Post on 07-Apr-2018
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Carbohydrates-structure and functionGeneral Biochemistry-II
(BCH 302)
Dr . Saba Abdi
Asst . Prof. Dept. Of Biochemistry
College Of Science
King Saud University. Riyadh.KSA
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Importance of the topicWhy 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
other structural features
} control biological properties
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Origin of CarbohydrateBefore 1900
}Monosaccharide: cannot be
hydrolyzed into simpler sugars
Glucose C6H12O6
Fructose C6H12O6 no changeH2O, H3O+
no changeH2O, H3O
+
Hydrolysis: waterbreaking; reaction with water,
often in the presence of acid or base
Sucrose C12H22O11H2O, H3O
+
glucose + fructose
Cellulose CnH2nOnH2O, H3O
+
many glucose
Disaccharide: saccharide composed of two simpler sugars
Polysaccharide: composed
of many monosaccharides}Starch CnH2nOn
H2O, H3O+
many glucose
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QuickTime and aDV/DVCPRO - NTSC decompressor
are needed to s ee this picture.
Origin of Carbohydrate
Sugar general formula = CnH2nOn
Confirmation
sucrose + H2SO4 C + H2O (steam)
dehydrating agent
= Cn(H2O)n
= carbohydrate
= carbon hydrate
sucrose
H2SO4
steam
carbon
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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 6Dr.Saba
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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.
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C HHO
C OHH
C OHH
CH2OH
CH2OH
C O
D-fructose
C
C OHH
C HHO
C OHH
C OHH
CH2OH
D-glucose
OH
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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 thecarbonyl 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 L-
sugars). 8Dr.Saba
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Levorotation and dextrorotation
Optical isomers rotate the beam of plane-polarized light for the same angle, but in
opposite direction.
Equimolar mixture of optical isomers hasno optical activity - racemic mixture
Dextrorotation and levorotation refer,
respectively, to the properties of rotatingplane polarized light clockwise (for
dextrorotation) or counterclockwise (for
levorotation).9Dr.Saba
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. A compound with dextrorotation is called
dextrorotatory ordextrorotary,while a
compound with levorotation is calledlevorotatory orlevorotary
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 FamilyThe (D)-Aldotrioses
C
C
CH2OH
HO H
H O
C
C
CH2OH
H OH
H O
One stereocenter two enantiomers
(L)-(-)-glyceraldehyde (D)-(+)-glyceraldehyde
C
C
CH2OH
H OH
H O
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Alternate representation
C
C
CH2OH
H OH
H O
The (D)-Aldose FamilyFischer Projections
Emil Fischer
Determined relative structure of(D)-aldoses
Nobel Prize in Chemistry 1902
Most natulral saccharides are (D)-form
(D)-(+)-glyceraldehyde
C
C
CH2OH
H OH
H O
Vertical lines
= broken wedges
Horizontal lines
= solid wedges
Fischer projection
CHO
H OH
CH2OH
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The (D)-Aldose FamilyThe (D)-Aldotetroses
Two stereocenters four stereoisomers
Two (D) and two (L)
(D)-(-)-erythrose (D)-(-)-threose
Not found in nature
C
C
CH2OH
H OH
C
OH
H OH C
C
CH2OH
H OH
C
OH
HO H
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The (D)-Aldose FamilyThe (D)-Aldopentoses
Three stereocenters eight stereoisomersFour(D) and four(L)
(D)-(-)-ribose
RNA (ribonucleic acid)
DNA (deoxyribonucleic acid)
(D)-(-)arabinose (D)-(-)-lyxose
Not found in nature
(D)-(+)-xyloseC
C
CH2OH
H OH
H OH
CH
C
OH
OH
C
C
CH2OH
H OH
H OH
CHO
C
H
OH
C
C
CH2OH
H OH
HO H
CH
C
OH
OH
C
C
CH2OH
H OH
HO H
CHO
C
H
OH
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The (D)-Aldose FamilyThe (D)-Aldohexoses
Four stereocenters 16 stereoisomers eight(D) and eight(L)
(D)-(+)-allosenot found in nature
C
C
CH2OH
H OH
H OH
CH OH
CH OH
C
OH
(D)-(+)-altrose
C
C
CH2OH
H OH
H OH
CH OH
CHO H
C
OH
(D)-(+)-glucosemost abundant
monosaccharide
C
C
CH2OH
H OH
H OH
CHO H
CH OH
C
OH
(D)-(+)-mannose
C
C
CH2OH
H OH
H OH
CHO H
CHO H
C
OH
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The (D)-Aldose FamilyThe (D)-Aldohexoses
Four stereocenters 16 stereoisomers eight(D) and eight(L)
(D)-(-)-gulose
not found in nature
C
C
CH2OH
H OH
HO H
CH OH
CH OH
C
OH
(D)-(-)-idose
C
C
CH2OH
H OH
HO H
CH OH
CHO H
C
OH
(D)-(+)-galactose
fairly common
C
C
CH2OH
H OH
HO H
CHO H
CH OH
C
OH
(D)-(+)-talose
C
C
CH2OH
H OH
HO H
CHO H
CHO H
C
OH
Most important aldoses: glucose, ribose, galactose
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Epimers
Pairs of monosaccharides different only in
configuration around only one specific C-
atom. For example, glucose and galactose are C-4
epimerstheir 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 carbonsare predominantly found in a ring (cyclic)
form, in which the aldehyde (or ketone)
group has reacted with an alcohol group onthe same sugar
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O C
H
R
OH
O C
R
R'
OHC
R
R'
O
aldehyde alcohol hemiacetal
ketone alcohol hemiketal
C
H
R
O R'R' OH
"R OH "R
+
+
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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 cyclefuran
with6 atoms in cyclepyranoses
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O
O
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Cyclic Structures
Glucose forms an
intra-molecular
hemiacetal, as the C1aldehyde & C5 OH
react, to form a 6-
member pyranose
ring, named afterpyran
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H O
OH
H
OHH
OH
CH2OH
H
OH
H H O
OH
H
OHH
OH
CH2OH
H
H
OH
-D-glucose -D-glucose
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4
5
6
1 1
6
5
4
3 2
H
CHO
C OH
C HHO
C OHH
C OHH
CH2OH
1
5
2
3
4
6
D-glucose(linear form)
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Cyclic Structures
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CH2OH
C O
C HHO
C OHH
C OHH
CH2OH
HOH2C
OH
CH2OH
H
OH H
H HO
O
1
6
5
4
3
2
6
5
4 3
2
1
D-fructose (linear) -D-fructofuranose
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 stablefor all ketones.
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Cyclization of glucose produces a new asymmetric centerat 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:
(OHbelow 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 O
OH
H
OHH
OH
CH2OH
H
-D-glucose
OH
H H O
OH
H
OHH
OH
CH2OH
H
H
OH
-D-glucose
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4
5
6
1 1
6
5
4
3 2
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Because of the tetrahedral nature of carbonbonds, pyranose sugars actually assume a
"chair" or "boat" configuration, depending
on the sugar.
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O
H
HO
H
HO
H
OHOH
HH
OH
O
H
HO
H
HO
H
HOH
HOH
OH
-D-glucopyranose -D-glucopyranose
1
6
5
4
3
2
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Reducing sugars
If the oxygen on the anomeric carbon of asugar is not attached to any other structure,
that sugar can act as a reducing agent and is
termed a reducing sugar. Such sugars canreact 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)25Dr.Saba
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Sugar derivative
sugar alcoholFormed 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).
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COOH
C
C
C
C
H OH
HO H
H OH
D-gluconic acid
D-glucuronic acid
CH2OH
OHH
CHO
C
C
C
C
H OH
HO H
H OH
COOH
OHH
CH2OH
C
C
C
CH2OH
H OH
H OH
H OH
D-ribitol
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i
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amino sugaran amino group substitutes for a hydroxyl. An example isglucosamine (component of chitin).The amino group may be acetylated, as in N-
acetylglucosamine.
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H O
OH
H
OH
H
NH2H
OH
CH2OH
H
-D-glucosamine
H O
OH
H
OH
H
NH
OH
CH2OH
H
-D-N-acetylglucosamine
C CH3
O
H
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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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NH O
H
COO
OH
H
HOH
H
H
R
CH3C
O
HC
HC
CH2OH
OH
OH
N-acetylneuraminate (sialic acid)
R =
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Glycosidic BondsThe anomeric hydroxyl and a hydroxyl of another sugar orsome other compound can join together, splitting out waterto form a glycosidic bond:
R-OH + HO-R' R-O-R' + H2OE.g., methanol reacts with the anomeric OH on glucose toform methyl glucoside (methyl-glucopyranose).
Glycosidic bonds are readily hydrolyzed by acid but resistcleavage by base
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O
H
HO
H
HO
H
OH
OHHH
OH
-D-glucopyranose
O
H
HO
H
HO
H
OCH3
OHHH
OH
methyl--D-glucopyranose
CH3-OH+
methanol
H2O
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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 N-
glycoside
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.
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DisaccharidesComposed of two monosaccharide molecules
Useful vocabulary:
Linked by glycoside (an ether), part ofacetal functional group
Other anomeric carbon = hemiacetal functional group
glycoside linkage
C-O-C
OO
O
CH2OH
HO
HO
HO
HOHO
CH2OH
OH
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DisaccharidesCarbohydrate Ring Numbering
Anomeric carbon receives lowest number
Carbon 1 in aldoses
Carbon 2 (rarely 3) in ketoses
All other carbons numbered in order
OO
CH2OH
HO
HO
12
3
45
6
O
Numbering for an aldohexose
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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) 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 bythe glycosidic bond, with an arrow connecting the two
numbers
(4) Name the second residue
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DisaccharidesLactose
1,4--D-galactopyranosyl-D-glucopyranose
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
Lactose
O
OO
CH2OHHO
HO
OH
HO
CH2OH
OHHOH3O
+/H2O
hydrolysis
+O
OH
CH2OH
HOHO
HO
Glucopyranose
(glucose)
Galactopyranose
(galactose)
O
CH2OHHO
HO
HO OH
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DisaccharidesSucrose
1,2--D-fructofuranosyl--D-glucopyranose
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.
Sucrose
O
OO
CH2OH
HOHO
HO
OH
OH
CH2OH
CH2OH
H3O+/H2O
hydrolysis
O
OH
CH2OH
HOHO
HO
Glucopyranose(glucose)
Fructofuranose(fructose)
+ O
OH
OH
CH2OHHO
CH2OH
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.
Maltose, a cleavage product of starch (e.g.,
amylose), is a disaccharide with an (14)glycosidiclink 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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H O
OH
H
OHH
OH
CH2OH
H
O H
OH
H
OHH
OH
CH2OH
H
O
HH
1
23
5
4
6
1
23
4
5
6
maltose
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.
Cellobiose, a product of cellulose breakdown,The (14) glycosidic linkage between twoglucose molecules.
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H O
OH
H
OHH
OH
CH2OH
H
O OH
H
H
OHH
OH
CH2OH
H
H
H
O1
23
4
5
6
1
23
4
5
6
cellobiose
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Trehalose is found in plants and insects
formed by an ,-1,1-glucoside bond
between two -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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PolysaccharidesCellulose
Linear 1,4--D-glucopyranose polymer
~5,000 - 10,000 glucopyranose molecules per cellulose molecule
repeating subunit:glucopyranose
OO
OO
OOO
HO
CH2OH
OH
HO
CH2OH
OH
CH2OH
HO
OH
Most abundant organic substance in nature
Function: support structure in plants
Wood is ~50% cellulose by weight
Strength due to intermolecular hydrogen bonding
H3O+/H2O
hydrolysis
O
OH
CH2OH
HOHO
HO
Many glucopyranose
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CelluloseCellulose, a major constituent ofplant cell walls, consists of long linear
chains of glucose with (14) 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. Osmoticswelling is prevented.
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Chitin
It is a structural homopolysaccharide
It is a linear molecule composed ofN-
acetylglucosamine monomers linked by (14) glycosidic bond
It forms extended fibers similar to that of
cellulose It is component of exoskeleton of insects
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Polysaccharides
OO
HO
CH2OH
HOO
OHO
CH2OH
HO O
O
OHO
CH2OH
HO
Amylose
Starch
Two forms: amylose, amylopectin
Amylose
Linear coiled polymer of glucopyranose linked by
-1,4 glycosidic bonds
20-25% of starch OO
HO
CH2OH
HO OO
HO
HO O
O
OHO
CH2OH
HO
O
O
OHO
CH2OH
HO
Amylopectin
Amylopectin
Branched polymer containing glucopyranose linked by
-1,4 glycosidic bonds. Branch points has
-1,6glycosidic bonds, 12 glucose in a branch
75-80% of starch
1,4--D-glucopyranose polymer
Function: plant glucose/energy storage
Hydrolysis glucopyranose
Easily digested by mammals
Glucose storage in polymeric form minimizes
osmotic effects
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.
Glycogen, the glucose storage polymer in animals, is similarin structure to amylopectin.
But glycogen has more(16) branches.
The highly branched structure permits rapid glucose releasefrom glycogen stores, e.g., in muscle during exercise.
The ability to rapidly mobilize glucose is more essential toanimals than to plants.
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H O
OH
H
OHH
OH
CH2OH
H
O H
H
OHH
OH
CH2OH
H
O
HH H O
OH
OHH
OH
CH2
HH H O
H
OHH
OH
CH2OH
H
OH
HH O
OH
OHH
OH
CH2OH
H
O
H
O
1 4
6
H O
H
OHH
OH
2
HH H O
H
OHH
OH
2
HH
O
1
OH
3
4
5
2
glycogen
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Glycosaminoglycans
Glycosaminoglycans (mucopolysaccharides) are
linear polymers ofrepeating 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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Hyaluronate(hyaluronan) is a glycosaminoglycan with a
repeating disaccharide consisting of 2 glucose derivatives,
glucuronate (glucuronic acid) &N-acetyl-glucosamine. Theglycosidic linkages are (13) & (14). Form ground
substance of connective tissue.
47
H O
H
H
OHH
OH
COO
H
H O
OH HH
NHCOCH3H
CH2OH
H
OO
D-glucuronate
O
1
23
4
5
61
23
4
5
6
N-acetyl-D-glucosamine
hyaluronate
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.
Inulin :Plant polysaccharide of fructose residues
connected by 2-1 linkage
Chitin: Polymer of N-acetylglucosamine linkedthrough -1,4 glycosidic bond. Found in fungal
cell wall and arthropod cuticle
Mannan: Polymer of mannose linked by -1,4
and -1,3 glycosidic bonds. Found in cell wall ofbacteria 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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Physical propreties of
carbohydrates Monosaccharides are colorless crystalline
solids, very soluble in water, but only
slightly soluble in ethanol, low molecularweight, sweet tasting
Disaccharides are low molecular weight,
sweet,crystalline, less soluble in water thanmonosaccharides
Polysaccharides are high molecular weight,
not sweet, not soluble and not crystalline.49Dr.Saba
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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 andglucosaminoglycans)
(d) Lipids (in glycolipids)50Dr Saba