Carbohydrases

CARBOHYDRASES, enzymes that produce the hydrolytic breakdown of complex carbohydrates (di-, tri-, and polysaccharides and glycosides) into simple sugars, or monosaccharides. The action of carbohydrases is directed at the ether or, more precisely, the glycosidic group of carbohydrates: R-O-R'+H₂O ⇌ R.OH+HO.R'. As this formula shows, the action of carbohydrases is reversible; it has been experimentally proven that under suitable conditions these enzymes, at least those of them which act on disaccharides and glycosides, can synthesize their substrate from a mixture of breakdown products. According to the modern classification, carbohydrases are divided as follows. 1. Polyases, which break down polysaccharides; this includes amylases (diastase, ptyalin, glycogenase of animal cells), which hydrolyze starch and glycogen into maltose; furthermore, cellulases, which break down cellulose, inulinase, etc. 2. Hexosidases. Under this name are grouped carbohydrases acting on disaccharides and glycosides, since all di- (and tri-) saccharides can be considered as glycosides consisting of two (respectively three) hexoses; hexosidases act only on those sugars and glycosides which contain naturally occurring fermentable isomers of hexoses (d-glucose, d-fructose, d-galactose, and d-mannose), and since each of these hexoses gives two series of stereoisomeric derivatives—α- and β-hexosides (see Glycosides, Disaccharides)—a distinction is made between α- and β-hexosidases, the action of which is specifically directed at derivatives of the corresponding series; moreover, even within these groups, it is possible to carry out a further differentiation of carbohydrases depending on which of the two components of the hexoside their affinity is more pronounced towards. Thus, among the enzymes that break down cane sugar (glycosido-fructoside), yeast sucrase is a fructo-sucrase, while the sucrase of animal tissues and the fungus Aspergillus niger is a gluco-sucrase. The most important hexosidases: 1) fructosidases—sucrase (invertase), raffinase; 2) α-glycosidases—maltase; 3) β-glycosidases—cellobiase, emulsin (the latter represents a mixture of enzymes that sequentially break down the glycoside of bitter almonds—amygdalin—into 2 particles of glucose, hydrocyanic acid, and benzaldehyde); 4) galactosidases—lactase. The biological role of carbohydrases is very diverse and important. On one hand, in animals, the carbohydrases of digestive juices, freely secreted by the glands of the digestive canal (amylase of saliva, pancreatic and intestinal juices, sucrase, maltase, and lactase of the intestinal canal), by breaking down food carbohydrates into simple monosaccharides, make them available for absorption and assimilation. On the other hand, intracellular carbohydrases, universally distributed in animal and plant organisms, break down poly- and disaccharides (and in plants also glycosides), which serve as a 'reserve' form of carbohydrates, and convert them into a 'working' form—monosaccharides, which the cell can use as a source of energy by oxidizing or fermenting them, or as material for various syntheses (e.g., fat or protein); examples are the glycogenase of the liver and muscles; amylase and maltase of germinating seeds. These same intracellular carbohydrases, in all probability, serve to convert simple sugars in cells into complex carbohydrates and glycosides stored in reserve, acting in this case as synthesizing enzymes. Some of these syntheses (the formation of many plant glycosides, in animals—the synthesis of paired glucuronic acids, which represent the product of oxidation of the corresponding glycosides) play the role of protective reactions, by means of which the organism detoxifies poisonous metabolic products (and poisons introduced from outside), converting them into more indifferent compounds (see also Hydrolytic enzymes, Carbohydrate metabolism, Digestion, and individual enzymes). A. Braunshtein