Chemistry of carbohydrates - PowerPoint PPT Presentation

Title: Chemistry of carbohydrates


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Chemistry of Carbohydrates

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

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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
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Constituents of nucleic acids
Ribose and deoxyribose

Constituents of nervous tissue
Glycolipids
Form some hormones and blood group substances
Glycoproteins
Constituent of mucus
Mucin
Mucopoly-saccharides
Structural constituents of tissues
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3
3
3
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Definition
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Classification
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Disaccharides are made up of two monosaccharides
The constituent monosaccharides may be identical
or different
The common disaccharides are sucrose, lactose and
maltose

Monosaccharides and disaccharides are called
sugars because of their sweet taste
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Monosaccharides
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Aldoses
Monosaccharides having an aldehyde group
Ketoses
Monosaccharides having a keto group
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Aldoses and ketoses may be sub-divided on the
basis of number of carbon atoms
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Some common monosaccharides
No. of carbon

atoms Trioses
3 Glyceraldehyde Dihydroxyacetone
Tetroses 4 Erythrose
Erythrulose Pentoses 5
Ribose Ribulose Hexoses 6
Glucose Fructose
Aldose Ketose
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Trioses
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C1 CH2OH CHO CH2OH    
C2 CHOH CHOH C O  
C3 CH2OH CH2OH
CH2OH Glycerol Glyceraldehyde Dihydroxyacetone
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CHO CHO HCOH HOCH
       CH2OH CH2OH D-Glyceraldehyd
e L-Glyceraldehyde
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E M B - R C G
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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
E M B - R C G
This will be carbon atom 3 in tetroses, carbon
atom 4 in pentoses and carbon atom 5 in hexoses
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If the OH group is on the right of these carbon
atoms, the isomer will be D
E M B - R C G
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A method to show the configuration of
mono-saccharides on paper was devised by Emil
Fischer
E M B - R C G
His formulas are known as Fischer projection
formulas
The monosaccharides are shown as linear molecules
in these formulas
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E M B - R C G
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E M B - R C G

• In aldoses, the carbon of the aldehyde group is
  C1

In ketoses, the carbon of the keto group is C2
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Tetroses
E M B - R C G

• This is formed as an intermediate (as
  erythrose-4-phosphate) during the metabolism of
  glucose via the hexose monophosphate shunt
  pathway

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Pentoses
E M B - R C G
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E M B - R C G
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E M B - R C G
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Hexoses
E M B - R C G
The important ketohexose is D-fructose which is
the ketoisomer of D-glucose
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• D-Glucose is the most important
  carbohydrate in human beings

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• 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

E M B - R C G
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• 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

E M B - R C G
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• Formed in some path-ways of carbohydrate
  metabolism
• Also present in seminal fluid provides
  nourish-ment to sperms

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Heptoses
The only heptose important in human beings is
D-sedoheptulose which is a ketoheptose
E M B - R C G
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Anomerism

• Aldose-ketose isomerism, stereoisomerism and
  optical isomerism have been seen earlier

E M B - R C G
These can be explained easily by Fischer
projection formulas
A problem arose with the discovery of two
different methyl glucosides derived from glucose.
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Glucose reacts with methanol in the presence of a
mineral acid to form two distinct methyl
glucosides
E M B - R C G
One is known as methyl-a-D-glucoside and the
other is known as methyl-b-D-glucoside
Both have cyclic structures
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• It was later found that higher monosaccharides
  also exist in solution in a cyclic hemi-acetal
  form

Ring is formed by a reaction between carbonyl
group and the ?OH group attached to C4 or C5
E M B - R C G
If cyclisation involves C4, it results in the
formation of a five-membered ring similar to
furan
A monosaccharide having this type of ring
structure is designated as a furanose
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E M B - R C G
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Pyran
Furan
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E M B - R C G
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E M B - R C G
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• The anomeric carbon produces an additional type
  of isomerism called anomerism

E M B - R C G
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?
?
The a and b anomers of glucose in pyran ring form
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E M B - R C G
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• The additional centre of asymmetry in
  keto-hexoses is at carbon atom 2

E M B - R C G
The ?OH group attached to C2 projects below the
plane of the ring in the a-anomer
It projects above the plane of the ring in the
b-anomer
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6
1
5
2
?
4
3
6
?
2
5
CH2OH
4
3
1
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• Aldopentoses, e.g. ribose, also exist in the form
  of five membered furan ring form

E M B - R C G
Sometimes even aldohexoses exist in furan ring
form
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5
1
4
6
3
2
5
1
4
5
3
2
4
1
3
2
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Mutarotation

• Carbohydrates possessing an asymmetric carbon
  atom are optically active

E M B - R C G
Before the ring structures of carbohydrates were
established, it had been shown that glucose
existed in two optically distinct forms
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E M B - R C G
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E M B - R C G
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E M B - R C G
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E M B - R C G
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Epimerism
E M B - R C G
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CH2OH
CH2OH
O
O
H
H
HO
H
H
H
4
H
OH
H
OH
OH
OH
H
OH
H
H
OH
OH
a-D-Gluco- pyranose
a-D-Galacto- pyranose
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CH2OH
CH2OH
O
O
H
H
H
H
H
H
H
OH
OH
OH
OH
OH
OH
OH
2
H
H
OH
H
a-D-Mannopyranose
a-D-Glucopyranose
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E M B - R C G
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Formed by replacement of hydroxyl group attached
to carbon atom 2 of ribose with hydrogen
E M B - R C G
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E M B - R C G
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Constituent of hyaluronic acid

Glucosamine

Constituent of chondroitin sulphate
Galactosamine
Found in glycoproteins
Mannosamine
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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
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E M B - R C G
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H
OH
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Glucuronic acid
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
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Reactions of monosaccharides
E M B - R C G
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Interconversion
E M B - R C G
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• The interconversion occurs via formation of a
  common enediol intermediate

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Dehydration
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CH2OH
CHO
O
O
H
H
H
? 3 H2O
OH
H
Furfural
OH
OH
D-Ribose
CH2OH
CHO
CH2OH
CH2OH
O
O
OH
H
? 3 H2O
OH
H
Hydroxymethyl
H
OH
furfural
D-Fructose
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• 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 (Molischs test) and
  resorcinol (Seliwanoffs test), to form
  characteristically coloured complexes

E M B - R C G
Molischs test
Seliwanoffs test
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Oxidation
Aldehyde group of aldoses is readily oxidised to
a carboxyl group by mild oxidizing agents in
acidic medium
E M B - R C G
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
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Reduction
E M B - R C G
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E M B - R C G
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E M B - R C G
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Reduction of metal ions
E M B - R C G
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E M B - R C G
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• 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

E M B - R C G
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Formation of osazones
E M B - R C G
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E M B - R C G
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E M B - R C G
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E M B - R C G
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Osazone crystals of glucose, mannose and fructose
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Reaction with hydroiodic acid
E M B - R C G
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Reaction with alcohols
E M B - R C G
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If the carbon atom of glucose has an
a-configuration, the bond is known as
a-glycosidic bond
E M B - R C G
A similar reaction occurs between other
carbohydrates and alcohols as well leading to the
formation of various glycosides
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E M B - R C G
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E M B - R C G
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Esterification
E M B - R C G
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E M B - R C G
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Caramelisation
E M B - R C G
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Disaccharides
Made up of two monosaccharide molecules linked
by a glycosidic bond
Mostly found in plants
Important ones are sucrose, maltose and lactose
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Sucrose
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CH OH
2
O
H
H
H
1
H
OH
OH
Carbon atom 1 of glucose linked to carbon
atom 2 of fructose by a glyco-sidic bond
H
OH
Sucrose
O
CH OH
2
O
2
OH
H
CH OH
H
2
H
OH
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• 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

E M B - R C G
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E M B - R C G
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Laevorotation caused by fructose is greater than
the dextrorotation caused by glucose
E M B - R C G
Therefore, the hydrolysate is laevorotatory
As the optical rotation is inverted on
hydrolysis, sucrose is described as invert sugar
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Carbon atom 1 of one molecule is linked to carbon
atom 4 of the second
E M B - R C G
The carbon atom 1 (anomeric carbon) of the second
glucose molecule is free, and may possess an a-
or a b-configuration
Therefore, maltose may exist as a-maltose or
b-maltose
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Anomeric carbon of the first glucose molecule,
involved in bonding, possesses a-configuration
E M B - R C G

• Therefore, the bond is an a-glycosidic bond

The a-form of maltose may be described as
a-D-glucopyranosyl-a-D glucopyranoside
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• 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

E M B - R C G
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4
1
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• 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

E M B - R C G
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Reactions of disaccharides
E M B - R C G
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E M B - R C G
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E M B - R C G
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E M B - R C G
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Disaccharides also give reactions characteristic
of hydroxyl, aldehyde and ketone groups
E M B - R C G
For example, maltose and lactose form distinctive
osazones
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Osazone crystals of maltose and lactose
Maltosazone
Lactosazone
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E M B - R C G
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Polysaccharides
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Homopolysaccharides
Most important homopolysaccharide in animals
(including man) is glycogen
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Glycogen (animal starch)
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• A long chain of glucose molecules is formed in
  this way

E M B - R C G
However, after every 8-12 glucose units, there is
a branch point
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
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E M B - R C G
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Starch
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Starch is made up of a large number of glucose
units
It contains two different types of molecules
amylose and amylopectin
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E M B - R C G
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Amylose
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Amylopectin
Constitutes the remaining 80-85 of starch
E M B - R C G
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
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Amylopectin
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Dextrin
E M B - R C G
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E M B - R C G
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Cellulose
E M B - R C G
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E M B - R C G
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Inulin
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E M B - R C G
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Heteropolysaccharides
E M B - R C G
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E M B - R C G
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Hyaluronic acid
Has a very wide tissue distribution
Forms the ground substance of mesenchymal tissue
Made up of glucuronic acid and N-acetyl
glucosamine
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• Carbon atom 1 of glucuronic acid forms
  a glycosidic bond with carbon atom 3 of
  N-acetylglucosamine

E M B - R C G
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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
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Chondroitin sulphate
Has a restricted tissue distribution
Is mainly found in cartilages and bones
Is made up of glucuronic acid and
N-acetylgalactosamine sulphate
Glycosidic bonds are similar to those in
hyaluronic acid
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• In chondroitin-4-sulphate, the sulphate group is
  esterified with carbon atom 4 of
  N-acetylgalactosamine

O
4
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• In chondroitin-6-sulphate, the sulphate group is
  esterified with carbon atom 4 of
  N-acetylgalactosamine

6
O
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Heparin
Is made up of glucuronic acid and glucosamine,
both of which are sulphated
These two are linked to each other by
a-1,4-glycosidic bonds
Some L-iduronic acid residues are also present in
heparin
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Heparin is secreted by mast cells (present in
walls of large arteries, lungs, liver etc)
It is an anticoagulant (prevents intra-vascular
clotting)
It releases lipoprotein lipase from walls of
capillaries (helps in catabolism of chylomicrons
and VLDL)
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E M B - R C G
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Heparan sulphate
E M B - R C G
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Dermatan sulphate
E M B - R C G
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Keratan sulphate
E M B - R C G
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E M B - R C G
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E M B - R C G
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Reactions of polysaccharides
E M B - R C G
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E M B - R C G
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E M B - R C G
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E M B - R C G
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E M B - R C G
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E M B - R C G
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E M B - R C G
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E M B - R C G
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Furfural derivatives condense with a-naphthol to
give violet colour (Molischs reaction)
E M B - R C G
Molischs test is, thus, given by all the
carbo-hydrates
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Thank you