. Iron containing proteins
30. Zinc containing proteins.
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CHAPTER - 16 | Water, Electrolytes & Acid-Base Balance
16
Chapter
Water, Electrolytes & Acid-Base Balance
1. Write functions of Water
A. WaterFunctions :
1. Human body is nothing but a water reservoir. About 80% of body mass is occupied by
water.
2. For all forms of life water is essential constituent.
3. Semi solid nature of human body is due to enormous amount of water present.
4. Every cell of human body and all other life forms contain water.
5. Water is medium in which all cellular events occurs.
6. Water is required for enzyme action.
7. Transport of several molecules requires water.
8. Body temperature is regulated by water.
9. Water aids folding of proteins, nucleic acids, enzyme etc.
10. Ions required for biochemical reactions of cells is provided by water.
2. Write about Maintenance of water balance and associated disorders.
A. In a normal healthy individual water intake equals water out put. Water intake per day is
about 2500ml. Drinking water contributes to about 1200ml, food water is about 1000ml
and water of metabolisms is about 300ml. Water out put per day is about 2500 ml. Urine is
major route of water elimination. About 1200ml of water is removed in urine. Other routes
of water removal are through lungs, skin and feces.
The body water homeostasis is achieved by combined action of hormonal and other factors.
ADH is hormone involved in water homeostasis. Water intake and water out put are two
factors involved in water balance maintenance. Decrease in water content in the body
leads to stimulation of thirst centre and thirst in caused. At the same time another centre
in the hypothalamus is stimulated anti diuretic hormone is released. Water is consumed
due to thirst. Through circulation ADH reaches kidney and act on renal tubular cells. As a
result more absorption of water takes place in kidney. So by the combined action of ADH
and thirst centre water balance is attained. Exactly reverse occurs when there is excess
water in body. Due to excess water thirst centre and other centre in hypothalamus are
inhibited. Water in take is stopped. ADH secretion is inhibited. Absence of ADH leads to
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BIOCHEMISTRY - Questions and Answers
elimination of water through kidney. These mechanisms bring back water content to
normal state. Thus the body water content of healthy adult is maintained.
Diseases of water balance :
1. Edema : Excess water in body leads to edema. It is also known as over hydration. It
occurs in excess secretion of ADH, water intoxication, cancer, drugs and overdose of
intravenous fluids.
2. Dehydration : In this condition water content of body is decreased. It occurs in vomiting,
diarrhoea, hypothalamus lesions, diabetes insipidus etc.
3. Define Electrolytes. Classify. Give examples for each class.
A. Charged solute molecules are known as electrolytes. There are two types of electrolytes.
Inorganic and organic solutes. Usually inorganic solutes are considered as electrolytes.
Inorganic solutes are further divided into cations and anions.
Cations : They are positively charged inorganic solutes. Sodium (Na+), Potassium (K+),
Calcium (Ca2+), and magnesium (Mg2+), are examples.
Anions : They are negatively charged inorganic solutes. Sulphate (So42-), Phosphate (Po43-),
Chloride (Cl-) and bicarbonate (HCO3-) are examples. Organic electrolytes are mainly
anions. They are contributed by protein, organic acid and organic phosphate.
4. Name plasma electrolytes. Write their normal level in plasma and
functions
A. Plasma electrolytes includes both anions and cations.
Anions of blood plasma : Bicarbonate, chloride, phosphate, sulphate, iodine and fluoride
are anions present in plasma. Normal plasma bicarbonate level is 24-30meq/ L. Normal
chloride level in plasma is 100-110 meq/L.
Cations of blood plasma : Sodium, potassium, calcium, magnesium, iron and copper are
cations present in plasma. Normal plasma sodium level is 135-145 meq/L. Normal
potassium level in plasma is 3-5 meq/L.
Functions of electrolytes :
1. Electrolytes are essential for several physiological process.
2. Electrolytes are involved in membrane potential maintenance.
3. Neuro muscular excitability and nerve impulse trans mission involves electrolytes.
4. Electrolytes are required for bone formation.
5. Enzyme catalysis is dependent on electrolytes.
6. Blood clotting is another physiological process dependent on electrolytes.
5. Write about electrolyte balance maintenance and disorders associated.
A. In the body electrolyte balance is maintained by many mechanisms.
1. Sodium pump is one mechanism of maintaining sodium balance. It keeps high extra
cellular sodium level against low intracellular sodium.
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CHAPTER - 16 | Water, Electrolytes & Acid-Base Balance
2. Aldosterone is hormone involved in handling of sodium and potassium by kidney. It
favours sodium absorption and potassium excretion by kidney.
3. Bicarbonate concentration is maintained by kidney. Kidney also maintains electrolyte
balance by eliminating salts.
4. Electrolyte concentration in body is also affected by diet, water intake and salt in take.
Diaseases of electrolyte balance : Electrolyte balance is disturbed due to loss of body fluids. It
occurs in diarrhoea, vomiting, burns, hemorrhages and sun stroke.
6. Describe Acid base balance or Hydrogen ion (PH) Homeostasis
A. Normal persons blood pH ranges from 7-35-7. 45. This range is maintained by specific
systems. These systems keeps blood pH within normal range by neutralizing acid or bases
produced in the body. If they are not neutralized then hydrogen homeostasis or acid –base
balance is disturbed which is not good for the well being of an individual.
Hydrogen ion homeostasis : Since proper pH is required body processes maintenance of pH
with in normal range is essential. Several systems participates in body hydrogen homeostasis.
They are 1. Buffer systems present in body. 2. Lungs 3. Kidneys.
Buffer systems
They are present in blood, plasma, extracellular fluids, intracellular fluid and tissues of the
body. Buffer is composed of weak acid and its salt or conjugate base. Buffer resist change in PH
of medium in which they are present when acid or base is added.
Buffers of blood plasma
1. Bicarbonate and carbonic acid buffer : It has major role in the maintenance of blood PH. It is
present in high concentration in blood. It function as an effective buffer in controlling pH
by maintaining conjugate (bicarbonate)and weak acid (carbonic acid) ratio 20 : 1. The blood
pH is with in normal limits as long as this ratio is maintained.
Mechanism of action of bicarbonate buffer : In the blood bicarbonate exist as sodium
bicarbonate (NaHCO3). When acid is added NaH CO3 reacts with acid and reduces its
strength i. e convert to weak acid. For example aceto acetic acid when added it is converted
to sodium acetate.
NaHCO3 +Acetoacetate
Sodium aceto acetate + carbonic acid.
The carbonic acid formed is weak acid compared to acetoacetate. So blood pH changes very
little and ratio of bicarbonate and carbonic acid alters. This is restored to normal level or
ratio by eliminating carbonic acid from lungs. Thus the ratio of bicarbonate and carbonic
acid returns to normal and blood pH remains 7. 4.
2. Phosphate buffer and protein buffer : These buffers are present in blood at low
concentrations. Hence their role in blood pH regulation is limited. However at normal
blood pH phosphate buffer is found as more effective than bicarbonate buffer.
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Erythrocytes Buffers : In erythrocytes hemoglobin is present at high concentration.
Hemoglobin (Hb) and its protonated form (HHb) constitutes buffer system in erythrocytes.
It is known as hemoglobin (Hb/HHb) buffer system. High hemoglobin concentration make
hemoglobin buffer system as major buffer of blood. Further constituent aminoacids
particularly histidine make hemoglobin an effective buffer at blood pH.
Respiratory system or lungs
Lungs have an important role in blood pH homeostasis. They maintain blood pH by affecting
carbonic acid part of bicarbonate buffer. They control carbonic acid concentration in blood by
carbon dioxide partial pressure in plasma. Further formation of carbonic acid from carbon
dioxide is catalyzed by carbonic anhydrase present in lungs. Decrease in blood pH occurs due
to neutralization of acids by bicarbonate. As a result ratio of bicarbonate and carbonic acid is
altered. Now respiratory centre is stimulated more of carbon dioxide is eliminated and
carbonic acid formation is decreased. This leads to restoration of bicarbonate and carbonic
acid ratio to normal and blood pH returns to normal. Thus hyper ventilation compensates
blood pH fall. When the blood pH increases exactly reverse process occurs. Due to raise in
blood pH carbonic acid concentration in blood decreases. This leads to change in bicarbonate
and carbonic acid ratio. To bring blood pH to normal respiratory rate decreases and more
carbonic acid is formed due to increased partial pressure of carbon dioxide. At the same time
ratio of bicarbonate and carbonic acid also returns to normal. So hypoventilation compensates
for raise in blood pH.
Kidneys
Kidneys play major role in maintenance of constant level of blood pH. It regulates blood pH
by several mechanisms.
1. Bicarbonate absorption : In proximal tubule of kidney bicarbonate of plasma filtrate is
absorbed. Usually luminal membrane is not permeable to bicarbonate. It enters tubular
cells in the form of carbon dioxide which is freely permeable. In the luman bicarbonate
combine with proton to form carbonic acid which is decomposed to water and carbon
dioxide.
H++HCO3 -
Filtrate
H2CO3
H2O + Co2
Enters luminal cell.
In tubular cells carbonic anhydrase rehydrates carbon dioxide to carbonic acid which
dissociates to bicarbonate and proton. The bicarbonate diffuses into blood.
Carbonic
Co2 + H2 o
H2CO3
An Hydrase
HCO3
H
Blood plasma
+
2. Excretion of hydrogen ions : Kidney regulates blood pH by eliminating hydrogen in distal
tubules. In tubular cells hydrogen ion is formed from dissociation of carbonic acid. As
mentioned above bicarbonate diffuses into plasma and hydrogen is eliminated into luman.
In the lumen hydrogen combines with (NaHPO4) monosodium hydrogen phosphate to form
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mono sodium dihydrogen phosphste (NaH2 PO4) which is excreted in urine. Due to this the
urine pH becomes acidic in distal tubules lumen.
HCO3NaHPO4
Distal tubular cells
H2CO3
+
H
NaH2 PO4
Urine
luman
Ammonia excretion : By eliminating ammonia in urine kidney plays important role in acid
base balance. In renal cells ammonia is released from glutamine by the action of
glutaminase enzyme. Glutamine enters renal cells from blood plasma.
Plasma
Glutamine
Glutaminase
renal cells
Ammonia
Glutamate
Ammonia formed in renal cells diffuses into lumen where it combines with proton to
form ammonium ion. The presence of ammonium ion makes urine acidic.
Proton
Ammonia
Ammonium ion
Urine
Acidic Urine.
Lumen
Thus kidney regulates blood pH by absorbing bicarbonate, eliminating hydrogen ion and
excreting ammonium ion. Further kidney also recover sodium along with bicarbonate.
Therefore kidneys are not only involved in acid base balance but also in electrolyte balance.
7. Explain functions of body normal pH
A. 1. For the action of enzymes appropriate pH is essential. If pH is altered then enzyme
action is affected. This disturbs body homeostasis.
2. For transport of solute molecules in the body proper pH is required.
3. For the maintenance of structure of folded state of proteins, enzymes and nucleic acids
proper pH is required. Alteration in pH causes structural changes in these
biomolecules which in turn affects functions of these macro molecules.
4. Proper pHis required for maintenance of structure of coenzymes and metabolites.
8. Write about Acid base imbalance or Acid base disturbances
A. Two types of acid base disturbances are known. They are acidosis and alkalosis.
a. Acidosis : It occurs due to low blood pH. It is due to accumulation of acids. It is further
sub divided into 1. Metabolicacidosis and 2. Respiratory acidosisbased on underlying
cause.
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BIOCHEMISTRY - Questions and Answers
1. Metabolic acidosis : Most common acid base disturbance is metabolic acidosis. More
acids are produced by metabolism. This leads to decreased bicarbonate concentration.
It occurs in uncontrolled diabetes mellitus and starvation. Loss of bicarbonate due to
diarrhoea and vomiting also cause metabolic acidosis. Increased elimination of
bicarbonate by kidneys also leads to metabolic acidosis. Ingestion of acids and
decreased elimination of proton by kidneys also leads to metabolic acidosis.
2. Respiratory acidosis :
Plasma partial carbon dioxide pressure is more due to abnormal lung function.
Decreased respiration or hypoventilation occurs due to depression of respiratory
centre. Sedatives like morphine and barbiturates depress respiratory centre.
Hypoventilation also occurs due to obstruction to air passage. In pnumonia,
emphysema, asthma air passage is narrowed. Therefore respiratory acidosis occurs
in diseases of lung and respiratory centre depression.
b. Alkalosis : It occurs due to high blood pH. It is due to accumulation of alkali. It is further
subdivided into 1. Metabolic alkalosis 2. Respiratory alkalosis.
1. Metabolic alkalosis : Bicarbonate concentration is more in blood. It occurs due to
loss of HCl. More HCl is lost in prolonged vomiting. Excessive excretion of ammonia
by kidney also leads to metabolic alkalosis. Ingestion of alkali cause metabolic
alkalosis.
2. Respiratory Alkalosis : Partial pressure of carbon dioxide is less. It occurs due to
hyperventilation. When respiratory centre is stimulated hyperventilation occurs.
Hyper ventilation occurs at high altitudes, head injury, drug poisoning, fever and
anxiety.
Other model questions are
9. Blood buffers
10. Role of kidneys in acid base balance
11. Acidosis
12. ADH
13. Water intoxication.
14. Alkalosis
15. Serum electrolytes
16. Normal levels of electrolytes in blood
17. Write normal blood PH range. Explain mechanisms of its regulation.
18. Metabolic acidosis
19. Respiratory acidosis
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CHAPTER - 17 | Energy Metabolism and Nutrition
17
Chapter
Energy Metabolism and Nutrition
1. Define calorie. How energy values of foods are measured? Write
calorific values of common foods.
A. Calorie (C) or Kilocalorie (Kcal) or nutritional calorie (C) is energy unit currently in use.
It is defined as amount of heat energy required to raise temperature of 1Kg of water by 1o c.
Direct colorimetric methods are used to determine energy values of foods. Energy out put
of food stuff is estimated by measuring heat production. The human subject is asked to stay
in a steel insulated chamber. When food is oxidized in his body heat is produced which is
transferred to water through chamber. Energy out put of food stuff is calculated from
temperature differences between in coming and out going water.
The calorific values of some common foods are carbohydrates (1g) -4C ;Lipids (1g) -9C ;
proteins (1g) -4C;bread (1kg) -2630C; sugar (1kg) -4100C and cake (1kg) -4000C.
2. Write a note on Respiratory quotient
A. It is defined as ratio of amount of carbon dioxide produced to amount of oxygen consumed
when food is oxidized in side the body.
Amount of carbon dioxide produced
Respiratory quotient (R. Q) = ––––––––––––––––––––––––––––––––––
Amount of oxygen consumed
Significance :
1. Carbohydrate, fat and protein (R. Q) values are 1, 0. 7 and 0. 8 respectively.
2. R. Q of mixed diet is about 0. 85.
3. Type of food being oxidized for energy production in the body can be known from R. Q
values.
4. In some diseases like diabetes mellitus and starvation R. Q values decreases due to
change of food used for energy production in the body.
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BIOCHEMISTRY - Questions and Answers
3. Write about BMR.
A. Basal metabolic rate (BMR) : It is defined as heat out put of an individual having normal
body temperature who is on fasting for the last 12 hours and lying in complete mental and
physical and emotional rest.
BMR of an individual is obtained using formula 24C/day /kg. Normal BMR Values for male
and female are 37. 5C/sqm/hr and 35C/sqm/hr respectively.
Factors affecting BMR :
1. Age, surface area, sex and environment influences BMR.
2. In children BMR is high but it is low in old people.
3. BMR increases with increasing surface area.
4. Males has high BMR than females.
5. In cold weather BMR is high but in hot climate BMR is low.
6. Physiological conditions and exercise also affects BMR. In pregnancy and lactation
BMR is high but it is low in sleep. BMR increases with exercise.
7. In several pathological conditions BMR is either increased or decreased.
8. In fever, hyperthyroidism and cushing's syndrome BMR is increased.
9. In starvation, hypothyroidism and addison's disease BMR is decreased.
4. Specific dynamic action of food (SDA)
A. Specific dynamic action (SDA) of food : It is defined as extra amount of energy produced
over normal energy out put of food stuff when it is oxidized in the body. For example when
carbohydrate of 100C is oxidized in the body 106C of energy is produced. The extra 6C of
carbohydrate oxidation is SDA of carbohydrate. Traditionally SDA is expressed as per
centage. Hence SDA of carbohydrate is 6% like wise SDA of other foods like protein and fat
is 30% and 4% respectively. For mixed diet SDA is 6-19%. This extra energy comes from
tissue metabolism. Since mixed diet is most commonly consumed to compensate for this
loss of energy by tissues about 10% of total calories are added to daily energy requirement
of an individual.
5. Calculate your energy requirement per day.
A. Energy requirement of college student per day
Daily energy need of 18 years old college student with weight of 70Kg has been determined.
A college student is a sedentary worker. His daily life consist of three phases. Each phase
has duration of 8 hours. They are a. Sleep. b. Personal activities which consist of 4 hours of
light work and 4 hours of moderate work. c. Sedentary work (college work). Energy
requirement for sleep is calculated as 600calaries (BMR X body surface area X 8 = 44C X
1. 7 X 8). Energy requirement for personal activities is calculated as 1120calaries (1. 5C X
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CHAPTER - 17 | Energy Metabolism and Nutrition
70 X 4 + 2. 5 X 70 X 4). For 8 hours of college work energy requirement is calculated as
840 calories (1. 5 X 70 X 8). Total energy requirement is obtained by taking sum of three
energy requirements. It is about 2560C(600+840+1120). Thus a college student requires
about 2500 C per day.
In the case of adult sedentary worker the daily energy requirement is 2500C and 2000C for
men and women respectively. For moderate worker daily energy requirement is 3000C for
men and 2500C for women. An adult male heavy worker needs about 4500C per day where
as female heavy worker needs about 3000 calories per day. The energy requirement of
pregnant and lactating women is more than sedentary worker. About 2300 calories and
2700 calories are required by pregnant and lactating women respectively.
6. Define nutrients. Give examples.
A. Nutrients : These are essential substances present in food which are not produced in the
body. They are required for growth and maintenance of the body.
Six major types of nutrients are identified. They are 1. Carbohydrates
2. Lipids. 3.
Proteins. 4. Vitamins. 5. Minerals and 6. Water.
7. Write about proximate principles of food
A. Carbohydrates, lipids and proteins are known as proximate principles of food and they
yield energy.
1. Carbohydrates : Food carbohydrate contributes to about 60% of body energy
requirement in many countries. Carbohydrates present in diet are majority of them
are polysaccharides and to some extant disaccharides. Cereals, legumes, vegetables
like potatoes, sweet potatoes etc and other fruits like apples, bananas etc and meat,
garlic etc. are source for polysaccharides. Starch, glycogen and inulin are
polysaccharides present in diet. Among all the polysaccharides starch is present in high
concentration. Further amylopectin content varies among starches of different origin.
Glycogen is absent in plant foods. In animal foods glycogen content is less.
Milk is major source of disaccharide lactose. In the infants lactose contributes to 50% of
energy requirement. Table sugar is major source of disaccharide sucrose. Other source
of sucrose are sweets, bakery products like jams, jellies, syrups, ice creams etc. Fruits
juices, honey and beverages contain sucrose. Some biscuits contain glucose. Fructose is
present some fruit juices.
Daily requirement : For proper health about 100gm of carbohydrate must be present in
diet. However for short periods carbohydrate is not essential because it is synthesized
from protein and lipids.
2. Lipids : Fat is common name for dietary lipids. Food fat contributes to 30% of energy
requirement of people in majority of countries. However in industrialized nations fat
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BIOCHEMISTRY - Questions and Answers
may contribute to more i. e about 40-50% of energy requirement. Fat of plants as well as
animal origin is present in diet. Animal fat tissue (Adipose tissue) is major source for
fat in diet.
Ground nut oil, rice bran oil, sunflower oil, coconut oil, soybean oil, corn oil, palm oil
etc are other oils that contributes to fat in diet. Further nuts, cereals and pulses of food
also contain lipid. Triglycerides, fatty acids, phospholipids and cholesterol are lipids
present in dietary fats. Animal foods are rich in cholesterol. Other functions of dietary
fat is to supply essential fatty acids and fat soluble vitamins. Fat reduces size of diet
and improves palatability of food. Dietary fat is immediate source of energy also apart
from glucose. Further dietary fat is responsible for fullness feeling.
Daily requirement : Diet must contain about 30-50gm of fat per day. And 5gm of
essential fatty acids.
3. Proteins : Protein in diet supplies about 10-15% of energy demand of body. However in
rich industrialized countries proteins contributes more to energy requirement. In the
case of low income people of developing countries protein contributes to less of energy
requirement. Animal meat, fish, eggs, milk, cereals and pulses are sources for protein
in diet. Vegetables and fruits contain less of protein. Main function of dietary protein is
to replace protein lost due to turn over. By supplying essential amino acids dietary
proteins replaces nitrogen lost from the body. Another function of dietary protein is
maintenance of nitrogen balance.
Daily requirement : About 70gm of protein is required per day.
8. Define balanced diet. Write balanced diet for a sedentary worker.
A. It is a diet containing all six types of nutrients in adequate amounts to meet energy
requirement as well as nutritional requirement of an individual. Further recommended
proportion of carbohydrates, fats and proteins in balanced diet are 70%, 20% and 10%
respectively. Balanced diet of a sedentary worker whose energy demand is 2500 C per day
must contain 440gm of carbohydrate, 50gm of fat, 70gm of protein and vitamins, minerals
and water in adequate amounts. However a balance diet of this nature is difficult to
prepare because foods we consume consist of cereals, legumes, vegetables and fruits etc in
which carbohydrates, fats and proteins are together. Hence composition of balanced diet
for vegetarians consisting of cereals, legumes, vegetables etc. is cereals, 450gm;pulses,
70gm; green vegetables, 100gm; other vegetables, 175gm;milk, 250ml; fruit, 30gm; fat,
45gm and sugar 40gm.
9. Write about Malnutrition
A. Consumption of food or diet low in calories or proteins or both is called as malnutrition.
People of developing countries are mostly exposed to malnutrition. Civil war, famines or
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CHAPTER - 17 | Energy Metabolism and Nutrition
drought, earth quakes, sunamies etc may cause malnutrition in general population.
Children and pregnant women are most susceptible. Two types of malnutrition are well
known. They are a. Marasmus b. Kwashiorkor.
Marasmus : It occurs due to consumption of food low in protein and calories. Children
below 2years of age are most affected. Weight of children affected with disease is below
60% of normal. It is due to feeding infants diet low in calories and proteins after withdrawn
from breast feeding. Main clinical symptoms are severe loss of body fat and muscle
protein, head is big, dry skin, growth and mental retardation are common.
Kwashiorkor : It occurs due to consumption of insufficient or inadequate protein only.
Children below 2years of age are most affected. Weight of children affected with this
disease in usually below 80% of normal weight. Usually it occurs in children when they are
given traditional foods after with drawn from breast feeding. These traditional foods are of
mostly plant based foods rich in starch but lacks adequate protein. So these foods meet only
energy requirement of body but protein requirement is not met. Low economic group
children are most affected. Clinical symptoms are edema, pot belly or distended abdomen,
dermatitis, anaemia, diarrhoea and susceptible to infections.
10. Write briefly about dietary fibre
A. Dietary fibre is mainly of plant origin. Indigestible plant polysaccharides like cellulose,
hemicellulose, pectin, gums and lignin constitutes dietary fibre. Importance of dietary
fibre in human
health is recently recognized. Dietary fibre has several protective,
preventive and curative effects. It is required for good health.
The incidence of colonic diseases like ulcerative colitis, piles, constipation and irritable
bowel syndrome decreases with use of fibre in the diet. Metabolic diseases like diabetes
mellitus, obesity, coronary artery disease, hypertension etc incidence decreases with use of
fibre diet. Dietary fibre lowers blood glucose, cholesterol and triglyceride levels. However
absorption of glucose, cholesterol and some minerals is slow in presence of dietary fibre.
11. Write a note on biological value
A. Biological value (BV) is defined as percentage of absorbed nitrogen that is retained by the
body.
N retained
Biological value (BV) = –––––––––––
N absorbed
x 100
Significance
The biological value of a protein measures the quantity of dietary protein used by animal for
growth and maintenance of body function. Biological value of protein may also indicates
essential amino acid content, digestibility of protein and availability of digested products for
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BIOCHEMISTRY - Questions and Answers
absorption. Biological values for some proteins are 96% for egg, 80% for milk and meat
proteins and 60% for rice and wheat proteins.
Based on biological values proteins are classified as
1. Good quality proteins
2. Poor quality proteins
Good quality proteins have high biological values. Examples are animal derived proteins
Poor quality proteins have low biological values. Most of plant derived proteins come under
this category.
Other model questions are
12. PEM
13 Kwashiorkor
14. Dietary fibre
15. PCM
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CHAPTER - 18 | Hormones
18
Chapter
Hormones
1. Define hormone. Write chemical nature of hormones.
A. Hormones are substances produced by various cells or glands of the body. They act on cells
which are
far away
from site or gland of their origin. Usually blood carries these
hormones from site of production to site of action. Organs on which hormones act are
known as target tissues. Endocrine glands produce hormones.
Chemical nature :Hormones are proteins, steroids, organic substances, peptides and
amino acid derivatives.
2. Describe general mechanisms of Hormone action
A. 1. Generally hormones are chemical substances with messages.
2. In target tissues these messages are translated into biological effects.
3. Several steps may be involved in translation of these messages and number of steps
depends on type of hormones.
4. Further second messenger molecules present in target tissues mediate hormone
action in some cases.
5. Translation of message begins with binding of hormones to receptor present on
membrane to form hormone receptor complex.
6. The remaining steps involved in signal transduction process varies from one
hormone to another
a. For example adrenal medullary hormones like catecholamines the hormone
receptor complex inter act with class of membrane proteins G-proteins to produce
second messenger molecules. G- proteins are heterotrimers containing three
different subunits Gα, Gß and Gγ and binding sites for GTP and GDP. Signal transduction of hormones involves association and dissociation of subunits. In resting
state GDP is bound to Gα subunits of G- protein. It is designated as GDP –G protein.
The hormone –receptor complex causes conformational change in GDP- G protein.
This lead to dissociation of Gß and Gγ subunits and exchange of GTP for GDP. Now
G-Protein designated as Gα-GTP.
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BIOCHEMISTRY - Questions and Answers
Hormone
GDP-G protein
Membrane receptor
Hormone receptor complex
Gα- GTP + Gß and Gγ.
Now Gα-GTP activates enzymes like adenylate cyclase, phospholipase C to produce
second messenger molecules like cAMP and inositol triphosphate (IP3).
Gα – GTP
Inactive adenylate cyclase
Active adenylate cyclase
Active adenylate cyclase
ATP
cAMP
Gα – GTP
In active phospholipase C
Active phospholipase C
Active phospholipaseC
PI P2
Inositol tri phosphate (IP3).
These second messenger molecules produce final biological responses by acting on
enzymes of biological process.
Enzymes
cAMP, IP3
Biological response.
Proteins
b. In the case of steroid hormones and thyroid hormones hormone – receptor complex
is translocated into nucleus where it binds selective genes and alters gene
expression. The increased expression of genes lead to biological effect.
Translocation
Hormone –receptor
Nucleus
complex
Gene product
c. In the case of
Gene expression
Biological response.
hormones like insulin binding of hormone to receptor leads to
activation of tyrosine kinase activity of receptor. Tyrosine kinase phosphorylates
tyrosine residues of receptor to form phosphorylated receptor. This inturn
phosphorylates insulin receptor substrate (IRS). Binding of phosphorylated IRS to
several proteins, enzymes leads to final biochemical effect.
Hormone
Receptor
Hormone receptor
Activated tyrosine kinase
Activation of receptor tyrosine kinase
Phosphorylated receptor
phosphorylation of IRS
Enzymes, proteins
Biochemical effect.
Binding
Phosphorylated IRS
To
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CHAPTER - 18 | Hormones
3. Name hormones of pancreas. Write their actions.
A. Insulin, glucagon and somatostatin are hormones of pancreas. They are proteins.
Actions of Insulin
Insulin affects carbohydrate metabolism, lipid metabolism and protein metabolism.
Carbohydrate Metabolism :It increases glucose entry into peripheral tissues like muscle
and adipose tissue by promoting translocation of glucose transporter. It stimulates
glycogensis by activating glycogen synthase of glycogenesis. It suppresses
gluconeogenesis. It increases rate of glycolysis by activating regulatory enzymes of
glycolysis. All these actions of insulin lowers blood glucose level.
Insulin
Adipose tissue, muscle
Translocation of glucose transporter
more glucose entry.
Insulin
Glycogen synthase
Activation of glycogen synthase
Increased glycogenesis
Insulin
Key enzymes of gluconeogenesis
Suppression of key enzyme
Decreased
gluconeogenesis.
Insulin
Regulatory enzymes of glycolysis
Enzymes
Activation of regulatory
Increased glycolysis rate.
Lipid Metabolism :Insulin promotes lipogenesis in adipose tissue. It increases fatty acid
biosynthesis by activating acetyl –CoA carboxylase.
Insulin
Adipose tissue
Increased lipogenesis
Insulin
Acetyl –CoA carboxylase (native)
Active acetyl –CoA carboxylase
Increased fatty acid biosynthesis.
By increasing uptake of glucose by adipose tissue insulin promotes triglyceride
biosynthesis.
Insulin
Adipose tissue
Activation of HMG –CoA reductase
cholesterol synthesis
increased.
Protein Metabolism :Insulin is an anabolic hormone. It promotes protein synthesis and
retards protein breakdown.
Decreased protein breakdown
Insulin
Increased protein synthesis.
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BIOCHEMISTRY - Questions and Answers
Actions of glucagon
Glucagon is an antagonist of insulin. It raises blood glucose level by promoting
glycogenolysis and gluconeogenesis. It inhibits fatty acid synthesis by inhibiting acetyl
–CoA carboxylase. By activating hormone sensitive lipase glucagon promotes lipolysis.
Cholesterol synthesis is inhibited by glucagon.
Actions of Somatostatin
It prolongs gastric emptying. It decreases acid secretion by stomach. It decreases secretion
of pancreatic enzymes. Absorption of glucose is decreased by somatostatin.
4. Name hormones of Adrenal cortex. How they are synthesized ?
Mention their functions.
A. Aderenal cortex produces glucocorticoids and mineralo corticoids. All of them are
steroids and produced from cholesterol.
Gluco corticoids :Cortisol is the major glucocorticoid. They affects carbohydrate, lipid
and protein metabolism.
Biosynthesis of cortisol :
It beings with cholesterol. Progesterone is an important intermediate.
Cholesterol
Pregnenolone
Progesterone
progesterone
Deoxycortisol
Cortisol.
Hydroxy
Carbohydrate Metabolism :Glucocorticoids promote gluconeogenesis by increasing
availability of glucogenic amino acids
and by increasing activity of key enzymes of
gluconeogenesis. They promote glycogenesis by activating glycogen synthase.
Glucocortucoids raises blood glucose levels by these actions.
Lipid Metabolism :Glucocorticoids promotes lipolysis by activating hormone sensitive lipase.
At high concentration they promote lipogenesis.
Protein Metabolism :Under normal conditions glucocorticoids stimulates protein metabolism.
At excess concentration they promote protein breakdown.
Other functions of glucocoticoids are immuno suppressive action, anti inflammatory action
and maintenance of blood pressure.
Mineralo corticoids :
1. Aldosterone promotes sodium retention in distal convoluted tubules. 2. It promotes sodium
retention by increasing formation of proteins involved in sodium absorption.
Synthesis of aldosterone :Cholesterol is converted to aldosterone via progesterone as out
lined below
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CHAPTER - 18 | Hormones
Cholesterol
Pregnenolone
Deoxy corticosterone
Progesterone
Corticosterone
]
Aldosterone.
5. Name hormones of adrenal Medulla. Write their synthesis and functions.
A. Hormones of adrenal medulla are adrenaline or epinephrine and nor adrenaline or nor
epinephrine. They are also called as catecholamines. They are amino acid derivatives.
Biosynthesis :Catecholamines are synthesized from amino acid tyrosine.
Reactions :
1. Hydroxylation of tyrosine by tyrosinase initiates catecholamine formation. Dihydroxy
phenyl alanine (DOPA) is product.
2. In the second reaction a pyridoxal phosphate dependent decarboxylase converts DOPA to
dopamine.
Tyrosinase
Tyrosine
Decarboxylase
dihydroxy phenyl alanine (Dopa)
O2 (1)
Dopamine.
(2)
Co2
3. Another vit. C dependent hydroxylation of dopamine by hydroxylase generates nor
adrenaline.
4. A transmethylation reaction using SAM as methyl donor converts nor adrenaline to
adrenaline
Hydroxylase
Dopamine
Transmethylase
Nor adrenaline
Adrenaline +SAH.
Vit. c (3)
(4)
SAM
Actions of catecholamines :The catecholamines act through two classes of receptors.
They are α –adrenergic and ß –adrenergic receptors. These two classes consist of two sub
classes. They are α1, α2 and ß1, ß2. Epinephrine can bind to both types of receptor but nor
epinephrine binds to only α-receptors.
1. In liver and muscle catecholamines increases glycogenolysis.
2. They cause contraction of smooth muscle.
3. In liver they inhibit glycogenesis
4. By activating hormone sensitive lipase they promote lipolysis.
5. They cause myocardial contraction.
6. They increase gluconeogenesis in liver.
7. They raise blood pressure by acting on smooth muscle of blood vessels and heart.
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BIOCHEMISTRY - Questions and Answers
6. Name thyroid Hormones. How they are formed? Mention their
functions.
A. Thyroid gland produces thyroxine (T4) and triiodotyronine (T3). They are amino acid
tyrosine derivatives.
Biosynthesis :Tyrosine serve as precursor of thyroid hormones.
Reactions :
1. Iodination of tyrosine of thyroglobulin (TG) initiates thyroid hormone synthesis.
Mono iodotyrosine (MIT) and diiodotyrosine (DIT) bound to TG are products.
І
Tyrosine –TG
Monoiodotyrosine (MIT) –TG+ Diiodotyrosine (DIT)-TG.
(1)
2. In the second reaction MIT and DIT condense to form tri iodotyromine (T3)-TG and
tetraiodo tyrosine (T4) –TG.
Condensation
MIT + DIT
Triiodotyronine (T3) – TG +Tetraiodotyronine (T4) –TG.
(2)
3. Proteolysis generates T3 and T4 from thyroglobulin.
Proteolysis
T3 –TG
Proteolysis
T3
T4 – TG
(3)
T4 (Thyroxine)
(3)
TG
TG
Functions :
1. Thyroid hormones are involved in maintenance of Basal Metabolic Rate (BMR)
2. They cause positive nitrogen balance by increasing protein synthesis.
3. They are required for over all development of humans.
4. They influences blood glucose through an unknown mechanism.
7. Name hormones of Gonads. Write their chemical nature, biosynthesis and actions.
A. Gonads produce sex hormones. Ovaries produce estrogen and progesterone. Testes
produce testosterone.
Chemical nature:They are all steroids. They are synthesized from cholesterol.
Biosynthesis of testosterone:
Cholesterol is converted to testosterone.
Cholesterol
Andostenediol
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Pregnenolone
Testosterone.
Hydroxy pregnenolone
CHAPTER - 18 | Hormones
Actions of Testosterone
1. It is an anabolic steroid hormone. It regulates gene expression.
2. It is required for spermatogenesis and for development of secondary sex characteristics.
3. It is involved in sexual differentiation.
4. It determines male pattern behaviour by influencing brain regions sensitive to this
hormone.
Synthesis of progesterone and oestrogen :Cholesterol is converted to oestrogen via
progesterone and testosterone.
Cholesterol
Pregnenolone
Andostenedione
Progesterone
Testosterone
Hydroxy progesterone
Estrogen.
Actions of oestrogen
1. It is an anabolic steroid hormone. It regulates gene expression and growth.
2. It is required for maturation of ovarian follicle and for development of tissues involved
in reproduction.
3. It promotes mammary gland maturation.
4. It is required for menstrual cycle.
Actions of Progesterone
1. It prepares uterine epithelium for implantation of fertilized ovum.
2. Progesterone decreases peripheral blood flow and reduces body temperature
8. Write about factors regulating hormone action.
A. Hormone action is regulated by two ways
a) Regulation of hormone action by releasing and trophic hormones
Higher centres of brain regulate hormone action by release or secretion or inhibition of
hormones by endocrine glands. Depending on needs of organism or individual a
particular hormone is produced or inhibited and this signal is delivered to target gland
through chemical substances known as releasing factors or release inhibiting factors
and trophic hormones. Releasing factors or release inhibiting factors are produced by
hypothalamus where as trophic hormones are produced by anterior pituitary gland
Usually releasing factors or release inhibiting factors of hypothalamus reach anterior
pituitary gland through direct circulatory connection where as trophic hormones of
pituitary reaches target gland through blood circulation.
The releasing or release inhibiting factors produced by hypothalamus are thyrotrophin
releasing factor (TRF), growth hormone release inhibiting factor (GRF) or
somatostatin, corticotrophin releasing factor (CRF) and gonadotrophin releasing factor
(GRF).
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BIOCHEMISTRY - Questions and Answers
When hypothalamus is activated by higher brain centres releasing or release inhibiting
factors are produced. In response to hypothalamic signal through releasing or release
inhibiting factor anterior pituitary either generates trophic hormone or stops its
release. The trophic hormone if released acts on target endocrine glands to produce
hormone.
For example thyrotrophin releasing factor (TRF) when released by hypothalamus in
response to higher centre stimulation acts on anterior pituitary to release thyrotropic
or thyroid stimulating hormone (TSH). This in turn acts on thyroid gland to release
thyroxine.
Hypothalamus
TRF
Thyroxine
Thyroid
Anterior Pituitary
TSH
b) Regulation of hormone action by feed back inhibition
This is another way of controlling hormone action. Usually hormone action is self
limiting. It controls its own production. For example hormones of adrenal cortex inhibit
release of CRF and ACTH
Release of CRF
Release of ACTH
Inhibition
Adrenal Cortex
Inhibition
Hormones
Other model questions are
9. Mechanism of steroid hormone action
10. Adrenal cortical hormones
11. G-Proteins
12. cAMP
13. Mechanism of insulin action.
14. Catacholamines
15. Glucagon.
16. Thyroid hormones
17. Insulin
18. Glucocorticoids
19. Regulation of hormone action
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CHAPTER - 19 | Organ Function Tests
19
Chapter
Organ Function Tests
1. Write about Kidney Function Tests (KFT)
A. Since urine is product of kidney function, urine analysis is most common kidney function
test.
1. Urine Volume : In kidney diseases urine volume decreases. Urine out put during day
time and night time is not same. Usually more urine is passed in the day time and less
in night time. Higher urine output in night time is suggestive of kidney dysfunction.
2. Urine specific gravity : If urine contains glucose or protein specific gravity increases.
If water re absorption in renal tubules is defective due to absence of ADH specific
gravity of urine decreases.
3. Urine proteins : Albumin is absent in normal persons urine. If glomerular permeability
increases albumin appears in urine. It is indicative of kidney disease.
Since kidney eliminates several substances from blood in kidney diseases levels of
these substances is affected.
1. Blood urea, uric acid and creatinine : The levels of these substances increases in blood
with progressing kidney disease.
2. Blood Phosphorus : Kidney is involved in phosphorus excretion. Phosphorus excretion
by kidney decreases in renal disease. Hyper phosphatemia (more phosphorus in blood )
occurs in chronic nephritis which may ultimately lead to renal failure.
3. Serum protein electrophoresis : It is useful in diagnosis of nephrotic syndrome. In
electro phoretogram albumin and gamma globulin bands size decreases and α- globulin
band size increases.
4. Blood cholesterol : In nephrotic syndrome blood cholesterol level is more.
5. Serum Cystatin C : It is a Cysteine protease inhibitor. It is cleared by kidneys. It is
elevated in kidney dysfunction.
Apart from blood and urine analysis kidney function is assessed by specific tests. These
tests may assess function of kidney as whole or its parts like glomerulus and renal tubules.
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BIOCHEMISTRY - Questions and Answers
1. Concentration Test : It is test for renal tubular function. This test indicates
concentrating ability of kidney. Specific gravity of urine is measured after
administration of anti diuretic hormone (ADH). If specific gravity is normal then
kidney function is normal. If specific gravity is decreased it is suggestive of impaired
concentrating ability of kidney.
2. Dilution test : It is another test that assesses renal tubular function. It this test a fixed
dose of water is given. Then urine sample is collected. Volume and specific gravity of
urine sample are determined. If volume of urine is equal to volume of water given it
indicates normal kidney function. In normals the specific gravity of urine collected is
below normal.
Clearance Tests
Since glomerulus is major functional unit of kidney and involved in filtration clearance
tests are used to assess glomerular filtration rate (GFR). The word clearance is defined as
ml of plasma from which a substance is cleared (eliminated) by kidneys in a minute. The
cleared substance is found in urine. Then clearance is calculated by estimating that
substance in blood and urine. If kidney function is not normal clearance values decreases.
Some common clearance tests are detailed below.
Urea clearance test (UCT)
Procedure : After a normal break fast about 200ml of water is given to patient under
investigation. Immediately urine is collected and discarded. After an hour his urine and
blood samples are collected. Urea level is estimated in the samples.
Calculation :
U V
Urea clearance (ml/min) = ––––––
P
U = Urine urea concentration
P = Blood urea concentration
V = Urine out put per minute.
Significance : Normal urea clearance value is 75ml/ min. Low clearance value suggest loss
of kidney function that occurs due to diseases.
Creatinine Clearance Test (CCT)
Procedure : Initially the patient under investigation is given 500ml of water to hydrate his
body completely. After one hour urine sample is collected and discarded. Again urine is
collected after 4 hours and urine volume is noted. Blood sample is also collected. Then blood
and urine samples creatinine is determined.
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CHAPTER - 19 | Organ Function Tests
Calculation :
UV
Creatinine clearance ml/ min = –––––––
P
U = Creatinine concentration in urine
V = Urine volume per minute
P = Creatinine concentration in blood.
Significance : Creatinine clearance normal value is 90-110 ml/min/ 1. 73m2 body surface
area. Low values are obtained when GFR is decreased due to diseases of kidney and
pre renal diseases.
2. Describe Liver Function Tests (LFT)
A. Liver is involved in most of the metabolisms of body. It has role in hemoglobin, protein,
lipid, carbohydrates and xenobiotics metabolisms. So they are affected in liver diseases. By
measuring various products of these metabolisms in blood liver function is assessed.
Hemoglobin (Bile pigment ) Metabolism
1. Serum bilirubin : Secretory function of liver is assessed by measuring serum bilirubin.
Van den Bergh Test : It is used to measure conjugated and unconjugated bilirubin level in
blood.
a. Conjugated bilirubin : It is more in obstructive jaundice.
b. Un conjugated bilirubin : In pre hepatic jaundice it is more.
c. Conjugated and unconjugated bilirubin : In hepatic jaundice both are elevated.
2. Urine bilirubin : In normal people urine bilirubin is absent. It is found in urine in post
hepatic or obstructive type jaundice. Fouchet's test is used for urine bilirubin.
3. Urine urobilinogen : Urobilinogen in urine increases in pre hepatic jaundice. Decrease
in urine urobilinogen occurs in post hepatic or obstructive jaundice. Ehrlich's test detects
urobilinogen in urine.
Protein Metabolism
These functional tests mostly assesses synthetic function of liver because many proteins,
urea and ammonia are handled by liver.
1. Serum albumin : Since liver synthesizes albumin low albumin level indicates liver
disease like cirrhosis.
2. Serum globulins : Increased globulin level occurs in blood in liver disease like cirrhosis,
auto immune hepatitis, biliary cirrhosis etc.
Method : Biuret method is used to estimate proteins in serum.
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BIOCHEMISTRY - Questions and Answers
3. Serum protein electrophoresis : Changes that takes place in various plasma proteins
is assessed by electrophoresis. In cirrhosis size of albumin and gamma globulin bands
decreases. Band sizes of other proteins remains same.
4. Prothrombin time (PT) : Prothrombin a blood clotting factor is synthesized by liver.
Decreased prothrombin synthesis increases prothrombin time. Hence increased
prothrombin time suggests liver dysfunction. In obstructive jaundice also prothrombin
time increases.
5. Alanine transaminase (ALT) : It is an enzyme involved in amino acid break down in
liver and other tissues. Since liver is rich in ALT this test is specific for liver dysfunction.
ALT is more in hepatitis. In alcoholic cirrhosis also ALT is more.
6. Gamma glutamyl trans peptidase (GGT) : It is an enzyme involved in peptide
metabolism. In all liver diseases the level of this enzyme is more in blood. Elevation is more
marked in biliary tree obstruction. It is also more in alcoholic cirrhosis, fatty liver and
infective hepatitis.
7. Blood Urea : Liver is the only organ involved in synthesis of urea. So low urea level in
blood is suggestive of liver diseases. This test also assesses synthetic capacity of liver.
8. Blood Ammonia : Liver maintains normal ammonia level by converting it to urea. It is
eliminated by kidney. Ammonia is toxic but urea is nontoxic. So liver detoxifies ammonia
to urea. Hence high ammonia level in blood suggests impaired liver function.
Lipid Metabolism
1. Serum bile acids : Liver converts cholesterol to bile acids and secretes into bile. So this
test assesses secretory and excretory function of liver. Serum bile acid level increases in
hepatitis and cirrhosis. In cholestasis or obstruction to bile flow or biliary tract bile acid
level is elevated to a greater extent.
2. Blood Cholesterol : Increased level of cholesterol is suggestive of liver disease because
liver catabolizes cholesterol to various products. In obstructive jaundice cholesterol level
in blood is increased.
3. Urine bile salts : Liver produce bile acids from cholesterol and secretes into bile. In the
bile, bile acids generates bile salts which are eliminated through bile. In normal people
bile salts are absent in urine. Presence of bile salts is suggestive of obstructive jaundice.
Hays test is used to detect bile salts in urine.
Carbohydrate Metabolism
Functional tests based on carbohydrate metabolism assesses synthetic function of liver. Liver
is responsible for the conversion of galactose to glucose. So in liver disease this capacity of liver
is disturbed.
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CHAPTER - 19 | Organ Function Tests
Galactose Tolerance Test (GATT) : It measures rate of conversion of galactose to glucose by
liver.
Procedure : A dose of
galactose is injected into blood. Then blood samples are collected for
every ten minutes for about 30 minutes. Then blood samples are analyzed for galactose.
Significance : Normally within 10 to 15 minutes galactose is cleared by liver. Decreased
clearance indicates liver dysfunction due to diseases like cirrhosis or hepatitis.
Xenobiotics Metabolism
Many xenobiotics are cleared by liver. So extent of removal of given xenobiotic from blood is
directly related to liver dys function. Some xenobiotics used to assess liver function are amino
pyrine, indocyanin, bromosulfophthalein, rose Bengal and sodium benzoate. Retention of the
administered xenobiotic in blood suggests liver dysfunction.
Nucleotide Metabolism
51 – Nucleotidase : It is an enzyme of nucleotide breakdown. It is present in bile canaliculus and
bound to membrane. It is increased in blood when there is obstruction to bile flow. Hence
elevated level of this enzyme suggests cholestasis.
Mineral Metabolism
Alkaline phosphatase : It is an enzyme involved in hydrolysis of organic phosphates at alkaline
pH. Portal vascular system and sinusoids are rich in this enzyme. However bile canaliculi
contain less. In portal vascular endothelium it exist as membrane bound enzyme. It generally
passes into bile. So in extra hepatic cholestasis it is elevated significantly and in intra hepatic
cholestasis moderately. However in infective hepatitis it may be normal. In metastasis also it
is elevated.
Serum hepcidin and prohepcidin : Hepcidin and prohepcidin are proteins related to iron
metabolism. In cirrohosis serum prohepcidin level is lowered. Serum hepcidin level indicates
degree of liver dysfunction.
3. Write a note on Thyroid function test
A. Thyroid function is assessed by determining thyroid hormones level in blood.
1. Thyroid hormone profile : It is most common thyroid function test in our country.
Thyroid hormones measured in blood are thyroid stimulating hormone (TSH),
thyroxine (T4 ) and tri iodotyronine (T3 ). Many methods are available to determine
these hormones in blood. Enzyme linked immuno absorbent assay (ELISA ) and radio
immuno assay (RIA) methods are currently in use.
In this test directly blood is collected from subject under investigation and result are
compared with hormonal status of normal person. In normal people T4 ranges from 4. 0 -6.
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BIOCHEMISTRY - Questions and Answers
0 mg/ 100 ml. T3 level is100-250ng /100ml. Normal TSH ranges from100-300 m U/100 ml.
Hyper thyroidism : T3, T4 level is more in hyper thyroidism. But TSH level decreases in
hyper thyroidism.
Hypothyroidism : T3, T4 levels diminishes in hypothyroidism. However TSH level is
raised in this condition. All three hormone levels diminishes in hypothyroidism that
occurs due to diseases of hypothalamus and pituitary gland.
2. Thyrotrophin releasing hormone (TRH) stimulation test : In this test TRH is
administered intravenously in a fixed dose. Then blood is collected and TRH is
measured. Normal levels of TRH indicates hyperthyroidism. Higher TRH level
indicates hypothyroidism.
3. Blood cholesterol : Normal blood cholesterol level is 150-200mg%. Its level is more in
hypothyroidism, In contrast level of cholesterol is less in hyper thyroidism.
Other model questions are
4. Enumerate liver function tests and kidney function tests. Give
detailed procedure f or any one.
5. Clearance tests
6. Liver function tests in jaundice
7. Creatinine clearance tests
8. Differntial diagnosis of jaundice
9. Enzyme tests in liver dysfunction
10. Urine urobilinogen in jaundice
11. Glomerular filtration tests
12. Urea clearance test
13. Urine protein
14. Bile salts in urine
15. Hemolytic jaundice
16. LFT
17. KFT
18. Van den Bergh test
19. PT
20. TRH Stimulation test
224
CHAPTER - 20 | Xenobiotics
Chapter
20
Xenobiotics
1. What are xenobiotics?
A. Many substances that are used in agriculture, food processing, drugs, environmental
pollutants, toxins etc may enter into human body through food and air. They may be
pesticides used in agriculture, food additives, food colors of food processing, drugs taken as
part of treatment of illness, insecticides used to eliminate domestic pests, adulteration of
food with cheaply available varieties and microbial contamination gives rise to microbial
toxins. Generally all these are referred as xenobiotics
2. What is detoxification? Write its significance.
A. Detoxification is word used for the chemical transformation of xenobiotics or toxins or
poisons or drugs or chemicals to less toxic form. . Since they cause damage to body they are
eliminated from body.
The elimination involves chemical modifications which reduces toxicity and increases
solubility of xenobiotics. Hence detoxification may be considered as protective mechanism.
It is one way of protecting body from potential carcinogens
3. Explain biotransformation and lethal synthesis.
A. Biotrans formation is another similar word for detoxification. Sometimes a less toxic
substance may be converted to highly toxic substance by biotransformation. Then it is
known as lethal synthesis
4. Write about different types of detoxification reactions with examples.
A. Liver is the major organ involved in detoxification. Due to detoxification of drugs and their
elimination from blood to maintain a level of drug in the body the drug must be taken in
fixed timings and in fixed dose. Other wise it may not be effective in controlling diseases.
Liver detoxifies xenobiotics by four types of reactions.
They are 1. Hydrolysis, 2. Oxidation, 3. Hydroxylation, 4. Conjugation.
A xenobiotic is detoxified by any one of these reaction types or combination. Generally
detoxification of highly hydrophobic substances, potentially carcinogenic substances and
formation of procarcinogens or carcinogens involves more than one type of these reactions.
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BIOCHEMISTRY - Questions and Answers
1. Hydrolysis :
a. Drug like aspirin which is used as pain killer or analgesic is detoxified by
hydrolysis. An esterase hydrolyses aspirin.
Esterase
Aspirin
Acetic acid + salicylic acid
H2 O
b. Drugs that work as sedatives or anesthetic like atropine is hydrolyzed.
Atropine +H2 o
Tropine +tropicacid.
c. Digitalis a cardiac glycoside is hydrolyzed.
Digitals
Aglycone + carbohydrate
d. Pethidine is hydrolyzed to ethanol and meperidinic acid.
e. Phenacetin a pain killer and anti inflammatory drug is hydrolyzed to acetic acid and
phenetidine.
f. Procaine an anesthetic is hydrolyzed to dimethyl amino ethanol and para amino
benzoic acid.
g. Acetanilide is hydrolyzed to acetic acid and aniline.
2. Oxidation :
a. Ethyl alcohol an essential part of beverages or hot drinks is detoxified by oxidation.
An alcohol dehydrogenase converts it to an aldehyde which inturn oxidized to acetic
acid by aldehyde oxidase.
Ethyl alcohol
Acetaldehyde
Acetic acid.
b. Methanol an adulterant of ethyl alcoholic drinks is also detoxified by oxidation,
initially it is converted to aldehyde which is oxidized.
Methanol
Formaldehyde
Formic acid.
c. In the intestine tryptophan is converted to skatole and indole by enzymes of
intestinal flora. They are responsible for characteristic odor or smell of feces. They
are detoxified by oxidation. Skatole is oxidized to skatoxyl and indole is oxidized to
indoxyl.
d. Benzaldehyde and toluene are oxidized to benzoic acid.
e. Codeine a common ingradiant of cough syrups is detoxified by oxidation to
morphine and formaldehyde.
3. Hydroxylation :Detoxification by hydroxylation is brought about by microsomal cyt P450
hydroxylase system.
Pentobarbitol an anticonvulsant is hydroxylated to hydroxy pentobarbital.
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CHAPTER - 20 | Xenobiotics
Cy t P450
Pentobarbitol
Hydroxy pentobarbital.
O2
H2 O
Butazone is detoxified by hydroxylation to hydroxybutazone.
Steroid hormone containing drugs are eliminated by hydroxylation. Many anti
inflammatory drugs contain steroid hormones.
Steroids
Hydroxy steroids.
Quinones are detoxified by hydroxylation
Quinone
Hydroxyquinone.
Reduction : Rarely toxins are eliminated by reduction.
a. Desulfiram an inhibitor of aldehyde dehydrogenase is detoxified to diethyl
dithio carbomic acid.
b. Chloroamphenicol or chloromycetin an antibiotic is detoxified by reduction.
c. Picric acid is reduced to picramic acid.
4. Conjugation :Many drugs are detoxified by conjugation. Variety of compounds are
used to combine with drug. Some of them are glucuronic acid, sulfate, acetyl-CoA,
methyl group, glutathione, aminoacids like glycine, glutamine etc.
a. An antibiotic chloroamphenicol is conjugated with glucuronic acid.
b. Morphine a sedative or pain killer is detoxified with glucuronic acid.
Morphine + UDP – Glucuronic acid
Conjugated product +UDP.
c. Chloral hydrate a sedative is detoxified by conjugation with glucuronic acid.
UDP- Glucuronic acid+ chloral hydrate
UDP + Conjugated product.
d. Phenol is detoxified by conjugation with sulphate
Phenol +PAPS
Phenyl sulphate +PAP.
e. Skatoxyl and indoxyl are detoxied by conjugation with sulphate.
f. Antibiotic sulfanilamide is detoxified by conjugation with acetate.
Sulfanilamide + Acetyl –CoA
Acetyl sulfanilamide+ CoA.
g. Glutathione is used to conjugate several hydrocarbons with substituted
halogens. Difluoro benzene is conjugated with glutathione.
Diflorobenzene + glutathione
Mercapturic acid.
h. Salicylic acid is conjugated with glycine.
Salicylic acid +glycine
Salicyluric acid.
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BIOCHEMISTRY - Questions and Answers
I. Indole acetic acid is conjugated with glutamine.
Indole acetic acid +glutamine
indoleacetyl glutamine.
j. Nicotinic acid is detoxified by methylation.
Nicotinic acid + SAM
Methyl nicotinic acid +SAH.
k. Arsenic is detoxified by methylation.
Arsenic +SAM
Methyl arsenic acid.
5. Name phase I and phaseII detoxification reactions.
A. The three types of detoxification reactions hydrolysis, oxidation and hydroxylation are also
known as phase I detoxification reactions. Conjugation detoxification reactions are known
as phase II detoxification reactions.
Other model questions are
6. Detoxification
7. Xenobiotics
8. Detoxification by conjugation
9. How aspirin and phenol are detoxified?
10. How skatole and indole are detoxified?
228
CHAPTER - 21 | Cancer
Chapter
21
Cancer
1. Define cancer, metastasis and invasion.
A. Cancer is malignant growth or uncontrolled growth of cells. Malignant growth of cell is
also called as tumor. Cancer of a particular organ or tissue develops when the cells of that
organ have lost growth control.
Metastasis and invasion is spreading of cancer from organ of its origin to other organs.
Cancer cells of an organ enters blood stream then enters other organs and turns other
normal organs into malignant tumors.
2. Write about cancer genes
A. Cancer genes are related to development of cancer as well as prevention of cancer.
They are oncogenes, proto oncogenes and tumour suppressor genes.
Oncogenes : Are genes responsible for development of cancer.
Proto oncogenes : They are precursors of oncogenes. They are converted to oncogenes by
activation. Both cellular and viral oncogenes are found.
Examples for oncogenes and proto oncogenes are given below.
1. Cellular oncogene that causes rat sarcoma is designated as c-ras oncogene. Like wise cras proto oncogene.
2. Viral oncogene that causes rat sarcoma is designated as v-ras oncogene. Like wise vras proto oncogene.
Tumour suppressor genes (TSG)
: They are present in normal healthy people.
Products of them prevent cancer development. They encode proteins involved in several
cellular processes. They are
1. Enzymes that participate in DNA repair
2. Proteins that promote apoptosis
3. Receptors or signal transducers for hormones or developmental signal that inhibit cell
proliferation
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BIOCHEMISTRY - Questions and Answers
4. Intra cellular proteins involved in cell cycle progression.
5. Check point control proteins that arrest cell cycle if DNA is damaged.
Mutations in these proteins leads to loss of function. In many cancers tumor suppressor
genes have deletions or point mutations that prevent protein synthesis or lead to synthesis
of non functional protein. Examples are ATM, ATR, Ch K 1, Ch K2, Rb, P16 and P53
3. What is carcinogenesis? Write about different types of Carcinogenesis
A. Cancer forming process is known as carcinogenesis or tumorigenesis. Cancer is primarily
due to DNA damage or damage of genes. DNA damage may result from the action of
biological, chemical, physical and environmental agents on DNA. Incidence of cancer also
depends on the genetic make up of an individual. By several ways carcinogenesis occurs in
humans and other animals. Usually they are named according causative agent or factor.
Different types of carcinogenesis are given below
1. Biological agents that cause cancer or biological (viral) carcinogenesis : Some
DNA and RNA viruses are carcinogenic and hence they are called as oncogenic viruses.
When normal cells are cultured with oncogenic viruses, the normal cells are transformed
into cancer (tumour) cells. Oncogenes of the viruses are responsible for the development of
cancer.
Examples:
1. Hepatitis B virus cause liver cancer in humans.
2. Retro viruses also cause cancer in humans.
2. Chemical carcinogens or mutagens or chemical carcinogenesis: Many chemical
substances cause mutations in DNA. They are called mutagens. Some times this mutation
in DNA may convert normal cell to cancer cell. Then they are called as carcinogens.
Examples:
1. Cigarette smoke causes lung cancer in humans.
2. Aflatoxins are carcinogens.
3. Nitrosamine, Benzapyrins and asbestos also cause cancer.
3. Physical agents that cause cancer or physical carcinogenesis: Exposure to
radiation may damage DNA. UV light exposure causes mutation in DNA of skin cells.
Mutant DNA mediates carcinogenesis by activation of oncogenes which leads to
development of cancer of skin or multiple tumours of skin.
4. Write a note on tumour markers.
A. Cancer ells produce abnormal substances. Usually these substances are not produced by
normal cells. The abnormal substances produced by the cancer cells are enzymes,
hormones and proteins. These substances are released into blood by cancer cells. As a
result their level in blood rises. Measurement of these substances in blood or serum
provides useful information about cancer. Hence, they are called as tumour markers.
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CHAPTER - 21 | Cancer
Some important tumour markers are
1. Feto protein (AFP): It is a plasma protein and usually absent in normal people
plasma. It is tumour marker for liver cancer and germ cell cancer.
2. Calcitonin: It is a hormone. It is tumour marker for thyroid cancer.
3. Carcino embryonic antigen (CEA): It is a protein and it is tumour marker for lung
cancer, breast cancer, colon cancer and pancreas cancer.
4. Human chorionic gonodotropin (HCG): It is tropic hormone. It is tumour marker
for germ cell cancer and trophoblast cancer.
5. Acid phosphatase: It is tumour marker for prostate cancer.
6. High mobility group chromosomal proteins (HMGCP): They are family of nonhistone chromosomal proteins that serve as architectural elements in chromatin. In
normal tissues these proteins are expressed at very low levels. Their level is elevated in
many human cancers. This small molecular weight protein's expression is increased in
neoplastic transformation of cells and metastatic of tumour progression. They can
serve as novel diagnostic tumour markers.
7. Prostate specific antigen(PSA): It is tumour marker for prostate cancer
8. CA125: It is cancer marker.
5. Write a note on anti cancer agents.
A. Several compounds are able to prevent growth of cancer cells by blocking nucleotide or
nucleic acid formation which are necessary for cell multiplication. They are used as
anticancer agents. Mercaptopurine, fluorouracil, methotrexate, acivicin, azaserine etc.
are examples of anticancer agents.
Other model questions are
6. Mutagens
7. Viral carcinogenesis
8. Alfa feto protein
9. Tumour suppressor gene
10. PSA
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BIOCHEMISTRY - Questions and Answers
232
Biochemistry
Questions and Answers
Ø This book contains questions, answers and model
questions.
Ø The book is written in such way that learning of
question and answers given in each chapter makes
student to a enquire concept of that topic
simultaneously.
Ø Answers are given in simple language with
illustrations.
Ø Complex path ways are presented in a easy to
remember way.
Ø This book helps students in their preparation for
examination.
Ø The book contains 495 questions and answers to 249
questions. Answers to essay questions, short and very
smart questions are given in this book.
Dr. N. Mallikarjuna Rao
ISBN 978-81-924169-3-9
# 225/B, 9th ‘A’ Main, Vijayanagar, Bengaluru - 560 040.
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