Chemistry of carbohydrates

Chemistry of Carbohydrates
R.C. Gupta
Professor and Head
Dept. of Biochemistry
National Institute of Medical Sciences
Jaipur, India
1

EMB-RCG Chemistry of Carbohydrate
2
Synthesized in plants by photosynthesis
Used as source of energy by animals
Largest source of energy in our daily diet
Some perform other functions also
Carbohydrates

3
Constituents of nucleic
acids
Constituents of nervous
tissue
Form some hormones and
blood group substances
Constituent of mucus
Structural constituents
of tissues
Ribose and
deoxyribose
Glycolipids
Glycoproteins
Mucin
Mucopoly-
saccharides

Definition
4
Carbohydrates are polyhydroxyaldehydes
or polyhydroxyketones or compounds
that give polyhydroxyaldehydes or poly-
hydroxyketones on hydrolysis
Carbohydrates are aldehyde or ketone
derivatives of polyhydric alcohols
OR

Classification
5
Carbohydrates can be
classified into:
Monosaccharides
Disaccharides
Polysaccharides

Monosaccharides are the smallest carbo-
hydrates
They can’t be hydrolysed into smaller
carbohydrates
Made up of carbon, hydrogen and oxygen
Have general formula CnH2nOn
6

Disaccharides are made up of two
monosaccharides
The constituent monosaccharides may be
identical or different
The common disaccharides are sucrose,
lactose and maltose
7
Monosaccharides and disaccharides are
called sugars because of their sweet taste

Polysaccharides are made up of a large
number of monosaccharide molecules
Those having 3-6 monosaccharide units are
called oligosaccharides
Those having more than 6 monosaccharide
units are called polysaccharides
8

Monosaccharides
9
Monosaccharides may be aldehyde or
ketone derivatives of polyhydric alcohols
Accordingly, they can be divided into
aldoses and ketoses

10
Monosaccharides having
an aldehyde group
Aldoses
Monosaccharides having
a keto group
Ketoses

Aldoses and ketoses may be sub-divided
on the basis of number of carbon atoms:
Trioses - Three carbon atoms
Tetroses - Four carbon atoms
Pentoses - Five carbon atoms
Hexoses - Six carbon atoms
Heptoses - Seven carbon atoms
11

12
No. of
carbon
atoms
Trioses 3 Glyceraldehyde Dihydroxyacetone
Tetroses 4 Erythrose Erythrulose
Pentoses 5 Ribose Ribulose
Hexoses 6 Glucose Fructose
Aldose Ketose
Some common monosaccharides

Trioses
The smallest monosaccharides
Include glyceraldehyde and
dihydroxyacetone
Aldehyde and ketone derivatives of
trihydric alcohol, glycerol
Formed during metabolism of hexoses
13

Glyceraldehyde and dihydroxyacetone share
the same molecular formula (C3H6O3)
They differ in their structural formulae
They are isomers of each other
This is a simple aldose-ketose isomerism
15

Glyceraldehyde shows another type of
isomerism
It contains an asymmetric carbon atom
All the four groups attached to C2
are different from each other
This produces two stereo-isomers of
glyceraldehyde
16

17
The ‒OH group is
on right hand side
of carbon 2
D-Glyceraldehyde
The ‒OH group is
on left hand side
of carbon 2
L-Glyceraldehyde

CHO CHO
| |
H—C—OH HO—C—H
| |
CH2OH CH2OH
D-Glyceraldehyde L-Glyceraldehyde

Asymmetric carbon atom also confers
optical activity
On passing polarised light, its plane is
rotated to the left or the right
One stereoisomers causes laevorotation
(rotation to left) and the other causes
dextrorotation (rotation to right)
19

20
They have several stereoisomers and
optical isomers
The higher monosaccharides possess more
than one asymmetric carbon atoms
Stereoisomerism and optical activity are
present in higher monosaccharides also

21
The D/L assignment depends upon the
orientation of –OH group relative to the
asymmetric carbon atom most remote from
the aldehyde or the ketone group
This will be carbon atom 3 in tetroses,
carbon atom 4 in pentoses and carbon
atom 5 in hexoses

22
Most of the carbohydrates important in
human biochemistry are D-isomers
If the –OH group is on the left, the isomer
will be L
If the –OH group is on the right of these
carbon atoms, the isomer will be D

23
A method to show the configuration of mono-
saccharides on paper was devised by Emil Fischer
The monosaccharides are shown as linear
molecules in these formulas
His formulas are known as Fischer projection
formulas

24
The orientation of the chain is such that carbon 1
(C1) is at the top and ‒CH2OH at the bottom
All the bonds are shown by horizontal or vertical
lines
The chain of carbon atoms is shown to be
oriented vertically

25
In aldoses, the carbon of the aldehyde group is
C1
In ketoses, the carbon of the keto group is C2
The hydrogen atoms and hydroxyl groups are on
left or right of the carbon atoms

This is formed as an intermediate (as erythrose-4-
phosphate) during the metabolism of glucose via
the hexose monophosphate shunt pathway
26
Tetroses
The only tetrose of some importance in human
beings is D-erythrose
The corresponding ketotetrose is D-erythrulose

Pentoses
28
D-Ribose and its corresponding ketopentose,
D-ribulose are formed as intermediates in the
hexose monophosphate shunt
D-Ribose and 2-deoxy-D-ribose are the most
important pentoses which are the constituents
of nucleic acids and nucleotides

30
Another pentose formed in HMP shunt pathway is
D-xylulose
Its corresponding aldopentose is D-xylose
D-Xylose is used as a diagnostic agent to study
intestinal absorption

31
An L-pentose occurring in human beings is
L-xylulose
It is excreted in urine in detectable amounts in a
hereditary disease, essential pentosuria
L-Xylulose is formed as an intermediate in the
uronic acid pathway of carbohydrate metabolism

33
Hexoses
The important ketohexose is D-fructose which
is the ketoisomer of D-glucose
The important aldohexoses in human beings
are D-glucose, D-galactose and D-mannose

35
The carbohydrates are transported in blood in the
form of D-glucose
This is the form in which carbohydrates are used
by the tissues to obtain energy
Most other carbohydrates are converted into
D-glucose in the body
The important polysaccharides, starch, dextrin
and glycogen are made up of D-glucose

D-Galactose is present in glycolipids which are an
important constituent of nervous tissue
It is also present in milk in the form of the
disaccharide, lactose
Amino derivatives of D-galactose and D-mannose
are present in mucopolysaccharides and glyco-
proteins
36

39
Heptoses
It is formed as an intermediate in HMP
shunt pathway of carbohydrate metabolism
The only heptose important in human
beings is D-sedoheptulose which is a
ketoheptose

41
A problem arose with the discovery of two
different methyl glucosides derived from glucose.
Anomerism
These can be explained easily by Fischer
projection formulas
Aldose-ketose isomerism, stereoisomerism and
optical isomerism have been seen earlier

42
One is known as methyl-a-D-glucoside and the
other is known as methyl-b-D-glucoside
Both have cyclic structures
Glucose reacts with methanol in the presence of a
mineral acid to form two distinct methyl glucosides

44
It was later found that higher monosaccharides
also exist in solution in a cyclic hemi-acetal form
If cyclisation involves C4, it results in the formation
of a five-membered ring similar to furan
Ring is formed by a reaction between carbonyl
group and the ‒OH group attached to C4 or C5
A monosaccharide having this type of ring structure
is designated as a furanose

45
If cyclisation involves C5, it results in the formation
of a six-membered ring
This ring (cyclic 1,5-oxide) is similar in structure to
pyran
Therefore, a monosaccharide having this type of
ring structure is called a pyranose

47
For showing the ring form of monosaccharides on
paper, Haworth introduced a projection formula
In this representation, the plane of the ring is
perpendicular to the plane of the paper
The substituent groups project upwards or
downwards from the ring
The ring oxygen is away from the viewer

48
This is C1 in case of aldoses and C2 in case of
ketoses
Cyclisation creates an additional asymmetric
carbon atom in the molecule.
This carbon is known as anomeric carbon atom

49
The anomeric carbon produces an additional type
of isomerism called anomerism
In b-anomer, it projects above the plane of the ring
In a-anomer, ‒OH group attached to anomeric
carbon projects below the plane of the ring
The additional isomers are called a-anomer and b-
anomer

50
The a and b anomers of glucose in pyran
ring form
↓
↑
CH2OH
H H
OH OH
H
HOH
O
H OH
CH2OH
H OH
OH H
H
HOH
O
H OH
O
1
2
4
5
6
1
23 3
4
5
6
Pyran a-D-Glucopyranose b-D-Glucopyranose

51
Ketohexoses exist in the form of a five
membered ring resembling furan
The monosaccharides in furan ring form
also exhibit anomerism

52
It projects above the plane of the ring in
the b-anomer
The additional centre of asymmetry in keto-
hexoses is at carbon atom 2
The ‒OH group attached to C2 projects
below the plane of the ring in the a-anomer

↓
↑
5
6
5
4 3
2
1
6
4 3
2
1
CH2OH

Aldopentoses, e.g. ribose, also exist in the
form of five membered furan ring form
54
Sometimes even aldohexoses exist in furan
ring form

55
H
H H
a-D-Ribofuranose
OH
HOH2C
OH
H
O
OH
OH
H H
b-D-Ribofuranose
H
HOH2C
OH
H
O
OH
CH2OH
|
H
OH H
a-D-Glucofuranose
OH
H‒C‒OH
H
H
O
OH
1
1
1
5
4
3 2
5
4
3 2 6
5
4
3 2

56
Mutarotation
Carbohydrates possessing an asymmetric carbon
atom are optically active
Before the ring structures of carbohydrates were
established, it had been shown that glucose
existed in two optically distinct forms
The specific rotation caused by each carbo-
hydrate is quite characteristic

57
When either form is allowed to stand, the specific
rotation gradually changes to +52.5°, and then
becomes constant
When glucose, crystallized from a concentrated
aqueous solution at 110°C, is dissolved in water,
it has a specific rotation of +19°
When glucose, crystallized from alcohol-water, is
dissolved in water, its specific rotation is +112°

58
Glucose crystallized from a concentrated
aqueous solution at 110°C, is b-D-glucose
On discovery of ring structures of carbohydrates,
it was found that the glucose crystallized from
alcohol-water is a-D-glucose
This change in specific rotation is known as
mutarotation

59
On standing, a-D-glucose changes into
b-D-glucose and vice versa
b-D-Glucose has a specific rotation of +19°
a-D-Glucose has a specific rotation of +112°
The inter-conversion continues until an
equilibrium mixture is formed

60
This equilibrium mixture has a specific rotation of
+52.5°
The equilibrium mixture contains 36% a-D-
glucose and 64% b-D-glucose

61
Epimerism
They are said to be epimers of each other and
this phenomenon is known as epimerism
Glucose and galactose differ from each other
with respect to orientation of the hydrogen and
hydroxyl groups around carbon atom 4 only
Glucose, galactose and mannose show another
type of isomerism

62
HO H
H OH
H
HOH
O
H OH
a-D-Gluco-
pyranose
CH2OH
H H
OH OH
H
HOH
O
H OH
CH2OH
a-D-Galacto-
pyranose
4

63
Similarly, glucose and mannose are also epimers
of each other
They differ with respect to the orientation of ‒H
and ‒OH groups around carbon atom 2
H H
OH OH
H
OHOH
O
H H
a-D-Glucopyranose
CH2OH
H H
OH OH
H
HOH
O
H OH
CH2OH
a-D-Mannopyranose
2

64
Thus, carbohydrates differing in the orientation of
substituent groups around a single carbon atom,
are known as epimers of each other
Galactose and mannose are not epimers as the
orientation of ‒H and ‒OH differs around two
carbon atoms i.e. carbon atoms 2 and 4

Derivatives of
monosaccharides
Deoxysugars
Amino sugars
Uronic acids
65

Important
deoxysugars are:
Deoxyribose
L-Fucose
66

67
Deoxyribose
Generally present as b-anomer
in deoxyribonucleic acid
Formed by replacement of
hydroxyl group attached to carbon
atom 2 of ribose with hydrogen

68
L-Fucose
6-Deoxy derivative of
L-galactose
Commonly found in
glycoproteins

H
OH
OHH
H
CH3
OH
OH
O
H
H
a
a
-L-Fucose
(6-Deoxy- -L-galactose)2-Deoxy- -D-riboseb
H
CH OH2
OH
OH
H
H
O
HH

Amino sugars
Formed by substitution of a hydroxyl
group of the sugar with an amino group
Also known as hexosamines as most
are derived from hexoses
Important ones are glucosamine,
galactosamine and mannosamine
70

71
CH OH2
OH H
H OH
H
HOH
H NH2
CH OH2
H H
OH OH
H
H N2OH
H H
Glucosamine Galactosamine Mannosamine
(2-Amino-a-D-glucose) (2-amino-a-D-galactose) (2-Amino-a-D-mannose)
CH OH2
H H
OH OH
H
HOH
O
H NH2
O O

Constituent of hyaluronic
acid
Galactosamine
Constituent of chondroitin
sulphate
Mannosamine Found in glycoproteins
Glucosamine

73
N-Acetylglucosamine
CH2OH
H H
OH OH
H
HOH
H NHCOCH3
O
N-Acetylgalactosamine
CH2OH
OH H
H OH
H
HOH
H NHCOCH3
O
The amino sugars are generally present
in mucopolysaccharides in the form of
their N-acetyl derivatives in which an acetyl
group is attached to the amino group

74
One of the carbon atoms (as in
chondroitin sulphate, dermatan
sulphate and keratan sulphate
Amino group (as in heparin)
The sulphate group may be attached to:
The amino sugars may also be sulphated

Uronic acids
Formed by substitution of the terminal
–CH2OH group of aldoses with a
carboxyl group
The most important is glucuronic acid, a
derivative of glucose
L-Iduronic acid is another uronic acid
found in some mucopolysaccharides
75

76
H
H OH
OH H
COOH
HOH
O
H OH
b-D-Glucuronic acid a-L-Iduronic acid
COOH
H OH
OH H
H
O
H OH
HOH

77
Is used in our body to detoxify a
number of harmful substances
Is a constituent of several mucopoly-
saccharides either as such or in the
form of its sulphate
Glucuronic acid

78
Reactions of monosaccharides
Study their properties
Identify the monosaccharides
A number of chemical reactions are
performed in the laboratory to:

79
Interconversion
If glucose or fructose is allowed to stand
in a dilute alkali for a few hours, a
mixture containing both the
monosaccharides is formed
This is due to very poor stability of the
monosaccharides in alkaline solutions

80
The interconversion occurs via formation
of a common enediol intermediate
CH OH2
|
C = O
|
HO — C — H
|
H — C — OH
|
H — C — OH
|
CH OH2
D-FructoseD-Glucose
CHO
|
H — C — OH
|
HO — C — H
|
H — C — OH
|
H — C — OH
|
CH OH2
C — OH
||
H — C — OH
|
HO — C — H
|
H — C — OH
|
H — C — OH
|
CH OH2
Enediol intermediate

81
Dehydration
Monosaccharides are dehydrated by
strong mineral acids e.g. hydrochloric
acid and sulphuric acid
They are converted into furfural or
hydroxymethyl furfural

82
D-Ribose
OH
CH2OH
OH
H
H
H
OH
H
O
CH2OHCHO
Hydroxymethyl
furfural
O
Furfural
CHO
‒ 3 H2O
O
CH2OH
OH
OH
H
H
OHH
CH2OH
D-Fructose
O
‒ 3 H2O

83
This reaction forms the basis of a number of
tests for identification of carbohydrates
Furfural or its derivatives condense with various
phenols, e.g. a-naphthol (Molisch’s test) and
resorcinol (Seliwanoff’s test), to form
characteristically coloured complexes
Molisch’s test Seliwanoff’s test

84
Oxidation
Aldehyde group of aldoses is readily
oxidised to a carboxyl group by mild
oxidizing agents in acidic medium
The general name of the resulting
product is aldonic acid
Strong oxidizing agents, e.g. nitric acid,
convert aldonic acid into aldaric acid by
oxidizing the primary alcohol group to
carboxyl group

86
Monosaccharides are reduced in the presence
of sodium amalgam to sugar alcohols
Glucose, mannose and galactose are reduced
to sorbitol, mannitol and dulcitol respectively
Fructose gives both sorbitol and mannitol on
reduction
Reduction

87
Ribose and ribulose are reduced to ribitol
Glyceraldehyde and dihydroxyacetone are
reduced to glycerol

88
CHO
R
CH2OH
R
+ 2H
Aldose Sugar alcohol
CH2OH
C=O
R
CH2OH
H‒C‒OH
R
+ 2H
Ketose Sugar alcohol
I I
I
I I
I

89
Aldehyde and ketone groups of mono-
saccharides possess reducing property
Many qualitative/quantitative tests for sugars
are based on reduction of metal ions
The metals reduced by the sugars include
copper, iron, bismuth, silver etc
Reduction of metal ions

90
The best known example of reduction of
metal ions is Benedict’s test
Sugar is boiled with a solution of cupric
hydroxide, stabilized by sodium citrate,
in Benedict’s test
Cupric hydroxide is reduced to cuprous
oxide which separates out of the solution
as a red precipitate

91
If the sugar solution is dilute, the precipitate may
be orange, yellow or green in colour depending
upon the concentration of the sugar
This test differentiates the carbohydrates that
possess a free aldehyde or ketone group from
those that do not

92
This is another reaction given by carbo-
hydrates that possess a free aldehyde or
ketone group
The reaction involves carbon atoms 1 and
2 of the aldoses and ketoses
Formation of osazones

93
The carbohydrate is heated with pheny-
hydrazine in a boiling water-bath
Acetate buffer is added to maintain the pH
at 4.3
A series of reactions occur leading to the
formation of osazone of the given
carbohydrate

H–C=O
|
H—C—OH
|
R C6H5NH-NH2 H2O
H—C=N—NH—C6H5
|
H—C—OH
|
R
Aldose Phenylhydrazone
C6H5NH-NH2
C6H5NH2 + NH3
H—C=N—NH—C6H5H—C=N—NH—C6H5
||
C=OC=N—NH—C6H5
||
RR C6H5NH-NH2H2O
Osazone Intermediate compound

95
The differences between carbon atoms 1
and 2 are obliterated during formation of
osazones
Those carbohydrates that differ only with
respect to these two carbon atoms form
identical osazones
Glucose, mannose and fructose are such
carbohydrates

96
The other carbohydrates form distinctive
osazones which differ in their:
Time of formation
Solubility
Melting point
Crystalline structure
These differences may be used to identify
the carbohydrates

97
Osazone crystals of glucose,
mannose and fructose

98
If an aldohexose, e.g. glucose, is heated with
hydroiodic acid, it results in the formation of
iodohexane (C6H13I)
Since iodohexane is a straight-chain
compound, this reaction shows that there are
no branches in the structures of aldoses
Reaction with hydroiodic acid

99
Methyl alcohol reacts with the –OH group
attached to carbon atom 1 of glucose
forming methyl glucoside
A molecule of water is eliminated
The bond which is formed between the
methyl group of alcohol and the carbon atom
of glucose is known as a glycosidic bond
Reaction with alcohols

100
CH OH2
H H
OH O–CH3
H
HOH
O
H OH
a-D-Glucose Methyl-a-D-Glucoside
CH OH2
H H
OH OH
H
HOH
O
H OH
CH3OH
Mineral
acid
H2O

101
If the carbon atom of glucose has an
a-configuration, the bond is known as
a-glycosidic bond
A similar reaction occurs between other
carbohydrates and alcohols as well leading
to the formation of various glycosides

102
The alcohol group reacting with the mono-
saccharide may be provided by an organic
alcohol or by another monosaccharide
In the latter case, the product will be a
disaccharide
In all disaccharides and polysaccharides,
the constituent monosaccharides are linked
with each other through glycosidic bonds

103
If the alcohol group is provided by a non-
carbohydrate, it is known as the aglycone
portion of the glycoside
Cardiac glycosides, such as digoxin and
ouabain, are a group of drugs that increase
the force of contraction of heart
The aglycone portion of these glycosides is
made up of sterols

104
The hydroxyl groups of monosaccharides
can form esters with acids
Phosphoric esters of monosaccharides
are seen commonly in living organisms,
and are formed by enzymatic reactions
Esterification

105
Monosaccharides reacts with acetyl chloride
(CH3COCl) to form their acetate esters
This reaction can be used to determine the
number of –OH groups in a monosaccharide
Since glucose possesses five –OH groups,
its reaction with acetyl chloride results in the
formation of a penta-acetate

106
On heating at high temperatures, carbo-
hydrates are converted into a brown
coloured degradation product, caramel
This occurs commonly during baking of
bread
The outermost layer, which is exposed to
a high temperature, is caramelised
Caramelisation

Disaccharides
Made up of two monosaccharide
molecules linked by a glycosidic bond
Mostly found in plants
Important ones are sucrose, maltose and
lactose
107

108
Sucrose
Is the common table
sugar (cane sugar)
Occurs in cane, beet,
maple and many fruits
Is made up of glucose
and fructose
Sucrose

109
CH OH2
H H
OH
H
HOH
O
H OH
CH OH2
CH OH2
OH
O
H
H
OHH
1
2
O
Sucrose
Carbon atom 1
of glucose linked
to carbon atom 2 of
fructose by a glyco-
sidic bond

110
Since the anomeric carbon of fructose (carbon
atom 2) has got a b-configuration, the glycosidic
bond is said to be a b-glycosidic bond
Therefore, sucrose may be described as
a-D-glucopyranosyl-b-D-fructofuranoside

111
Sucrose is dextrorotatory (+66.5º)
When it is hydrolysed, an equi-molar mixture of
glucose and fructose is formed
Of these, glucose is dextrorotatory (+52.5º) and
fructose is laevorotatory (–92.3º)

112
As the optical rotation is inverted on hydrolysis,
sucrose is described as invert sugar
Laevorotation caused by fructose is greater than
the dextrorotation caused by glucose
Therefore, the hydrolysate is laevorotatory

Maltose
Does not occur as such in nature
usually
Formed during the hydrolysis of
polysaccharides
Made up to two glucose molecules
linked by an a-glycosidic bond
113

114
CH OH2
H H
OH
H
HOH
O
H OH
O
CH OH2
H H
OH
H
HOH
O
H OH
a-Maltose
1 4

115
Therefore, maltose may exist as a-maltose or
b-maltose
The carbon atom 1 (anomeric carbon) of the
second glucose molecule is free, and may
possess an a- or a b-configuration
Carbon atom 1 of one molecule is linked to
carbon atom 4 of the second

116
Therefore, the bond is an a-glycosidic bond
The a-form of maltose may be described as a-D-
glucopyranosyl-a-D glucopyranoside
Anomeric carbon of the first glucose molecule,
involved in bonding, possesses a-configuration

117
Lactose
Found only in mammals
Principal sugar of milk
Made up of galactose and glucose

118
In lactose, carbon atom 1 of galactose is linked
with carbon atom 4 of glucose by a b-glycosidic
bond
Lactose may exist in a- and b-forms depending
upon the orientation of –H and –OH groups
around carbon atom 1 of glucose which is free

119
CH OH2
OH
HH
H
HOH
O
H OH
O
CH OH2
H
H
OH
H
HOH
O
H OH
b-Lactose
41

120
Galactose is required for the formation of
glycolipids of the nervous tissue
Its presence in the diet of the young ones
of mammals is very important

121
The glycosidic bond of disaccharides can
be hydrolysed by specific enzymes
Sucrase hydrolyses sucrose
Maltase hydrolyses maltose
Lactase hydrolyses lactose
Reactions of disaccharides

122
As mentioned earlier, hydrolysis of sucrose
changes the direction of its optical rotation
Therefore, sucrose is known as invert
sugar
Sucrase, which hydrolyses sucrose, is also
known as invertase

123
Disaccharides are also hydrolysed on
heating them with mineral acids
Mineral acids first hydrolyse disaccharides
into monosaccharides
Then they dehydrate the
monosaccharides into furfural derivatives

124
The furfural derivatives condense with
phenols e.g. a-naphthol to form coloured
complexes (Molisch’s reaction)
Therefore, Molisch’s reaction can be used
for identification of disaccharides also

125
Disaccharides also give reactions
characteristic of hydroxyl, aldehyde and
ketone groups
For example, maltose and lactose form
distinctive osazones

126
Maltosazone Lactosazone
Osazone crystals of maltose and lactose

127
Maltose and lactose reduce metal ions,
e.g. cupric ions, in hot alkaline solutions
However, sucrose has no free aldehyde
or ketone group
It does not give any of the reactions
characteristic of these groups

Polysaccharides
Made up of a large number of
monosaccharide molecules
Very large in size (macromolecules)
May be homopolysaccharides or
heteropolysaccharides
128

129
Homopolysaccharides
Yield same type of
monosaccharide on
hydrolysis
Include glycogen,
starch, dextrin,
cellulose, inulin etc

130
Heteropolysaccharides
Yield more than
one type of mono-
saccharides on
hydrolysis
Include mucopoly-
saccharides (the
commonest
hetero-
polysaccharides)

131
Most important homopolysaccharide in
animals (including man) is glycogen
Plant homopolysaccharides include
starch, cellulose, inulin etc
Homopolysaccharides

Is the form in which carbohydrates are
stored in our body
Is made up of a large number of glucose
molecules
Glucose molecules are linked by a-1,4-
glycosidic bond
132
Glycogen (animal starch)

CH OH2
H H
— O
H
HOH
O
H OH
O
CH OH2
H H
O —
H
HOH
O
H OH n
14 14
Repeating unit of glycogen

134
A long chain of glucose molecules is formed
in this way
At branch points, a glucose molecule is
attached to one of the glucose units in the
linear chain by an a-1,6-glycosidic bond
However, after every 8-12 glucose units,
there is a branch point

CH OH2
H H
O
H
HOH
O
H OH
O
CH2
H H
O
H
HOH
O
H OH n
14 14
O
CH OH2
H H
H
HOH
O
H OH
14
O
6
A branch point in glycogen

136
The branch also continues linearly until a
secondary branch arises from it after 8-12
glucose units
An a-1,4 bond links it with the main chain,
and an a-1,6 bond links it with a branch
Thus, the glucose molecule at branch point
is involved in two glycosidic linkages

Glycogen
Reducing end
Branches

Most abundant source of energy in
our daily diet
Synthesized in plants by the process
of photosynthesis
Potatoes, other tubers, cereals and
legumes are rich in starch
138
Starch

139
Starch is made up of a large number
of glucose units
It contains two different types of
molecules – amylose and amylopectin

140
Linear molecule made up of glucose units
linked by a-1,4-glycosidic bonds
It is coiled to form a helical structure
The structure is similar to that of glycogen
but it has no branches
It constitutes about 15-20% of starch
Amylose

142
Amylopectin
Constitutes the remaining 80-85 % of starch
Has linear portions in which glucose units are
linked by a-1,4-glycosidic bonds
Contains branches arising from the straight
chains by a-1,6-glycosidic bonds
Branch points are 24-30 glucose units apart

143
Amylopectin
Branches
‒ Reducing end

144
Dextrin
Dextrin is not a naturally occurring polysaccharide
The intermediate products, between starch and
maltose, are known as dextrins
Upon hydrolysis, size of starch decreases
progressively until it is converted into maltose
It is a hydrolytic product of starch

145
Amylodextrin (violet)
Erythrodextrin (red)
Achrodextrin (no colour)
They are generally divided on the basis of
the colours they give with iodine into:
Dextrin is a mixture of several products with
progressively decreasing molecular sizes

146
Cellulose
Cellulose forms the structural framework of plants
It is a straight-chain molecule made up of glucose
units linked through b-1,4-glycosidic bonds

CH OH2
H
H
O H
HOH
O
H OH
O
CH OH2
H
H
O
H
HOH
O
H OH
n
14 14
Repeating unit of cellulose

148
There is no enzyme capable of hydrolysing the
b-1,4-glycosidic bond of cellulose in the human
digestive tract
Therefore, we cannot use cellulose as a source
of energy
However, cellulose provides roughage in our diet
and helps bowel movement by stimulating
peristalsis

Inulin
Present in Jerusalem artichoke and some
other plants
Straight-chain molecule made up of fructose
units joined by b-1,2-glycosidic bonds
There are 33-35 fructose residues in each
molecule of inulin
The molecular weight is about 5,000
149

Repeating unit of inulin
CH2
CH OH2
OH
O
H
H
OHH
CH2
CH OH2
OH
H
H
OH
H
O
n
O
O

151
Therefore, inulin clearance is used to measure
glomerular filtration rate
If it is injected intravenously, it is completely
filtered by the glomeruli, and is neither secreted
nor reabsorbed by the renal tubules
Like cellulose, inulin cannot be metabolized by
human beings

152
Heteropolysaccharides
These are present in connective tissues and
mucous secretions
The most important heteropolysaccharides in
human beings are the mucopolysaccharides
(glycosaminoglycans)
The heteropolysaccharides are made up of more
than one kind of monosaccharides and/or
monosaccharide derivatives

153
Mucopolysaccharides are often combined with
proteins
Hexosamines and uronic acids are the
prominent constituents of mucopolysaccharides
These are usually present in the form of
repeating disaccharide units

Important
mucopoly-
saccharides:
Hyaluronic acid
Chondroitin sulphate
Heparin
Heparan sulphate
Dermatan sulphate
Keratan sulphate
154

155
Has a very wide tissue distribution
Hyaluronic acid
Forms the ground substance of
mesenchymal tissue
Made up of glucuronic acid and N-acetyl
glucosamine

Carbon atom 1 of glucuronic acid forms
a glycosidic bond with carbon atom 3 of
N-acetylglucosamine
156
This basic structure is repeated a number of
times to form a very big molecule (MW 100,000-
3,000,000)
Carbon atom 1 of the latter forms a similar bond
with carbon atom 4 of the next glucuronic acid
residue in the chain

COOH
H
H
O H
HOH
O
H OH
O
CH OH2
H
H
OH
H
O
H NHCOCH3 n
OH
Repeating unit of hyaluronic acid

158
Hyaluronic acid acts as a cementing
substance
It helps in retaining water in the
interstitial spaces
It is a very efficient lubricant e.g. in the
synovial fluid

159
Chondroitin sulphate
Is made up of glucuronic acid and
N-acetylgalactosamine sulphate
Has a restricted tissue distribution
Is mainly found in cartilages and bones
Glycosidic bonds are similar to those in
hyaluronic acid

Types of
chondroitin
sulphate:
Chondroitin-4-sulphate
(chondroitin sulphate A)
Chondroitin-6-sulphate
(chondroitin sulphate C)
160

161
In chondroitin-4-sulphate, the sulphate
group is esterified with carbon atom 4 of
N-acetylgalactosamine
COOH
H
H
O H
HOH
O
H OH
O
CH2OH
HO3SO
H
O
H
H
H NHCOCH3 n
H
O
4

162
In chondroitin-6-sulphate, the sulphate
group is esterified with carbon atom 4 of
N-acetylgalactosamine
COOH
H
H
O H
HOH
O
H OH
O
CH2OSO3H
HO
H
O
H
H
H NHCOCH3 n
H
O
6

163
Is made up of glucuronic acid and
glucosamine, both of which are sulphated
Heparin
These two are linked to each other by
a-1,4-glycosidic bonds
Some L-iduronic acid residues are also
present in heparin

164
It is an anticoagulant (prevents intra-
vascular clotting)
It releases lipoprotein lipase from walls of
capillaries (helps in catabolism of
chylomicrons and VLDL)
Heparin is secreted by mast cells (present
in walls of large arteries, lungs, liver etc)

COOH
H
O
H
HOH
O
H OSO H3
O
CH OSO H2 3
H
O
H
H
O
H NH-SO H3
n
H
OH
H
Repeating unit of heparin

166
A low molecular weight heparin (MW
5,000) is used clinically as an anti-
coagulant drug
Naturally occurring heparin comprises
molecules of varying length ranging in
molecular weight from 3,000 to 30,000

167
Heparan sulphate
Differs from heparin in that some of the
glucosamine residues carry an N-acetyl group
instead of a sulphate group on carbon atom 2
Less powerful anticoagulant
Has a much wider tissue distribution than heparin

168
Dermatan sulphate
Found in skin, tendons and valves of the
heart
Differs from chondroitin sulphate in that it
has L-iduronic acid as the uronic acid
component instead of D-glucuronic acid

169
Keratan sulphate
It is of two types:
• Keratan sulphate I
• Keratan sulphate II
It is found in cornea, costal cartilages,
inter-vertebral discs and walls of aorta
Keratan sulphate is widely distributed in
tissues

170
N-Acetylglucosamine-6-sulphate is attached
to the next galactose residue by b-1,3-
glycosidic bond
Galactose is attached to N-acetylglucosa-
mine-6-sulphate by b-1,4-glycosidic bonds
Keratan sulphate I is made up of galactose
and N-acetylglucosamine-6-sulphate

171
The glycosidic bonds are similar to those in
keratan sulphate I
Keratan sulphate II is made up of galactose
and N-acetylgalactosamine-6-sulphate

172
A polysaccharide molecule possesses
only one free aldehyde or ketone group at
one end of the molecule
Thus, the number of reducing groups
relative to the size of the molecules is
practically negligible
Reactions of polysaccharides

173
Formation of osazones
Reduction of metal ions
Oxidation to aldonic acids
Therefore, polysaccharides do not give
reactions dependent upon the presence
of free aldehyde or ketone groups e.g.

174
Some important reactions of polysaccha-
rides depend upon their large size
As polysaccharides are macromolecules,
they form colloidal solutions

175
Polysaccharides can be precipitated from
their colloidal solutions by adding neutral
salts e.g. ammonium sulphate
Each polysaccharide molecule possesses
a number of electric charges on its
surface and is surrounded by a film of
water (shell of hydration)

176
Charges and shell of hydration prevent
coalescence of molecules and keep them
in solution
When ammonium sulphate is added, it
neutralizes the electric charges and
removes the shell of hydration
The molecules come together and are
precipitated

177
The amount of salt required to precipitate
polysaccharides is inversely proportional
to their molecular weights
Starch is precipitated when its solution is
half-saturated with ammonium sulphate
Glycogen and dextrin are precipitated on
full saturation with ammonium sulphate

178
If iodine is added to a polysaccharide
solution, it is adsorbed on the surface of
the polysaccharide
A complex having a characteristic colour
is formed
Starch gives a blue colour with iodine,
glycogen gives a red colour, and dextrin
gives a violet colour

179
Strong mineral acids, e.g. sulphuric acid,
convert:
Polysaccharides into monosaccharides
Monosaccharides into furfural derivatives

180
Furfural derivatives condense with a-naphthol
to give violet colour (Molisch‘s reaction)
Molisch’s test is, thus, given by all the carbo-
hydrates

181

Chemistry of carbohydrates

Recomendados

Recomendados

Más contenido relacionado

La actualidad más candente

La actualidad más candente (20)

Similar a Chemistry of carbohydrates

Similar a Chemistry of carbohydrates (20)

Más de Ramesh Gupta

Más de Ramesh Gupta (20)

Último

Último (20)

Chemistry of carbohydrates