polysaccharide(redirected from Polysaccharides)
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any member of a large class of chemical compounds that includes sugars, starches, cellulose, and related compounds. These compounds are produced naturally by green plants from carbon dioxide and water (see photosynthesis).
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A class of high-molecular-weight carbohydrates, colloidal complexes, which break down on hydrolysis to monosaccharides containing five or six carbon atoms. The polysaccharides are considered to be polymers in which monosaccharides have been glycosidically joined with the elimination of water. A polysaccharide consisting of hexose mono-saccharide units may be represented by the reaction below.
The term polysaccharide is limited to those polymers which contain 10 or more monosaccharide residues. Polysaccharides such as starch, glycogen, and dextran consist of several thousand d -glucose units. Polymers of relatively low molecular weight, consisting of two to nine monosaccharide residues, are referred to as oligosaccharides. See Dextran, Glucose, Glycogen, Monosaccharide, Starch
Polysaccharides are often classified on the basis of the number of monosaccharide types present in the molecule. Polysaccharides, such as cellulose or starch, that produce only one monosaccharide type ( d -glucose) on complete hydrolysis are termed homopolysaccharides. On the other hand, polysaccharides, such as hyaluronic acid, which produce on hydrolysis more than one monosaccharide type (N-acetylglucosamine and d -glucuronic acid) are named heteropolysaccharides. See Carbohydrate
a macromolecular compound of the carbohydrate class, composed of monosaccharide residues (M) joined by glycoside bonds. The molecular weight of polysaccharides ranges from several thousand for laminarin and inulin to several million for hyaluronic acid and glycogen; it can be determined only approximately, since individual polysaccharides are usually mixtures of components that differ in degree of polymerization. The chemical classification of polysaccharides is based on the structure of their monosaccharide components, hexoses (glucose, galactose, and mannose) and pentoses (arabinose and xylose), as well as amino sugars (glucosamine and galactosa-mine), deoxy sugars (rhamnose and fucose), and uronic acids.
Acid residues (acetic, pyruvic, lactic, phosphoric, and sulfuric acids) or alcohol residues (usually methanol) may be attached to hyroxyl (—OH) and amino (—NH2) groups in the molecules of natural polysaccharides. Homopolysaccharides are formed from residues of only one monosaccharide (for example, glucans and fructans), whereas heteropolysaccharides are composed of residues of two or more monosaccharides (for example, arabinoga-lactans, glycuronoxylans). Many common polysaccharides or polysaccharide groups bear long-established names, such as cellulose, starch, chitin, and pectins. (The name of a polysaccharide sometimes is associated with its source of extraction—for example, nigeran from the fungus Aspergillus niger and odontalan from the seaweed Odontalia corymbifera.)
Polysaccharides, unlike other classes of biopolymers, can exist in both a linear structural form (Figure 1,a) and branched forms (Figure 1,b and c).
Examples of linear polysaccharides are cellulose, amylose, and mucopolysaccharides; yeast mannans and vegetable gums have structure (b), and glycogen, amylopectin, and galactan from the edible snail Helix pomatia have structure (c). The structural type of a polysaccharide largely determines its physicochemical properties, particularly solubility in water. Regular linear polysaccharides (that is, those containing only one type of inter-monosaccharide bond), such as cellulose and chitin, are insoluble in water, since the energy of molecular interaction is greater than the hydration energy. Highly branched polysaccharides that do not have an ordered structure are readily soluble in water. The chemical reactions characteristic of many monosaccharides—for example, acylation, alkylation, oxidation of hy-droxyl groups, reduction of carboxyl groups, and the introduction of new groups—are also effected in polysaccharides, although the rates of these reactions are usually lower. Chemically modified polysaccharides often exhibit new properties of practical importance that are absent in the original compound.
Most polysaccharides are resistant to alkalies; their depolym-erization, or hydrolysis, takes place under the action of acids to yield free monosaccharides or oligosaccharides, depending on the conditions of acid hydrolysis. Molecules of heteropolysaccharides containing glycoside bonds of differing acid resistance can be split selectively; specific enzymes are also used for this purpose. Determination of the structure of low-molecular weight decomposition products simplifies the task of establishing the structure of the polysaccharide itself, reducing it to determination of the structure of the repeating units; it is believed—and has been shown using several examples—that all polysaccharides are composed of such units. Research on the secondary polysaccharide structure is conducted using physicochemical methods, particularly X-ray structural analysis, which has been successfully used in studying cellulose.
The biological functions of polysaccharides are extremely diverse. Starch and glycogen are reserve polysaccharides in plants and animals, whereas plant cellulose and the chitin in insects and fungi are structural polysaccharides. Hyaluronic acid, which occurs in the ovicellular membrane, synovia, and the vitreous humor, is a highly effective “lubricant.” Gums and mucosa in plants and capsular polysaccharides in microorganisms fulfill protective functions; the highly sulfated polysaccharide heparin inhibits blood clotting. Polysaccharide components in mixed carbohydrate-containing biopolymers such as glycoproteins and lipopolysaccharides, which are present in the surface cell layer, govern the specific immune reactions of the organism. Extracellular polysaccharides and other carbohydrate-containing biopolymers ensure intercellular reactions and the fixing of cells in plants (pectins) and animals (hyalin).
Polysaccharide biosynthesis is mainly effected with the participation of nucleoside diphosphate sugars, which act as donors of the monosaccharide or, less frequently, disaccharide residues that are transferred onto the corresponding oligosaccharide fragments of the polysaccharide being formed. The biosynthesis of heteropolysaccharides is effected by the successive addition of monosaccharides from the corresponding nucleoside diphosphate sugars to the polysaccharide chain. Another mechanism is also observed during the formation of polysaccharides in bacterial antigens: first the specific repeating units are synthesized, with the participation of lipid and nucleotide carriers of sugars; polysaccharides are then synthesized from the units under the action of the enzyme polymerase. Branched polysaccharides of the glycogen and amylopectin type are formed by the intramolecular enzymatic rearrangement of a linear polysaccharide. Approaches to the directed chemical synthesis of polysaccharides are being developed.
In living organisms, polysaccharides—which are the main energy reserves—are split by intracellular and extracellular enzymes to yield monosaccharides and their derivatives, which decompose further to liberate energy. The accumulation and decomposition of glycogen in the liver of humans and higher animals is a means of controlling the glycogen level in the blood. Monomeric products are formed indirectly by the successive splitting off of polysaccharides from the molecule or as a result of the step-by-step decomposition of polysaccharides, with the intermediate formation of oligosaccharides. Many polysaccharides, including starch, cellulose, and pectins, are used in the food and chemical industries, as well as in medicine.
REFERENCESStacey, M., and S. Barker. Uglevody zhivykh tkanei. Moscow, 1965. (Translated from English.)
Khimiia uglevodov. Moscow, 1967.
L. V. BAKINOVSKII