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 8- 10 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