Properties of Polysaccharides

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These are also compound sugars and yield more than 10 molecules of monosaccharides on hydrolysis. These may be further classified depending on whether the monosaccharide molecules produced as a result of the hydrolysis of polysaccharides are of the same type (homopolysaccharides) or of different types (heteropolysaccharides).

Polysaccharides are high molecular weight carbohydrates which, on hydrolysis, yield mainly monosaccharides or products related to monosaccharides. They may also be regarded as polymeric anhydrides of simple sugars.

D-glucose is the commonest component of polysaccharides. However, D-and L-galactose, D-mannose, D-xylose, L-arabinose as well as D-glucuronic, D-galacturonic, D-mannuronic acids, D-glucosamine, D-galactosamine and amino uronic acids also occur as constituents of polysaccharides.

Starch

Starch contains two types of glucose polymer, amylose and amylopectin.

Amylose consists of long, unbranched chains of D-glucose residues connected by (α1 -> 4) linkages (as in maltose). Such chains vary in molecular weight from a few thousand to more than a million.

Amylose

Amylopectin also has a high molecular weight (up to 200 million) but unlike amylose is highly branched. The glycosidic linkages joining successive glucose residues in amylopectin chains are (α1 -> 4); the branch points (occurring every 24 to 30 residues) are (α1 -> 6) linkages.

Amylopectin

Starch is a white soft amorphous powder and lacks sweetness. It is insoluble in water, alcohol and ether at ordinary temperature. The specific rotation of starch, [α]D 20 is + 196°. The microscopic form of the starch grains is characteristic of the source of starch. Starch, on partial hydrolysis by boiling with water under pressure at about 250°C, breaks down into large fragments called dextrins.

Glycogen

Glycogen is the main storage polysaccharide of animal cells. Like amylopectin, glycogen is a polymer of (α1 -> 4)-linked subunits of glucose, with (α1 -> 6)-linked branches, but glycogen is more extensively branched (on average, every 8 to 12 residues) and more compact than starch.

Glycogen

Glycogen is especially abundant in the liver, where it may constitute as much as 7% of the wet weight; it is also present in skeletal muscle. In hepatocytes glycogen is found in large granules, which are themselves clusters of smaller granules composed of single, highly branched glycogen molecules with an average molecular weight of several million. Such glycogen granules also contain, in tightly bound form, the enzymes responsible for the synthesis and degradation of glycogen.

It is a white powder and is more soluble in water than amylopectin. Hence, it readily forms suspension even in cold water. Glycogen is precipitated from aqueous solutions by addition of ethyl alcohol and is fairly stable in hot alkali. It is a nonreducing sugar and gives red colour with iodine. The red colour, however, disappears on boiling and reappears on cooling.

Cellulose

The molecular weight of cellulose ranges between 200,000 and 2,000,000, thus corresponding to 1,250–12,500 glucose residues per molecule. It may be formed by taking out a molecule of water from the glycosidic OH group on carbon atom 1 of one β-D-glucose molecule and the alcoholic OH group on carbon atom 4 of the adjacent β-D-glucose molecule. It, thus, resembles in structure with amylose except that the glucose units are linked together by β-1, 4-glucoside linkages. Cellulose may, henceforth, be regarded as an anhydride of β-D-glucose units.

Cellulose

It is a fibrous, tough, white solid, insoluble in water but soluble in ammoniacal cupric hydroxide solution (Schweitzer’s reagent). It gives no colour with iodine and lacks sweetness. Although insoluble in water, cellulose absorbs water and adds to the bulk of the fecal matter and acilitates its removal.

Hyaluronic acid

Hyaluronic acid has the least complicated structure among mucopolysaccharides. It is a straight-chain polymer of D-glucuronic acid and N acetyl- D-glucosamine (NAG) alternating in the chain. Its molecular weight approaches approximately, 5,000,000. Here, apparently two linkages are invloved, β-1 → 3 and β-1 → 4. Hyaluronic acid is an acidic substance, because the carboxyl groups are largely ionized at cellular pH.

Hyaluronic acid

Hyaluronic acid, upon hydrolysis, yields an equimolar mixture of D-glucuronic acid, Dglucosamine and acetic acid.

Heparin

Heparin is a heteropolysaccharide composed of D-glucuronic acid units, most of which (about 7 out of every 8) are esterified at C2 and D glucosamine-N-sulfate (= sulfonylaminoglucose) units with an additional O-sulfate group at C6. Both the linkages of the polymer are alternating α-1 → 4. Thus, the sulfate content is very high and corresponds to about 5–6 molecules per tetrasaccharide repeating unit. The relative positions of the sulfate residues may also vary. Its molecular weight ranges between 17,000 and 20,000.

Heparin