Kazakh Russian Medical University
Task No. 2
Semester: 2nd
GROUP: 103A
Lakhani Mansi
1. Enumerate the general properties of proteins
Answer:
1) Solubility :
Most of the amino acids are usually soluble in water and
insoluble in organic solvents.
2) Melting points :
Amino acids generally melt at higher temperatures, often above
200°C.
3) Taste: Amino acids may be sweet (Gly, Ala, Val), tasteless
(Leu) or bitter (Arg, Ile). Monosodium glutamate (MSG;
ajinomoto) is used as a flavoring agent in food industry, and
Chinese foods to increase taste and flavor. In some individuals
intolerant to MSG, Chinese restaurant syndrome (brief and
reversible flu-like symptoms) is observed.
4) Optical properties :
All the amino acid except glycine possess optical isomers due
to the presence of asymmetric carbon atom. Some amino acids
also have a second asymmetric carbon e.g. isoleucine,
threonine. The structure of L- and D-amino acids in comparison
with glyceraldehyde has been given (See Fig.4.1).
5) Amino acids as ampholytes :
Amino acids contain both acidic (COOH) and basic (NH2)
groups. They can donate a proton or accept a proton, hence
amino acids are regarded as ampholytes.
2. Phosphoproteins: Representatives, Structure, Role. The color
reactions on phosphoric acid
Answer: A phosphoprotein is a protein that is post-translationally
modified by the attachment of either a single phosphate group, or a
complex molecule such as 5'-phospho-DNA, through a phosphate
group. The target amino acid is most often serine, threonine, or
tyrosine residues (mostly in eukaryotes), or aspartic acid or histidine
residues (mostly in prokaryotes).
Phosphoric acid is the Prosthetic group e.g. casein (milk), Vitelline
(egg yolk).
3. The classification of proteins
Answer: Proteins are classified in several ways. Three major types of
classifying proteins based on their function, chemical nature and
solubility properties and nutritional importance are discussed here.
A. Functional classification of proteins Based on the functions they
perform, proteins are classified into the following groups (with
examples)
1. Structural proteins: Keratin of hair and nails, collagen of bone.
2. Enzymes or catalytic proteins: Hexokinase, pepsin.
3. Transport proteins: Hemoglobin, serum albumin.
4. Hormonal proteins: Insulin, growth hormone.
5. Contractile proteins: Actin, myosin.
6. Storage proteins: Ovalbumin, glutelin.
7. Genetic proteins: Nucleoproteins.
8. Defense proteins: Snake venoms, Immunoglobulins.
9. Receptor proteins for hormones, viruses.
B. Protein classification based on chemical nature and solubility
This is a more comprehensive and popular classification of proteins.
It is based on the amino acid composition, structure, shape and
solubility properties. Proteins are broadly classified into 3 major
groups
1. Simple proteins: They are composed of only amino acid residues.
2. Conjugated proteins: Besides the amino acids, these proteins
contain a non-protein moiety known as prosthetic group or
conjugating group.
3. Derived proteins: These are the denatured or degraded products of
simple and conjugated proteins.
The above three classes are further subdivided into different groups.
4. Chromoproteins: classification, structure, role
Answer: A chromoprotein is a conjugated protein that contains
a pigmented prosthetic group (or cofactor). A common example
is haemoglobin, which contains a heme cofactor, which is the ironcontaining molecule that makes oxygenated blood appear red. Other
examples of chromoproteins include
other hemochromes, cytochromes, phytochromes and flavoproteins.
In hemoglobin there exists a chromoprotein (tetramer MW:4 x 16.125
=64.500), namely heme, consisting of Fe++ four pyrrol rings.
A single chromoprotein can act as both a phytochrome and
a phototropin due to the presence and processing of multiple
chromophores. Phytochrome in ferns contains PHY3 which contains
an unusual photoreceptor with a dual-channel possessing both
phytochrome (red-light sensing) and phototropin (blue-light sensing)
and this helps the growth of fern plants at low sunlight.
The GFP protein family includes both fluorescent proteins and nonfluorescent chromoproteins. Through mutagenesis or irradiation, the
non-fluorescent chromoproteins can be converted to fluorescent
chromoproteins. An example of such converted chromoprotein
is "kindling fluorescent proteins" or KFP1 which was converted
from a mutated non-fluorescent Anemonia sulcata chromoprotein to a
fluorescent chromoprotein.
Sea anemones contain purple chromoprotein shCP with its GFPlike chromophore in the trans-conformation. The chromophore is
derived from Glu-63, Tyr-64 and Gly-65 and the phenolic group of
Tyr-64 plays a vital role in the formation of a conjugated system with
the imidazolidone moiety resulting a high absorbance in
the absorption spectrum of chromoprotein in the excited state. The
replacement of Tyrosine with other amino acids leads to the alteration
of optical and non-planer properties of the chromoprotein.
Fluorescent proteins such as anthrozoa chromoproteins emit long
wavelengths.
5. Hemoglobin: structure, role.
Answer: Hemoglobin, (haemoglobin BrE) (from the Greek word
αἷμα, haîma 'blood' + Latin globus 'ball, sphere' + -in)
(/ˌhiːməˈɡloʊbɪn, ˈhɛmoʊˌ-/), abbreviated Hb or Hgb, is the ironcontaining oxygen-transport metalloprotein in red blood
cells (erythrocytes) of almost all vertebrates (the exception being the
fish family Channichthyidae) as well as the tissues of
some invertebrates. Hemoglobin in blood carries oxygen from the
respiratory organs (e.g. lungs or gills) to the rest of the body
(i.e. tissues). There it releases the oxygen to permit aerobic
respiration to provide energy to power functions of an organism in the
process called metabolism. A healthy individual human has 12 to
20 grams of hemoglobin in every 100 mL of blood.
In mammals, the chromoprotein makes up about 96% of the red
blood cells' dry content (by weight), and around 35% of the total
content (including water). Hemoglobin has an oxygen-binding
capacity of 1.34 mL O2 per gram, which increases the total blood
oxygen capacity seventy-fold compared to dissolved oxygen in blood.
The mammalian hemoglobin molecule can bind (carry) up to four
oxygen molecules.
Hemoglobin is involved in the transport of other gases: It carries
some of the body's respiratory carbon dioxide (about 20–25% of the
total) as carbaminohemoglobin, in which CO2 is bound to the heme
protein. The molecule also carries the important regulatory
molecule nitric oxide bound to a thiol group in the globin protein,
releasing it at the same time as oxygen.
Hemoglobin is also found outside red blood cells and their progenitor
lines. Other cells that contain hemoglobin include the A9
dopaminergic neurons in the substantia nigra, macrophages, alveolar
cells, lungs, retinal pigment epithelium, hepatocytes, mesangial
cells in the kidney, endometrial cells, cervical cells and vaginal
epithelial cells. In these tissues, hemoglobin has a non-oxygencarrying function as an antioxidant and a regulator of iron
metabolism. Excessive glucose in one's blood can attach to
hemoglobin and raise the level of hemoglobin A1c.
Hemoglobin and hemoglobin-like molecules are also found in many
invertebrates, fungi, and plants. In these organisms, hemoglobins
may carry oxygen, or they may act to transport and regulate other
small molecules and ions such as carbon dioxide, nitric oxide,
hydrogen sulfide and sulfide. A variant of the molecule,
called leghemoglobin, is used to scavenge oxygen away
from anaerobic systems, such as the nitrogen-fixing nodules
of leguminous plants, lest the oxygen poison (deactivate) the system.
Hemoglobinemia is a medical condition in which there is an excess of
hemoglobin in the blood plasma. This is an effect of intravascular
hemolysis, in which hemoglobin separates from red blood cells, a
form of anemia.
6. Denaturation, renaturation. Denaturating agents, mechanism of their
action
Answer: The phenomenon of disorganization of native protein
structure is known as denaturation. Denaturation results in the loss of
secondary, tertiary and quaternary structure of proteins. This involves
a change in physical, chemical and biological properties of protein
molecules.
Agents of denaturation
Physical agents : Heat, violent shaking, X-rays, UV radiation.
Chemical agents : Acids, alkalies, organic solvents (ether, alcohol),
salts of heavy metals (Pb, Hg), urea, salicylate, detergents (e.g.
sodium dodecyl sulfate).
Characteristics of denaturation
1. The native helical structure of protein is lost.
2. The primary structure of a protein with peptide linkages remains
intact i.e., peptide bonds are not hydrolysed.
3. The protein loses its biological activity.
4. Denatured protein becomes insoluble in the solvent in which it was
originally soluble.
5. The viscosity of denatured protein (solution) increases while its
surface tension decreases.
6. Denaturation is associated with increase in ionizable and sulfhydryl
groups of protein. This is due to loss of hydrogen and disulfide bonds.
7. Denatured protein is more easily digested. This is due to increased
exposure of peptide bonds to enzymes. Cooking causes protein
denaturation and, therefore, cooked food (protein) is more easily
digested. Further, denaturation of dietary protein by gastric HCl
enchances protein digestion by pepsin.
8. Denaturation is usually irreversible. For instance, omelet can be
prepared from an egg (protein-albumin) but the reversal is not
possible.
9. Careful denaturation is sometimes reversible (known as
renaturation). Hemoglobin undergoes denaturation in the presence of
salicylate. By removal of salicylate, hemoglobin is renatured. 10.
Denatured protein cannot be crystallized.
Coagulation : The term ‘coagulum’ refers to a semi-solid viscous
precipitate of protein. Irreversible denaturation results in coagulation.
Coagulation is optimum and requires lowest temperature at
isoelectric pH. Albumins and globulins (to a lesser extent) are
coagulable proteins. Heat coagulation test is commonly used to
detect the presence of albumin in urine.
Flocculation : It is the process of protein precipitation at isoelectric
pH. The precipitate is referred to as flocculum. Casein (milk protein)
can be easily precipitated when adjusted to isoelectric pH (4.6) by
dilute acetic acid.
Flocculation is reversible. On application of heat, flocculum can be
converted into an irreversible mass, coagulum.
7. Myoglobin: structure, role
Answer: Myoglobin Myoglobin (Mb) is monomeric oxygen binding
hemoprotein found in heart and skeletal muscle. It has a single
polypeptide (153 amino acids) chain with heme moiety. Myoglobin
(mol. wt. 17,000) structurally resembles the individual subunits of
hemoglobin molecule. For this reason, the more complex properties
of hemoglobin have been conveniently elucidated through the study
of myoglobin.
Myoglobin functions as a reservoir for oxygen. It further serves as
oxygen carrier that promotes the transport of oxygen to the rapidly
respiring muscle cells.
8. The structures of protein’s molecules and bonds stabilizing them
Answer: Proteins are the polymers of L-D-amino acids. The structure
of proteins is rather complex which can be divided into 4 levels of
organization
1. Primary structure : The linear sequence of amino acids forming the
backbone of proteins (polypeptides).
2. Secondary structure : The spatial arrangement of protein by
twisting of the polypeptide chain.
3. Tertiary structure : The three dimensional structure of a functional
protein.
4. Quaternary structure : Some of the proteins are composed of two
or more polypeptide chains referred to as subunits. The spatial
arrangement of these subunits is known as quaternary structure.
[The structural hierarchy of proteins is comparable with the structure
of a building. The amino acids may be considered as the bricks, the
wall as the primary structure, the twists in a wall as the secondary
structure, a full-fledged self-contained room as the tertiary structure.
A building with similar and dissimilar rooms will be the quaternary
structure].
The term protein is generally used for a polypeptide containing more
than 50 amino acids. In recent years, however, some authors have
been using ‘polypeptide’ even if the number of amino acids is a few
hundreds. They prefer to use protein to an assembly of polypeptide
chains with quaternary structure.
9. Glycoproteins: classification, differences in the structure of a main
groups, representatives, role
Answer: Several proteins are covalently bound to carbohydrates
which are referred to as glycoproteins. The carbohydrate content of
glycoprotein varies from 1% to 90% by weight.
Sometimes the term mucoprotein is used for glycoprotein with
carbohydrate concentration more than 4%. Glycoproteins are very
widely distributed in the cells and perform variety of functions. These
include their role as enzymes, hormones, transport proteins,
structural proteins and receptors. A selected list of glycoproteins and
their major functions is given in Table 2.4.
The carbohydrates found in glycoproteins include mannose,
galactose, N-acetylglucosamine, N-acetylgalactosamine, xylose, Lfucose and N-acetylneuraminic acid (NANA). NANA is an important
sialic acid (See Fig.2.11).
Antifreeze glycoproteins : The Antarctic fish live below –2°C, a
temperature at which the blood would freeze. It is now known that
these fish contain antifreeze glycoprotein which lower the freezing
point of water and interfere with the crystal formation of ice.
Antifreeze glycoproteins consist of 50 repeating units of the tripeptide,
alanine-alanine-threonine. Each threonine residue is bound to Egalactosyl (1 o 3) D N-acetylgalactosamine.
10.
GAG: representatives, structure, role
Answer: Mucopolysaccharides are heteroglycans made up of
repeating units of sugar derivatives, namely amino sugars and uronic
acids. These are more commonly known as glycosaminoglycans
(GAG). Acetylated amino groups, besides sulfate and carboxyl
groups are generally present in GAG structure. The presence of
sulfate and carboxyl groups contributes to acidity of the molecules,
making them acid mucopolysaccharides. Some of the
mucopolysaccharides are found in combination with proteins to form
mucoproteins or mucoids or proteoglycans (Fig.2.16). Mucoproteins
may contain up to 95% carbohydrate and 5% protein.
Mucopolysaccharides are essential components of tissue structure.
The extracellular spaces of tissue (particularly connective tissuecartilage, skin, blood vessels, tendons) consist of collagen and elastin
fibers embedded in a matrix or ground substance. The ground
substance is predominantly composed of GAG.
The important mucopolysaccharides include hyaluronic acid,
chondroitin 4-sulfate, heparin, dermatan sulfate and keratan sulfate
(Fig.2.17).
Hyaluronic acid
Hyaluronic acid is an important GAG found in the ground substance
of synovial fluid of joints and vitreous humor of eyes. It is also present
as a ground substance in connective tissues, and forms a gel around
the ovum. Hyaluronic acid serves as a lubricant and shock absorbant
in joints
The lysosomal storage diseases caused by enzyme defects in the
degradation of glycosaminoglycans (GAGs) are known as
mucopolysaccharidoses. Mucopolysaccharidoses are characterized
by the accumulation of GAGs in various tissues that may result in
skeletal deformities, and mental retardation. Mucopolysaccharidoses
are important for elucidating the role of lysosomes in health and
disease.
11.
True glycoproteins: structure, representatives, role. Color
reactions on constituents and certain representatives
Answer: Several proteins are covalently bound to carbohydrates
which are referred to as glycoproteins. The carbohydrate content of
glycoprotein varies from 1% to 90% by weight.
Sometimes the term mucoprotein is used for glycoprotein with
carbohydrate concentration more than 4%. Glycoproteins are very
widely distributed in the cells and perform variety of functions. These
include their role as enzymes, hormones, transport proteins,
structural proteins and receptors. A selected list of glycoproteins and
their major functions is given in Table 2.4.
The carbohydrates found in glycoproteins include mannose,
galactose, N-acetylglucosamine, N-acetylgalactosamine, xylose, Lfucose and N-acetylneuraminic acid (NANA). NANA is an important
sialic acid (See Fig.2.11).
Antifreeze glycoproteins : The Antarctic fish live below –2°C, a
temperature at which the blood would freeze. It is now known that
these fish contain antifreeze glycoprotein which lower the freezing
point of water and interfere with the crystal formation of ice.
Antifreeze glycoproteins consist of 50 repeating units of the tripeptide,
alanine-alanine-threonine. Each threonine residue is bound to Egalactosyl (1 o 3) D N-acetylgalactosamine.
12.
Uniformity of the structure of protein’s molecules. The kinds of
bonds, examples
Answer: Proteins are biological polymers constructed from amino
acids joined together to form peptides. These peptide subunits may
bond with other peptides to form more complex structures. Multiple
types of chemical bonds hold proteins together and bind them to
other molecules. Take a closer look at the chemical
bonds responsible for protein structure.
Peptide Bonds
The primary structure of a protein consists of amino acids chained to
each other. Amino acids are joined by peptide bonds. A peptide bond
is a type of covalent bond between the carboxyl group of one amino
acid and the amino group of another amino acid. Amino acids
themselves are made of atoms joined together by covalent bonds.
Hydrogen Bonds
The secondary structure describes the three-dimensional folding or
coiling of a chain of amino acids (e.g., beta-pleated sheet, alpha
helix). This three-dimensional shape is held in place by hydrogen
bonds. A hydrogen bond is a dipole-dipole interaction between a
hydrogen atom and an electronegative atom, such as nitrogen or
oxygen. A single polypeptide chain may contain multiple alpha-helix
and beta-pleated sheet regions.
Each alpha-helix is stabilized by hydrogen bonding between the
amine and carbonyl groups on the same polypeptide chain. The betapleated sheet is stabilized by hydrogen bonds between the amine
groups of one polypeptide chain and carbonyl groups on a second
adjacent chain.
Hydrogen Bonds, Ionic Bonds, Disulfide Bridges
While secondary structure describes the shape of chains of amino
acids in space, tertiary structure is the overall shape assumed by the
entire molecule, which may contain regions of both sheets and coils.
If a protein consists of one polypeptide chain, a tertiary structure is
the highest level of structure. Hydrogen bonding affects the tertiary
structure of a protein. Also, the R-group of each amino acid may be
either hydrophobic or hydrophilic.
Hydrophobic and Hydrophilic Interactions
Some proteins are made of subunits in which protein molecules bond
together to form a larger unit. An example of such a protein is
hemoglobin. Quaternary structure describes how the subunits fit
together to form the larger molecule.
13.
Proteoglycans: representatives, structure, occurrence in the
body, role
Answer: Proteoglycans are conjugated proteins containing
glycosaminoglycans (GAGs). Several proteoglycans with variations in
core proteins and GAGs are known e.g. syndecan, betaglycan,
aggrecan, fibromodulin. For more information on the structure and
functions of proteoglycans Refer Chapter 2. GAGs, the components
of proteoglycans, are affected in a group of genetic disorders namely
mucopolysaccharidoses (Chapter 13).
14.
The general nature of products of proteins hydrolysis. The
classification of amino acids according their chemical nature,
examples. The concept of essential and non-essential amino acids,
value and non-value proteins
Answer: The mixture of amino acids liberated by protein hydrolysis
can be determined by chromatographic techniques. The reader must
refer Chapter 41 for the separation and quantitative determination of
amino acids. Knowledge on primary structure of proteins will be
incomplete without a thorough understanding of chromatography.
Proteans : These are the earliest products of protein hydrolysis by
enzymes, dilute acids, alkalies etc. which are insoluble in water. e.g.
fibrin formed from fibrinogen.
Metaproteins : These are the second stage products of protein
hydrolysis obtained by treatment with slightly stronger acids and
alkalies e.g. acid and alkali metaproteins.
Secondary derived proteins : These are the progressive hydrolytic
products of protein hydrolysis. These include proteoses, peptones,
polypeptides and peptides.
A mixture of amino acids (protein hydrolysate) or proteins can be
conveniently separated by ion-exchange chromatography. The amino
acid mixture (at pH around 3.0) is passed through a cation exchange
and the individual amino acids can be eluted by using buffers of
different pH. The various fractions eluted, containing individual amino
acids, are allowed to react with ninhydrin reagent to form coloured
complex. This is continuously monitored for qualitative and
quantitative identification of amino acids. The amino acid analyser,
first developed by Moore and Stein, is based on this principle.
15.
Metalloproteins: structure, representatives, role
Answer: Metalloprotein is a generic term for a protein that contains a
metal ion cofactor. A large proportion of all proteins are part of this
category. For instance, at least 1000 human proteins (out of ~20,000)
contain zinc-binding protein domains although there may be up to
3000 human zinc metalloproteins.
It is estimated that approximately half of all proteins contain
a metal. In another estimate, about one quarter to one third of all
proteins are proposed to require metals to carry out their
functions. Thus, metalloproteins have many different functions
in cells, such as storage and transport of
proteins, enzymes and signal transduction proteins, or infectious
diseases. The abundance of metal binding proteins may be inherent
to the amino acids that proteins use, as even artificial proteins without
evolutionary history will readily bind metals.
Most metals in the human body are bound to proteins. For instance,
the relatively high concentration of iron in the human body is mostly
due to the iron in hemoglobin.
16.
The chemical properties, peculiarities of amino acid’s
composition, occurrence in the nature and role of albumins
Answer: The general reactions of amino acids are mostly due to the
presence of two functional groups namely carboxyl ( COOH) group
and amino ( NH2) group.
Reactions due to COOH group
1. Amino acids form salts ( COONa) with bases and esters ( COORc)
with alcohols.
2. Decarboxylation : Amino acids undergo decarboxylation to produce
corresponding amines.
R CH2 R CH COO + CO2 – + NH3 NH3 +
This reaction assumes significance in the living cells due to the
formation of many biologically important amines. These include
histamine, tyramine and J-amino butyric acid (GABA) from the amino
acids histidine, tyrosine and glutamate, respectively.
3. Reaction with ammonia : The carboxyl group of dicarboxylic amino
acids reacts with NH3 to form amide
Aspartic acid + NH3 •o Asparagine
Glutamic acid + NH3 •o Glutamine
Reactions due to NH2 group
4. The amino groups behave as bases and combine with acids (e.g.
HCl) to form salts ( NH3 +Cl–).
5. Reaction with ninhydrin : The D-amino acids react with ninhydrin to
form a purple, blue or pink colour complex (Ruhemann’s purple).
Amino acid + Ninhydrin •o Keto acid + NH3 + CO2 + Hydrindantin
Hydrindantin + NH3 + Ninhydrin o Ruhemann’s purple
Ninhydrin reaction is effectively used for the quantitative
determination of amino acids and proteins. (Note : Proline and
hydroxyproline give yellow colour with ninhydrin).
6. Colour reactions of amino acids : Amino acids can be identified by
specific colour reactions (See Table 4.3).
7. Transamination : Transfer of an amino group from an amino acid to
a keto acid to form a new amino acid is a very important reaction in
amino acid metabolism (details given in Chapter 15).
8. Oxidative deamination : The amino acids undergo oxidative
deamination to liberate free ammonia (Refer Chapter 15).
17.
Lipoproteins: the concept of the structure, differences between
serum and tissue lipoproteins
Answer: Lipoproteins are molecular complexes of lipids with proteins.
They are the transport vehicles for lipids in the circulation. There are
five types of lipoproteins, namely chylomicrons, very low density
lipoproteins (VLDL), low density lipoproteins (LDL), high density
lipoproteins (HDL) and free fatty acidalbumin complexes. Their
structure, separation, metabolism and diseases are discussed
together (Chapter 14).
Lipoproteins are molecular complexes that consist of lipids and
proteins (conjugated proteins). They function as transport vehicles for
lipids in blood plasma. Lipoproteins deliver the lipid components
(cholesterol, triacylglycerol etc.) to various tissues for utilization.
Structure of lipoproteins
A lipoprotein basically consists of a neutral lipid core (with
triacylglycerol and/or cholesteryl ester) surrounded by a coat shell of
phospholipids, apoproteins and cholesterol (Fig.14.33). The polar
portions (amphiphilic) of phospholipids and cholesterol are exposed
on the surface of lipoproteins so that lipoprotein is soluble in aqueous
solution.
Classification of lipoproteins
Five major classes of lipoproteins are identified in human plasma,
based on their separation by electrophoresis (Fig.14.34).
1. Chylomicrons : They are synthesized in the intestine and transport
exogenous (dietary) triacylglycerol to various tissues. They consist of
highest (99%) quantity of lipid and lowest (1%) concentration of
protein. The chylomicrons are the least in density and the largest in
size, among the lipoproteins.
2. Very low density lipoproteins (VLDL) : They are produced in liver
and intestine and are responsible for the transport of endogenously
synthesized triacylglycerols.
3. Low density lipoproteins (LDL) : They are formed from VLDL in the
blood circulation. They transport cholesterol from liver to other
tissues.
4. High density lipoproteins (HDL) : They are mostly synthesized in
liver. Three different fractions of HDL (1, 2 and 3) can be identified by
ultracentrifugation. HDL particles transport cholesterol from peripheral
tissues to liver (reverse cholesterol transport).
5. Free fatty acids—albumin : Free fatty acids in the circulation are in
a bound form to albumin. Each molecule of albumin can hold about
20-30 molecules of free fatty acids. This lipoprotein cannot be
separated by electrophoresis.
Answer: The general reactions of amino acids are mostly due to the
presence of two functional groups namely carboxyl ( COOH) group
and amino ( NH2) group.
Reactions due to COOH group
1. Amino acids form salts ( COONa) with bases and esters ( COORc)
with alcohols.
2. Decarboxylation : Amino acids undergo decarboxylation to produce
corresponding amines.
R CH2 R CH COO + CO2 – + NH3 NH3 +
This reaction assumes significance in the living cells due to the
formation of many biologically important amines. These include
histamine, tyramine and J-amino butyric acid (GABA) from the amino
acids histidine, tyrosine and glutamate, respectively.
3. Reaction with ammonia : The carboxyl group of dicarboxylic amino
acids reacts with NH3 to form amide
Aspartic acid + NH3 •o Asparagine
Glutamic acid + NH3 •o Glutamine
Reactions due to NH2 group
4. The amino groups behave as bases and combine with acids (e.g.
HCl) to form salts ( NH3 +Cl–).
5. Reaction with ninhydrin : The D-amino acids react with ninhydrin to
form a purple, blue or pink colour complex (Ruhemann’s purple).
Amino acid + Ninhydrin •o Keto acid + NH3 + CO2 + Hydrindantin
Hydrindantin + NH3 + Ninhydrin o Ruhemann’s purple
Ninhydrin reaction is effectively used for the quantitative
determination of amino acids and proteins. (Note : Proline and
hydroxyproline give yellow colour with ninhydrin).
6. Colour reactions of amino acids : Amino acids can be identified by
specific colour reactions (See Table 4.3).
7. Transamination : Transfer of an amino group from an amino acid to
a keto acid to form a new amino acid is a very important reaction in
amino acid metabolism (details given in Chapter 15).
8. Oxidative deamination : The amino acids undergo oxidative
deamination to liberate free ammonia (Refer Chapter 15).
18.
The chemical properties, peculiarities of amino acid’s
composition, fractions, occurrence in the nature and role of globulins
Answer: The general reactions of amino acids are mostly due to the
presence of two functional groups namely carboxyl ( COOH) group
and amino ( NH2) group.
Reactions due to COOH group
1. Amino acids form salts ( COONa) with bases and esters ( COORc)
with alcohols.
2. Decarboxylation : Amino acids undergo decarboxylation to produce
corresponding amines.
R CH2 R CH COO + CO2 – + NH3 NH3 +
This reaction assumes significance in the living cells due to the
formation of many biologically important amines. These include
histamine, tyramine and J-amino butyric acid (GABA) from the amino
acids histidine, tyrosine and glutamate, respectively.
3. Reaction with ammonia : The carboxyl group of dicarboxylic amino
acids reacts with NH3 to form amide
Aspartic acid + NH3 •o Asparagine
Glutamic acid + NH3 •o Glutamine
Reactions due to NH2 group
4. The amino groups behave as bases and combine with acids (e.g.
HCl) to form salts ( NH3 +Cl–).
5. Reaction with ninhydrin : The D-amino acids react with ninhydrin to
form a purple, blue or pink colour complex (Ruhemann’s purple).
Amino acid + Ninhydrin •o Keto acid + NH3 + CO2 + Hydrindantin
Hydrindantin + NH3 + Ninhydrin o Ruhemann’s purple
Ninhydrin reaction is effectively used for the quantitative
determination of amino acids and proteins. (Note : Proline and
hydroxyproline give yellow colour with ninhydrin).
6. Colour reactions of amino acids : Amino acids can be identified by
specific colour reactions (See Table 4.3).
7. Transamination : Transfer of an amino group from an amino acid to
a keto acid to form a new amino acid is a very important reaction in
amino acid metabolism (details given in Chapter 15).
8. Oxidative deamination : The amino acids undergo oxidative
deamination to liberate free ammonia (Refer Chapter 15).
19.
Phosphoproteins: structure, representatives, role. The color
reaction on phosphoric acid
Answer: A phosphoprotein is a protein that is post-translationally
modified by the attachment of either a single phosphate group, or a
complex molecule such as 5'-phospho-DNA, through a phosphate
group. The target amino acid is most often serine, threonine, or
tyrosine residues (mostly in eukaryotes), or aspartic acid or histidine
residues (mostly in prokaryotes).
Phosphoric acid is the Prosthetic group e.g. casein (milk), Vitelline
(egg yolk).
20.
Scleroproteins: representatives, structure, properties, role
Answer: These are fiber like in shape, insoluble in water and resistant
to digestion. Albuminoids or scleroproteins are predominant group of
fibrous proteins.
(i)
Collagens are connective tissue proteins lacking tryptophan.
Collagens, on boiling with water or dilute acids, yield gelatin
which is soluble and digestible (Chapter 22).
(ii) Elastins : These proteins are found in elastic tissues such as
tendons and arteries.
(iii)
Keratins : These are present in exoskeletal structures e.g. hair,
nails, horns. Human hair keratin contains as much as 14%
cysteine (Chapter 22).
21.
Construct the tripeptide: N-alanylvalylleucine-C.
Answer:
22.
Construct the tripeptide: N-seryltreonylisoleucine-C.
Answer:
23.
Construct the tripeptide: N-cysteylmethionylhistidine-C.
Answer:
24.
Construct the tripeptide: N-glutamyllysylglycine-C.
Answer:
25.
Construct the tripeptide: N-prolyltryptophyltyrosine-C.
Answer: