chitin
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chitin
Chitin
A polysaccharide found abundantly in nature. Chitin forms the basis of the hard shells of crustaceans, such as the crab, lobster, and shrimp. The exoskeleton of insects is also chitinous, and the cell walls of certain fungi contain this substance.
Chitin is a long, unbranched molecule consisting entirely of N-acetyl- d -glucosamine units linked by β-1,4 bonds (see illustration). It may be thought of as cellulose in which the hydroxyl groups on the second carbon are replaced with NHCOCH3 groups. Chitin is considered to be synthesized in nature by an enzyme which is capable of effecting a glycosyl transfer of the N-acetyl- d -glucosamine from uridinediphosphate-N-acetyl- d -glucosamine to a preformed chitodextrin acceptor, forming the polysaccharide. This stepwise enzymic transfer results in the production of the long chain of β-N-acetyl- d -glucosamine units, which is insoluble chitin. See Polysaccharide
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chitin
[′kīt·ən]Chitin
a natural polysaccharide; the basic component of the exoskeleton, or cuticula, of arthropods and certain other invertebrates; also found in the cell walls of fungi and bacteria. Chitin performs protective and supportive functions by ensuring cell rigidity. The term “chitin” was proposed by the French scientists A. Audier, who in 1823 investigated the hard outer covering of insects.
Chitin consists of residues of N-acetyl glucosamine linked by β1,4 glycoside bonds. Its molecular weight may be as high as 260,000. It is insoluble in water, dilute acids, alkalies, and organic solvents, including alcohol; it is soluble in concentrated solutions of salts (thiocyanate of lithium and calcium), and it breaks down in concentrated solutions of mineral acids when heated. As found in nature, chitin is always associated with proteins; it is analogous to the cellulose of plants in its structure, physical and chemical properties, and biological functions.
The biosynthesis of chitin in an organism is effected by the transfer of a donor residue of N-acetylglucosamine from uridine-diphosphate-N-acetylglucosamine to chitodextrin acceptors—with the participation of the enzymatic glycosyltransferase system that is associated with the intracellular membranes. The biological decomposition of chitin down to free N-acetylglucosamine is effected by chitinase, an enzyme found in a number of bacteria and among the digestive enzymes of soil amoebas and certain snails and earthworms, as well as in crustaceans during the molting period. When an organism dies, chitin and the productions of its decomposition are converted to humin-like compounds in the soil and sea sediments, thus promoting the accumulation of nitrogen in the soil.
N. D. GABRIELIAN