Download Oligosaccharides Disaccharides

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Oligosaccharides
Oligosaccharides are carbohydrates formed of 2-10 monosaccharide units
covalently bonded to each other by glycosidic bonds.
According to the number of sugar units, they are classified into disaccharides,
trisaccharides and so on.
Disaccharides
Disaccharides are oligosaccharides formed of two monosaccharide units
covalently bonded by glycosidic bond.
Classification
Disaccharides are classified according to the type of its monosaccharide
unites into:
1- Homodisaccharides
These are disaccharides in which the two monosaccharide units are the
same e.g.:
• Maltose which formed of two α-glucose units linked together by α(1,4) glycosidic bond
It is the major degradation product of starch
It has a free aldehyde group so; it is a reducing sugar
• Cellobiose which is formed of two units of β glucose linked together
by β-(1,4) glycosidic bond.
It has a free aldehyde group so; it is a reducing sugar
2- Heterodisaccharide
These are disaccharides in which the two monosaccharide units are different
e.g.:
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• Sucrose which is formed of one α glucose molecule and one β
fructose molecule linked by α-(1,2)β-glycosidic bond
It is prevalent in cane sugar and beets.
Sucrose is a non-reducing sugar as it contains no free aldehyde
or ketone group.
• Lactose is found exclusively in the milk
It is formed of one β- galactose and one β- glucose linked together by
β-(1,4) glycosidic bond.
It has a free aldehyde group so; it is a reducing sugar.
Trisaccharides
Trisaccharides are oligosaccharides formed of three monosaccharide units
covalently bonded to each other by glycosidic bonds.
Maltotriose is a trisaccharide formed of three α-glucose units
Polysaccharides
Polysaccharides are carbohydrates that are formed of more than ten
monosaccharide units. They may contain hundreds or even thousands of
monosaccharide units.
The predominant monosaccharide found in polysaccharides is D-glucose.
Classification
1- According to the type of their monosaccharide unites:
A- Homopolysaccharides
Homopolysaccharides are composed of the same monosaccharide
units e.g. starch, glycogen, cellulose and inulin
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B- Heteropolysaccharides
Heteropolysaccharides are composed of more than one type of
monosaccharide units e.g. glycosaminoglycans, proteoglycans,
glycoproteins, agar and gum arabic
2- According to their physiological function:
A- Structural polysaccharides
Polysaccharides that inter in the structure of cells e.g. cellulose which
inters in the structure of plant cell wall
B- Storage polysaccharides
Polysaccharides that serve as energy stores e.g. starch in plants and
glycogen in animals.
Glycogen
Glycogen is the major form of stored carbohydrate in animals. It is formed
of many α-D glucose units linked to each other by α-(1,4) linkage.
It is also highly branched, with α-(1,6) branch linkages occurring every 810 glucose units.
It is found mainly in liver and skeletal muscles
Starch
Starch is the major form of stored carbohydrate in plant cells. Its structure is
identical to glycogen, except for a much lower degree of branching.
Starch granule consists of 2 layers;
Inner unbranched layer called amylose which constitutes about 10-30%
of the starch granule and is formed of α-D-glucose units in α(1,4)
linkages.
Outer highly branched layer called amylopectin which constitutes about
70 to 90% of the starch granule and is formed of α-D glucose units in
α(1,4) linear linkages and α(1,6) linkages at the branching points
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Amylose
Amylopectin
Inulin
It is a homopolysaccharide formed of fructose
It is used in to determine the rate of glomerular filtration by inulin
clearance test
It can be used as a diet for diabetics.
Cellulose
It is a homopolysaccharide formed of β-D-glucose units linked together by
β(1,4) glycosidic bonds.
It is found in the cell walls of nearly all plants. Cellulose is extremely
resistant to hydrolysis by the digestive tract enzymes. So, it can stimulate
peristaltic movement and prevent constipation.
Glycosaminoglycans (GAGs)
(Mucopolysaccharides)
Glycosaminoglycans are long linear unbranched heteropolysaccharide
chains generally composed of a repeating disaccharide unit
The disaccharide unit is usually formed of sugar acid and amino sugar.
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Examples of Glycosamineoglycans:
1- Heparin
Heparin is a mucopolysaccharide repeating disaccharide unit disaccharide
unit is formed of glucosamine and glucuronic acid. Almost all glucosamine
residues are. Sulphated
Unlike other mucopolysaccharides that are extracellular compounds,
heparin is an intracellular component of mast cells that line arteries. It is an
anticoagulant preventing blood clotting.
2- Hyaluronic acid
Its disaccharide unit is formed of N-acetylglucosamine and glucuronic acid.
It is different from heparin in being unsulphated
It is found in synovial fluid of joints, vitreous humour of the eye, umbilical
cord, and loose connective tissue.
It serves as a lubricant and shock absorber.
3- Chondroitin sulphate
Its disaccharide unit is formed of N-acetylgalactosamine and glucuronic
acid, with sulphate on either C-4 or C-6.
It is the most abundant mucopolysaccharide in the body.
It is found in cartilage, tendons and ligaments
4- Keratan sulphate
Its disaccharide unit is formed of N-acetyl glucosamine and galactose (no
uronic acid). Sulphate content is variable
It is found in cornea and in cartilage
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Proteoglycans
All of the mucopolysaccharides (except hyaluronic acid) are found
covalently attached to protein, forming proteoglycans.
Glycoproteins
Glycoproteins are proteins to which oligosaccharides are covalently
attached.
Membrane-bound glycoproteins have a role in:
1- Cell surface recognition by other cells and hormones
2- Cell surface antigenicity such as the blood group antigens
3- Structure of the extracellular matrix and of the mucin acting as protective
biologic lubricants.
Examples of glycoproteins include blood group antigens, enzymes and
some hormones