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Carbohydratesstructure and function
General Biochemistry-II
(BCH 302)
Dr . Saba Abdi
Asst . Prof. Dept. Of Biochemistry
College Of Science
King Saud University. Riyadh.KSA
Importance of the topic
Why is this topic important?
•All organisms utilize carbohydrates  important biomolecules
•Nutrition: “carbos” are more than just starch and sugar
•Application of previous concepts:
functional groups
stereochemistry
control biological properties
other structural features
}
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2
Origin of “Carbohydrate”
Before 1900
Glucose C6H12O6
Fructose C6H12O6
Sucrose C12H22O11
Hydrolysis: “water breaking;” reaction with water,
often in the presence of acid or base
H2O, H3O+
H2O, H3O+
H2O, H3O+
no change
no change
}
Monosaccharide: cannot be
hydrolyzed into simpler sugars
glucose + fructose
Disaccharide: saccharide composed of two simpler sugars
H2O, H3O+
Cellulose CnH2nOn
many glucose
Polysaccharide: composed
H2O, H3O+
of many monosaccharides
Starch CnH2nOn
many glucose
}
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Origin of “Carbohydrate”
Sugar general formula = CnH2nOn = Cn(H2O)n = “carbon hydrate”
= carbohydrate
Confirmation
C + H2O (steam)
sucrose + H2SO4
dehydrating agent
H2SO4
steam
carbon
QuickTime™ and a
DV/DVCPRO - NTSC decompressor
are needed to see this picture.
sucrose
Dr.Saba
4
Carbohydrates
• Widely distributed in nature
• Function
– Structural
– Source of energy
– Storage of energy
• Chemical structure
– Polyhydroxyaldehydes - aldoses
– Polyhydroxyketones - ketoses
• Classification
– monosaccharides (1 unit)
– oligosaccharides (2-10 units)
– polysaccharides (> 10 units)
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Monosaccharides-molecular structure
• Chemical structure

– Polyhydroxyaldehydes - aldoses
– Polyhydroxyketones - ketoses
Number of carbon atoms
 trioses (C-3)
 tetroses (C-4)
 pentoses (C-5)
 hexoses (C-6)
 heptoses (C-7)
Contain asymmetric carbon atoms C* =>
optically active
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Aldoses (e.g., glucose) have an aldehyde group at one end.
Ketoses (e.g., fructose) have a keto group, usually at C2.
H
O
C
CH2OH
H
C
OH
HO
C
H
H
C
H
C
C
O
HO
C
H
OH
H
C
OH
OH
H
C
OH
CH2OH
CH2OH
D-glucose
D-fructose
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Enantiomers
• Mirror image isomers are called enantiomers.
• There are two series: D- and L-.
• In the D isomeric form, the OH group on the
asymmetric carbon (a carbon linked to four
different atoms or groups) farthest from the
carbonyl carbon is on the right.
The number of stereoisomers is 2n, where n is the
number of asymmetric centers.
The 6-C aldoses have 4 asymmetric centers. Thus
there are 16 stereoisomers (8 D-sugars and 8 Lsugars).
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Levorotation and dextrorotation
• Optical isomers rotate the beam of planepolarized light for the same angle, but in
opposite direction.
• Equimolar mixture of optical isomers has
no optical activity - racemic mixture
Dextrorotation and levorotation refer,
respectively, to the properties of rotating
plane polarized light clockwise (for
dextrorotation) or counterclockwise (for
levorotation).
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9
.
• A compound with dextrorotation is called
dextrorotatory or dextrorotary ,while a
compound with levorotation is called
levorotatory or levorotary
• Both D- and L- isomers can be dextrotatory
or leavorotatory. A dextrorotary compound
is often prefixed "(+)-" or "d-". Likewise, a
levorotary compound is often prefixed "(–)" or "l-".
• These "d-" and "l-" prefixes should not be confused with the "D-"
and "L-" prefixes which is based on the actual configuration of
each enantiomer, with the version synthesized from naturally
occurring (+)-glyceraldehyde being considered the D- form
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The (D)-Aldose Family
The (D)-Aldotrioses
One stereocenter  two enantiomers
H
O
H
C
HO
C
O
C
H
H
CH2OH
C
OH
CH2OH
(L)-(-)-glyceraldehyde
(D)-(+)-glyceraldehyde
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The (D)-Aldose Family
Fischer Projections
H
H
O
CHO
C
C
H
C
O
H
OH
C
OH
CH2OH
CH2OH
(D)-(+)-glyceraldehyde
Horizontal lines
= solid wedges
Vertical lines
= broken wedges
Fischerrepresentation
projection
Alternate
Emil Fischer
•Determined relative structure of (D)-aldoses
•Nobel Prize in Chemistry 1902
•Most natulral saccharides are (D)-form
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The (D)-Aldose Family
The (D)-Aldotetroses
Two stereocenters  four stereoisomers
Two (D) and two (L)
H
H
O
O
C
C
H
C
OH
HO
C
H
H
C
OH
H
C
OH
CH2OH
CH2OH
(D)-(-)-erythrose
(D)-(-)-threose
Not found in nature
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The (D)-Aldose Family
The (D)-Aldopentoses
Three stereocenters  eight stereoisomers
Four (D) and four (L)
H
O
H
C
C
H
C
OH
H
C
OH
(D)-(-)-ribose
H
C
OH
RNA (ribonucleic acid)
DNA (deoxyribonucleic acid)
CH2OH
H
O
H
C
OH
HO
C
H
H
C
OH
(D)-(+)-xylose
CH2OH
O
H
C
O
C
HO
C
H
H
C
OH
H
C
OH
CH2OH
(D)-(-)arabinose
Dr.Saba
HO
C
H
HO
C
H
H
C
OH
CH2OH
(D)-(-)-lyxose
Not found in nature
14
The (D)-Aldose Family
The (D)-Aldohexoses
Four stereocenters  16 stereoisomers  eight (D) and eight (L)
H
O
H
O
C
C
H
O
C
H
C
OH
HO
C
H
OH
HO
C
H
HO
C
H
C
OH
H
C
OH
H
C
OH
C
OH
H
C
OH
H
C
OH
C
OH
HO
C
H
H
C
OH
H
C
H
C
OH
H
H
C
OH
H
(D)-(+)-allose
O
C
H
CH2OH
H
CH2OH
CH2OH
(D)-(+)-altrose
(D)-(+)-glucose
not found in nature
CH2OH
(D)-(+)-mannose
most abundant
monosaccharide
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The (D)-Aldose Family
The (D)-Aldohexoses
Four stereocenters  16 stereoisomers  eight (D) and eight (L)
H
O
H
H
O
O
C
C
OH
HO
C
H
OH
HO
C
H
HO
C
H
C
H
HO
C
H
HO
C
H
C
OH
H
C
OH
H
C
OH
C
OH
HO
C
H
H
C
OH
H
C
HO
C
H
HO
H
C
OH
H
(D)-(-)-gulose
H
H
H
CH2OH
O
C
C
C
CH2OH
CH2OH
(D)-(+)-galactose
(D)-(-)-idose
CH2OH
(D)-(+)-talose
fairly common
not found in nature
•Most important aldoses: glucose, ribose, galactose
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Epimers
• Pairs of monosaccharides different only in
configuration around only one specific Catom.
• For example, glucose and galactose are C-4
epimers—their structures differ only in the
position of the -OH group at carbon 4.
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Cyclization of monosaccharides
• monosaccharides with five or more carbons
are predominantly found in a ring (cyclic)
form, in which the aldehyde (or ketone)
group has reacted with an alcohol group on
the same sugar
H
C
H
O
+
R'
OH
R'
O
R
OH
R
aldehyde
alcohol
hemiacetal
R
C
C
R
O
+
"R
OH
R'
ketone
"R
O
C
OH
R'
alcohol
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hemiketal
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Cyclic Structures
• most stable are rings with 5 and 6 members
• most common in nature
• in accordance to oxygen containing
heterocycles monosaccharides are called
• with 5 atoms in cycle furan
• with 6 atoms in cycle pyranoses
Dr.Saba
O
O
19
Cyclic Structures
• Glucose forms an
intra-molecular
hemiacetal, as the C1
aldehyde & C5 OH
react, to form a 6member pyranose
ring, named after
pyran
1
H
HO
H
H
2
3
4
5
6
CHO
C
OH
C
H
C
OH (linear form)
C
OH
D-glucose
CH2OH
6 CH2OH
6 CH2OH
5
H
4
OH
O
H
OH
3
H
H
2
OH
-D-glucose
Dr.Saba
H
1
OH
5
H
4
OH
H
OH
3
H
O
OH
H
1
2
H
OH
-D-glucose
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Cyclic Structures
Fructose forms either
 a 6-member pyranose ring,
by reaction of the C2 keto
group with the OH on C6,
or
 a 5-member furanose ring,
by reaction of the C2 keto
group with the OH on C5.
This ring is more stable
for all ketones.
Dr.Saba
CH2OH
1
HO
H
H
2C
O
C
H
C
OH
C
OH
3
4
5
6
HOH2C 6
CH2OH
D-fructose (linear)
H
5
H
1 CH2OH
O
4
OH
HO
2
3
OH
H
-D-fructofuranose
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6 CH2OH
6 CH2OH
5
H
4
OH
O
H
OH
3
H
H
2
5
H
H
1
4
OH
OH
H
OH
3
H
OH
O
OH
H
1
2
H
OH
-D-glucose
-D-glucose
Cyclization of glucose produces a new asymmetric center at C1 (in
frucose at C2) . The 2 stereoisomers are called anomers,  & .
Haworth projections represent the cyclic sugars as having essentially
planar rings, with the OH at the anomeric C1:
  (OH below the ring)  (OH above the ring).
• The  and  anomers of D-glucose interconvert in aqueous solution by
a process called mutarotation.
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H OH
4
H OH
6
H O
HO
HO
H O
HO
H
HO
5
3
H
H
2
H
OH 1
OH
H
OH
OH
H
-D-glucopyranose
-D-glucopyranose
• Because of the tetrahedral nature of carbon
bonds, pyranose sugars actually assume a
"chair" or "boat" configuration, depending
on the sugar.
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Reducing sugars
• If the oxygen on the anomeric carbon of a
sugar is not attached to any other structure,
that sugar can act as a reducing agent and is
termed a reducing sugar. Such sugars can
react with chromogenic agents (for
example, Benedict's reagent or Fehling's
solution) causing the reagent to be reduced
and colored, with the anomeric carbon of
the sugar becoming oxidized to a carboxyl
group.
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Reactions of monosaccharides*
• 1. Esterification
• 2.Oxidation (only aldose sugar)
• 3. Reduction
• 4. Cyanohydrin
• 5.Osazone (test for identification of sugar)
• 6. Furfurals
• 7. Enolization
(* Refer to hand out of reactions)
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Sugar derivative
sugar alcohol – Formed by reduction of an aldehyde
or ketone group of monosaccharide; e.g., ribitol.
sugar acid - the aldehyde at C1, or OH at C6, is
oxidized to a carboxylic acid; e.g., gluconic acid,
glucuronic acid, ascorbic acid (vitamin C).
CHO
COOH
H
CH2OH
C OH
HO C H
H
C OH
HO C
H
H
C
OH
H
C
OH
H
C OH
H
C OH
H
C
OH
H
C OH
H
C OH
CH2OH
D-ribitol
CH2OH
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COOH
D-gluconic acid D-glucuronic acid
26
amino sugar –
an amino group substitutes for a hydroxyl. An example is
glucosamine (component of chitin).
The amino group may be acetylated, as in Nacetylglucosamine.
CH2OH
CH2OH
O
H
H
OH
H
H
OH
H
OH
OH
H
H
H
O OH
OH
H
NH2
-D-glucosamine
O
H
N
C
CH3
H
-D-N-acetylglucosamine
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H
O
H3C
C
O
NH
R
H
COO
H
R=
OH
H
HC
OH
HC
OH
CH2OH
OH
H
N-acetylneuraminate (sialic acid)
N-acetylneuraminate (N-acetylneuraminic acid, also
called sialic acid) is often found as a terminal residue
of oligosaccharide chains of glycoproteins.
Sialic acid imparts negative charge to glycoproteins,
because its carboxyl group tends to dissociate a
proton at physiological pH, as shown here.
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Glycosidic Bonds
The anomeric hydroxyl and a hydroxyl of another sugar or
some other compound can join together, splitting out water
to form a glycosidic bond:
R-OH + HO-R'  R-O-R' + H2O
E.g., methanol reacts with the anomeric OH on glucose to
form methyl glucoside (methyl-glucopyranose).
Glycosidic bonds are readily hydrolyzed by acid but resist
cleavage by base
H OH
H OH
H
HO
HO
H
H
H2O
O
H
+
CH3-OH
H
HO
HO
H
OH
H
OH
-D-glucopyranose
methanol
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O
H
OH
OCH3
methyl--D-glucopyranose
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O- and N-glycosides
• If the group on the non-carbohydrate
molecule to which sugar is attached is -OH
group the structure is an O-glycoside.
• If the group is an-NH2, the structure is Nglycoside
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.
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Disaccharides
•Composed of two monosaccharide molecules
•Useful vocabulary:
Linked by glycoside (an ether), part of acetal functional group
Other anomeric carbon = hemiacetal functional group
CH2OH
O
HO
HO
HO
HO
CH2OH
O
OH
O
HO
glycoside linkage
C-O-C
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Disaccharides
Carbohydrate Ring Numbering
4
HO
3
O
HO
6
CH2OH
5 O O
2
•Anomeric carbon receives lowest number
•Carbon 1 in aldoses
1
Numbering for an aldohexose
•Carbon 2 (rarely 3) in ketoses
•All other carbons numbered in order
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Naming disaccharides
• By convention the name describes the compound with its
nonreducing end to the left, and we can “build up” the
name in the following order.
• (1) Give the configuration ( or β) at the anomeric
carbon joining the first monosaccharide unit (on the left) to
the second.
• (2) Name the nonreducing residue; to distinguish five- and
six-membered ring structures, insert “furano” or “pyrano”
into the name.
• (3) Indicate in parentheses the two carbon atoms joined by
the glycosidic bond, with an arrow connecting the two
numbers
• (4) Name the second residue
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Disaccharides
Lactose
1,4’--D-galactopyranosyl-D-glucopyranose
HO
CH2OH
O
HO
HO
HO
O
OH
O
CH2OH
Lactose
OH
H3O+/H2O
HO
CH2OH
O
+
HO
hydrolysis
HO
OH
Galactopyranose
(galactose)
HO
HO
HO
CH2OH
O
OH
Glucopyranose
(glucose)
•Present in mammalian milk (up to 8 % by weight; varies with species)
•Readily digested by infant mammals; requires enzyme lactase
•Adults often less tolerant due to low levels of lactase
•It is a reducing sugar
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Disaccharides
Sucrose
1,2’--D-fructofuranosyl--D-glucopyranose
HO
HO
OH
CH2OH
O
H3O+/H2O
OH
HO
O
O
OH
CH2OH
CH2OH
HO
HO
CH2OH
O
HO
OH
hydrolysis
Sucrose
Glucopyranose
(glucose)
+
HO
O
CH2OH
OH
CH2OH
Fructofuranose
(fructose)
•Unusual structure: 1,2’--glycoside
•Most common disaccharide in nature
•Produced only by plants such as sugar cane, sugar beats
•An -glycoside: readily digested by mammals
Sucrose contains no free anomeric carbon atom; the anomeric carbons of
both monosaccharide units are involved in the glycosidic bond . Sucrose
is therefore a nonreducing sugar.
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36
6 CH2OH
6 CH2OH
H
4
OH
5
O
H
OH
3
H
H
2
OH
.
H
H
1
4
5
O
H
OH
H
O
3
maltose
H
H
1
2
OH
OH
• Maltose, a cleavage product of starch (e.g.,
amylose), is a disaccharide with an (1 4)
glycosidic link between C1 - C4 OH of 2 glucoses.
• Because the disaccharide retains a free anomeric
carbon (C-1 of the glucose residue on the right) ,
maltose is a reducing sugar
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6 CH2OH
H
4
OH
5
6 CH2OH
O
H
OH
H
.
H
1
O
4
5
O
H
OH
H
H
3
H
2
OH
3
cellobiose
H
2
OH
1
H
OH
Cellobiose, a product of cellulose breakdown,
The (1 4) glycosidic linkage between two
glucose molecules.
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.
• Trehalose is found in plants and insects
• formed by an α,α-1,1-glucoside bond
between two α-glucose units
• It is non reducing sugar
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Polysaccharides
• Generally called glycans
• Contains a number of monosaccharide units linked
by glycosidic bonds
• Divided into two broad groups:
(i) Homopolysaccharides-contains only one type of
monomer. Examples:Starch,cellulose,glycogen
(ii) Heteropolysaccharides- Contain two or more
types of monomers. Examples: hyaluronic acid,
chondriotin sulphate, heparin, and mureins.
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Polysaccharides
Cellulose
•Linear 1,4--D-glucopyranose polymer
CH2OH
O
O
HO
OH
CH2OH
O
O
HO
CH2OH
O
O
HO
OH
H3O+/H2O
HO
HO
O
HO
hydrolysis
OH
repeating subunit:
glucopyranose
CH2OH
O
OH
Many glucopyranose
•~5,000 - 10,000 glucopyranose molecules per cellulose molecule
•Most abundant organic substance in nature
•Function: support structure in plants
Wood is ~50% cellulose by weight
Strength due to intermolecular hydrogen bonding
•Not easily digested by mammals
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Cellulose
Cellulose, a major constituent of plant cell walls, consists of long linear
chains of glucose with (14) linkages.
Every other glucose is flipped over, due to  linkages.
This promotes intra-chain and inter-chain H-bonds and van der Waals
interactions, that cause cellulose chains to be straight & rigid, and pack
with a crystalline arrangement in thick bundles - microfibrils
Multisubunit Cellulose Synthase complexes in the plasma membrane spin
out from the cell surface microfibrils consisting of 36 parallel, interacting
cellulose chains.
These microfibrils are very strong.
The role of cellulose is to impart strength and rigidity to plant cell walls,
which can withstand high hydrostatic pressure gradients. Osmotic
swelling is prevented.
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Chitin
• It is a structural homopolysaccharide
• It is a linear molecule composed of Nacetylglucosamine monomers linked by β
(1→4) glycosidic bond
• It forms extended fibers similar to that of
cellulose
• It is component of exoskeleton of insects
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Polysaccharides
Starch
•Two forms: amylose, amylopectin
•1,4’--D-glucopyranose polymer
•Function: plant glucose/energy storage
•Hydrolysis  glucopyranose
•Easily digested by mammals
•Glucose storage in polymeric form minimizes
osmotic effects
Amylose
•Linear coiled polymer of glucopyranose linked by
α-1,4 glycosidic bonds
•20-25% of starch
Amylopectin
•Branched polymer containing glucopyranose linked by
α-1,4 glycosidic bonds. Branch points has
α-1,6glycosidic bonds, 12 glucose in a branch
• 75-80% of starch
Dr.Saba
O
HO
CH2OH
O
HO
O
HO
CH2OH
O
HO
O
HO
HO
Amylose
O
HO
O
CH2OH HO
O
HO
O
HO
CH2OH
O
O
CH2OH
O
HO
O
O
HO
O
HO
Amylopectin
CH2OH
O
HO
O
44
CH2OH
CH2OH
O
H
H
OH
H
H
OH
H
O
OH
CH2OH
H
H
OH
H
H
OH
H
H
OH
CH2OH
O
H
OH
O
H
OH
H
H
O
O
H
OH
H
H
OH
H
O
glycogen
H
1
O
.
6 CH2
H 5
H
4 OH
3
H
CH2OH
O
H
2
OH
H
H
1
O
CH2OH
O
H
4 OH
H
H
H
H
O
OH
O
H
OH
H
OH
H
OH
Glycogen, the glucose storage polymer in animals, is similar
in structure to amylopectin.
But glycogen has more (16) branches.
The highly branched structure permits rapid glucose release
from glycogen stores, e.g., in muscle during exercise.
The ability to rapidly mobilize glucose is more essential to
animals than to plants.
Dr.Saba
H
45
Glycosaminoglycans
Glycosaminoglycans (mucopolysaccharides) are
linear polymers of repeating disaccharides.
The constituent monosaccharides tend to be
modified, with acidic groups, amino groups, sulfated
hydroxyl and amino groups, etc.
Glycosaminoglycans tend to be negatively charged,
because of the prevalence of acidic groups.
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46
CH 2OH
D-glucuronate
6

6COO
H
4
5
H
OH
3
H
H
2
OH
1
H
O
H
4
O
H
5
H
OH
3
H
O
2
1 O
H
NHCOCH 3
N-acetyl-D-glucosamine
hyaluronate
Hyaluronate (hyaluronan) is a glycosaminoglycan with a
repeating disaccharide consisting of 2 glucose derivatives,
glucuronate (glucuronic acid) & N-acetyl-glucosamine. The
glycosidic linkages are (13) & (14). Form ground
substance of connective tissue.
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47
• Inulin :Plant polysaccharide of fructose residues
connected by 2-1 linkage
.
• Chitin: Polymer of N-acetylglucosamine linked
through β-1,4 glycosidic bond. Found in fungal
cell wall and arthropod cuticle
• Mannan: Polymer of mannose linked by α-1,4
and α-1,3 glycosidic bonds. Found in cell wall of
bacteria yeast and some plants.
• Heparin: Consists of glucosamine N-sulphate and
sulphate esters of glucuronic acid. Found in
granules of mast cells has anticoagulant
properties.
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h
Physical propreties of
carbohydrates
• Monosaccharides are colorless crystalline
solids, very soluble in water, but only
slightly soluble in ethanol, low molecular
weight, sweet tasting
• Disaccharides are low molecular weight,
sweet,crystalline, less soluble in water than
monosaccharides
• Polysaccharides are high molecular weight,
Dr.Sabaand not crystalline.
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not sweet, not soluble
Complex Carbohydrates
• Carbohydrates can be attached by
glycosidic bonds to non-carbohydrate
structures :
(a) Nitrogen bases (in nucleic acids)
(b) Aromatic ring (in steroid and bilirubin)
(c) Proteins (in glycoproteins and
glucosaminoglycans)
(d) Lipids (in glycolipids)
Dr.Saba
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