Download dr shamim shaikh mohiuddin dept of biochemistry university

CARBOHYDRATE
CHEMISTRY
DR SHAMIM SHAIKH MOHIUDDIN
DEPT OF BIOCHEMISTRY
UNIVERSITY OF DAMMAM
KSA
OVERVIEW
-Carbohydrate primarily composed of Carbon, Hydrogen
and Oxygen
-Carbohydrates may be defined as polyhydroxyaldehyde or
polyhydroxyketones or compounds which produce them on
hydrolysis
CLASSIFICATION OF CARBOHYDRATE
-Mainly classified into three major groups depends on
number of sugar units
-Monosaccharides
-Oligosaccharides
-Polysaccharides
Monosaccharides
-Monosaccharides are the simplest group of carbohydrate
and cannot be further hydrolyzed.
-Monosaccharides are divided into different categories,
based on the functional group and number of C atoms.
Aldose:
When functional group in monosaccharides is an
aldehyde group(-CHO) they are known as Aldose
example- glucose
Ketose:
When functional group in monosaccharide is an
ketone group(-CO) they are known as Ketose
example-fructose
Oligosaccharides
-Oligosaccharides contain 2-10 monosaccharide molecule
which are liberated on hydrolysis
-Based on the number of monosaccharide units they are
Further classified into disaccharides, trisaccharides etc
Polysaccharides
-Polysaccharides are polymers of monosaccharides unit
With high molecular weight
-Polysaccharides are of two types
-Homopolysaccharides
-Heteropolysaccharides
STRUCTURAL ASPECT OF MONOSACCHARIDE
=Stereoisomer=
Stereo isomers refers to a compound that have the same
structural formula but differs in their spatial configuration.
-Stereoisomerism is a very important character of
monosaccharides
=Asymmetric carbon atom=
A carbon atom is said to be asymmetric when it is attached
to four different atoms or group.
D and L isomers
-D and L isomers are mirror images of each other
-The spatial orientation of –H and –OH groups on the
carbon atom that is adjacent to the primary alcohol carbons
determines whether. the sugar is D- or L- form.
-OH in right side : D-series sugar
-OH in left side :
L-series sugar
Optical activity of sugars
-Optical activity is a characteristic feature of compounds
with asymmetric carbon
-A beam of polarized light when passed through a solution
of an optical isomer, it will be rotated either to the right or
left.
Rotation of polarized light to right- Dextrorotatory(+)
Rotation of polarized light to left- Levorotatory(-)
Racemic mixture
-If D and L isomers are present in equal concentration
is known as Racemic mixture
-Racemic mixture does not show any optical activity since
the dextro and levorotatory activities cancel each other
Epimers
-If two monosaccarides differ from each other in their
configuration around a single specific carbon(other than
anomeric) atom, they are referred to as epimers to each
other.
-Example: Glucose and Galactose are epimers with regards
to carbon no 4
Enantiomers
Enantiomers are a special type of stereoisomerism that are
mirror images of each other.
-The two members are designated as D and L sugars
-Majority of the sugars in the higher animals(including man)
are of D type
Diastereomers
-This represents the stereo isomers that are not mirror image
of one another.
Anomer
-The α and β cyclic forms of D-glucose are known as anomer.
-They differ from each other in configuration only around
C1 which is known as anomeric carbon atom
-In α anomer, the –OH group held by anomeric carbon is on
the opposite side of the group –CH2OH of sugar ring
-In β anomer, the –OH group held by anomeric carbon is on
the same side of the group –CH2OH of sugar ring
Mutarotation
-Mutarotation is defined as the change in the specific optical
rotation representing the interconversion of α and β forms of
D glucose to an equilibrium mixture
α –D-glucose ↔ Equilibrium mixture ↔ β-D-glucose
+ 112.2º
+ 52.7º
+ 18.7º
-The Equilibrium mixture contains
β anomer – 63℅
α anomer – 36℅
open chain form - 1℅
-β form is more predominant due to its stable conformation
DISACCHARIDES
Disaccharides consists of two monosaccharides units held
together by a glycosidic bond.
-They are crystalline, water soluble and sweet to taste.
-Disaccharides are of two types
-Reducing disaccharides with free aldehyde or keto gr
e.g. Maltose, Lactose
-Non reducing disaccharides with no free aldehyde or keto
gr.
e.g. Sucrose
Maltose
-Composed of two α-D-glucose units held together by
Glycosidic bond.
-Maltose can be hydrolyzed by dilute acid or enzyme maltase
to liberate two molecules of α-D-glucose
Lactose
-Lactose is more commonly known as milk sugar.
-It is composed of β-D-galactose and β-D-glucose held
together by glycosidic bond.
-It is hydrolyzed by intestinal enzyme Lactase to glucose and
galactose.
Sucrose
-Sucrose is made up of α-D-glucose and β-D-fructose
-Two monosaccharides are held together by a glycosidic bond,
between C1 of α-glucose and C2 of β-fructose.
-Sucrose is a non-reducing sugar because the reducing group
of glucose and fructose are involved in glycosidic bond.
-Sucrose is most abundant among the naturally occurring
sugars and important sources of dietary carbohydrate
-It is sweeter than most other common sugar except fructose.
-the intestinal enzyme, sucrase, hydrolyzed sucrose to glucose
and fructose.
POLYSACCHARIDES
-It consist of repeat units of monosaccharides or their
derivatives, held together by glycosidic bonds.
-Polysaccharides are linear as well as branched polymer
-Two types
1. Homopolysaccharides
-Homopolysaccharides on hydrolysis yield only a single
type of monosaccharides.
-They are named based on the nature of the
monosaccharide units. e.g.
-Glucans are polymers of glucose
-Fructosans are polymers of fructose
2. Heteropolysaccharides
-Heteropolysaccharides on hydrolysis yield a mixtureof
few monosaccharides or their derivatives
Starch
-Also known as glucosan or glucan
-It is a homopolysaccharide composed by D-glucose units
which is held by α(1→4)-glycosidic bonds
-Starch consists of two polysaccharide components
a. Water soluble amylose(15-20%)
b. Water insoluble amylopectin(80-85%)
-Chemically amylose is a long unbranched chain with 200-1000
D-glucose units held by α-(1→4) glycosidic linkage
H
H
H
5
4
5
1
3
D-Glucose
H
O
1
4
2 α(1→4)Glycosidic
bond
3
2
D-Glucose
O
n
-Amylopectin containing few thousand glucose units looks like
a branched tree(20-30 glucose units per branch).It is a
branched chain with α(1→6) glycosidic bonds at the branching
points and α(1→4) glycosidic bond everywhere
Else.
H
5
4
H5
1
3
O
4
2
1
2
3
O
6
α(1→6)glycosidic bond
CH2OH
6 CH2
H
1
4
O
H
H
H 5
H
3
2
5
4
O
1
3
α(1→4)glycosidic bond
2
Glycogen
-Glycogen is the carbohydrate reserve in animals.
-Present in Liver, muscle and brain
-Structure of glycogen is similar to that of amylopectin with
more number of branches.
-Glucose is the repeating unit in glycogen joined together by
α(1→4)glycosidic bond andα(1→6)glycosidic bonds at
branching points.
Cellulose
-Cellulose occurs exclusively in plants and totally absent in
animal body.
-It is composed of β-D-glucose units linked by
β(1→4)glycosidic bonds
-Cellulose cannot be digested in human, due to lack of the
enzyme that cleaves β-glycosidic bond
5
H
1
4
5
H
4
1
O
3
2
3
2
β(1→4)glycosidic bond
β-D-Glucose
β-D-Glucose
n