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Biochemistry
Dr. Anwar J Almzaiel
Carbohydrates
Carbohydrates: are poly hydroxyl aldehydes or ketones or substances that yield
such compounds on hydrolysis. It widely distributed in plants and animals. In
plants, carbohydrate (glucose) synthesized from CO2 and water by
photosynthesis and stored as starch while in animal, carbohydrates synthesized
from amino acid, glycerol precursor of lipid, pyruvate or lactate
Carbohydrates are classified as follow:
1- Monosaccharides, or simple sugar, consist of a single polyhydroxy
aldehyde or ketone unit. The empirical formula is (CH2O)n where n=3 or
more the ratio of C:H:O is 1:2:1 example: D-glucose C6H12O6 or (CH2O)6
or C6(H2O)6they classified as trioses, tetroses, pentose, hexoses or
heptoses depending upon the number of carbon atoms (3-7) and aldoses
or ketoses, depending upon whether they have an aldehyde or ketone
group.
2- Disaccharides: are condensation products of two monosaccharide units
for example, lactose, maltose, sucrose.
3- Oligosaccharides: are condensation products (3-10) monosaccharide most
are not digested by enzymes
4- Polysaccharides: are condensation products of more than ten
monosaccharide units, examples are starche and dexrins
1- Monosaccharaides
Can be further classified according to the number of c atoms they
contain, and according to the aldehydes or ketone present into aldoses
or ketoses
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Lec:1
Biochemistry
Dr. Anwar J Almzaiel
Examples of aldose and ketose of physiological significance:Aldoses
Ketoses
Trioses
C3H6O3
glycerose
Dihydroxy
acetone
Tetroses
C4H8O4
Erythrose
Erythrulose
Pentose
C5H10O5
Ribose
Ribulose
Hexoses
C6H12O6
Glucose
Fructose
Heptoses
C7H14O7
Sedoheptose sedoheptulose
Biomedical importance of carbohydrates:
12345678-
Chief source of energy
Constituents of compound lipids and conjugated proteins
Degradation products act as promoters or catalysts
Certain carbohydrates derivatives are used as drugs like cardiac
glycosides and antibiotics
Lactose principle sugar of milk-in lactating mammary gland
Degradation products utilized for synthesis of other substances
such as fatty acids, cholesterol, amino acids
Constituents of mucopolysaccharides which form the ground
substances of mesenchymal tissues
Inherited deficiency of certain enzymes in metabolic pathways of
different carbohydrates can cause diseases.
General properties in references to favoured glucose
Biomedical, glucose is the most important monosaccharide. The structure
of glucose can be represented in three ways:
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Lec:1
Biochemistry
Dr. Anwar J Almzaiel
The straight chain structural formula (A) can account for some of
properties of glucose, but a cyclic structure (a hemiacetal formed by
reaction between the aldehyde group and hydroxyl group) is
thermodynamically favoured and one oxygen atom is actually in the form
of a chair (C).
The six –membered ring containing:
Asymmetric carbon (chiral carbon atom)
A carbon atom to which four different atoms or groups of atoms are attached is
said to be asymmetricatoms
CHO
H
C
R
3
OH
Lec:1
Biochemistry
Dr. Anwar J Almzaiel
The number of possible isomers of any given compound depends upon the
number of asymmetric C-atoms the molecule possesses
According to vant Hoff’s rule of 2n= the possible isomers where n=number of
asymmetric c-atoms in a compound gives rise to the formation of isomers of
that compound such compounds which are identical in composition and differ
only in special configuration are called “sterioisomers”
Two such isomers of glucose –D and L-glucose are mirror image of each other
O
H
OH
C
C
C
H
C
OH
C
OH
H
O
H
H
HO
H
OH
H
HO
HO
C
C
H
C
OH
C
H
C
H
CH2OH
CH2OH
L-glucose
D-glucose
Glucose with four asymmetric carbon atoms can form 16 isomers. The more
important types of isomerism found with glucose are as follow:
1- D and L isomerism: The orientation of the –H and –OH groups around
the carbon atom adjacent to the terminal primary alcohol carbon (carbon 5
in glucose) determines whether the sugar belongs to the D or L series. When
the –OH group on this carbon is on the right, the sugar is the D isomers,
when it is on the left, It is the L isomer
Most of the naturally occurring monosaccharaides are D sugars and enzymes
responsible for their metabolism are specific for this configuration.
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Lec:1
Biochemistry
Dr. Anwar J Almzaiel
The presence of asymmetric carbon atoms also confersof optical activity on the
compound (when a beam of plane polarized light is passed through a solution of
an optical isomer, it rotates either to the right, dextrorotatory (+), or to the left,
levorotatory (-). The direction of rotation of polarized light is independent of
stereochemistry of sugar, so it may be designated D(-), D (+), L(-) or L(+). For
example, the natural occurring form of fructose is D (-), while glucose in
solution is the D (+), and sometimes known as dextrose.
All the monosaccharaides except dihydroxyacetone contain one or more
asymmetric or chiral C-atoms and thus occur in optically active isomeric forms.
The simplest aldose is glyceraldehyde, contains only one chiral centre.
H
H
O
O
C
H
Chiral C
atom
C
OH
C
CH2OH
D-glyceraldehyde
HO
C
H
CH2OH
L-glyceraldehyde
The common monosaccharides have several asymmetric centres. The
aldohexoses have four chiralhave four chiral centres and can exist in the form of
2n=24=16 different stereoisomers for example D-glucose
2- Pyranose and furanose ring structures (Haworth projection): the ring
structures of monosaccharides are similar to the ring structures of either
pyrane (a six –membered ring) or furan (a five-membered ring). For
glucose in solution, more than 99% is in the pyranose form.
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Lec:1
Biochemistry
Dr. Anwar J Almzaiel
3- Alpha and beta anomers: The ring structure of an aldose is a hemiacetal,
since it is formed by combination of an aldehyde and an alcohol group,
similarly, the ring structure of a ketose is a hemiketal
Aldehydes and ketones can react with alcohols and yield hemiactals and
hemiketals. The carbonyl carbon becomes chiral in these reactions. The cyclic
structure is retained in the solution, but isomerism occurs about position 1, the
carbonyl or anomeric carbon atom
O
R
C
OH
+
H
HO
R2
alcohol
R1
C
OR2
H
aldehyde
hemiacetal
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Biochemistry
H
OH
C
C
C
OH
OH
H
C
OH
H
H
H
O
Dr. Anwar J Almzaiel
H
OH
CHO
C
C
H
C
C
H
C
CH2OH
CH2OH
D-glucose
OH
H
OH
OH
H
OH
C
C
C
OH
H
C
OH
H
HO
H
H
O
C
CH2OH
β-D-glucose
α-D-glucose
Two C anomers of D-glucose
(cyclic hemiacetal structure
4- Epimers: isomers that are result from the difference in the configuration
of the –OH and –H on carbon atoms2,3 and 4 of glucose, these isomers
are known as epimers. Biologically the most important epimers of
glucose are mannose (emiperized at carbon 2) and galactose (epimerized
at carbon 4). A process in which one epimer is converted to other known
as epimerization for example conversion of galactose to glucose in liver.
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Lec:1
Biochemistry
Dr. Anwar J Almzaiel
5- Aldose-ketose isomerism: Fructose has the same molecular formula as
glucose but differs in its structure, since there is a potential keto group in
position 2, the anomeric carbon atom of fructose, whereas in glucose
there is a potential aldehyde group in position 1 the anomeric carbon
atom
Monosaccharaides of biologically importance
Trioses: Both D-glyceraldehyde and dihdroxy acetone occur in the form of
phosphate esters, as intermediates in glycolysis, they are precursors of glycerol,
which the organism synthesizes and incorporates into various types of lipids
-Tetroses:
Erythrose-4-phosphat
occur
as
an
intermediate
in
hexosmonophosphate shunt pathway which is an alternative pathway for
glucose oxidation
-Pentoses:
a- D-ribose: is a constituent of nucleic acid RNA, also of certain coenzymes
e.gATP, NAD(P) and flavine coenzyme (FAD)
b- D -2-deoxyribose: is a constituent of DNA
-Hexoses:
a- D-glucose (dextrose) blood sugar
It is the chief physiological sugar present in normal blood continually and
at fairly constant level
All tissues utilize glucose for energy, RBCs and brain cells utilize glucose
as source of energy which result from hydrolysis of glycogen
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Biochemistry
Dr. Anwar J Almzaiel
(polysaccharides) stored in liver and muscle extracted in urine in poorly
controlled diabetes mellitus
b- D-galactose
Seldom found free in nature, in combination it occurs both in plants and
animals as constituent of galacto-lipid and glycoprotein. It is epimer of
glucose formed in body from glucose by epimerization by the enzyme
epimerase in liver. Hereditary galactosemia as a result of failure to
metabolized galactose leads to cataracts
c- D-fructose (fruit sugar)
It is a keto-hexose, it is sweeter than sucrose and more reactive than
glucose. It is a constituent of sucrose and also of polysaccharide (Inulin).
Readily metabolized either via glucose or directly. Hereditary fructose
intolerance leads to fructose accumulation and hypoglycaemia
d- D-mannose
It is widely distributed in combination as the polysaccharides. In the body
it is found as constituent of glycoproteins
e- Sedoheptulose
It is a keto-heptose found in plants. It’s serving as intermediate in
hexosmonophosphate shunt pathway
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