Enzymes which cleave glucosidic linkages in e.g. STARCH or GLYCOGEN.
α-Amylases ((1 → 4)-α-D-glucan 4-glucanhydrolases, EC 18.104.22.168) are endoenzymes which have little action on terminal (1 → 4)-α-bonds or on bonds adjacent to (1 → 6)-α branch points. They act on amylopectin and glycogen to form glucose, maltose and branched α-limit DEXTRINS, and on amylose to form first maltose and maltotriose, then slowly on maltotriose to form maltose and glucose. α-Amylases are common among microorganisms. They are obtained commercially mainly from Bacillus spp (see also IMMOBILIZATION (sense 1)) and are used e.g. for processing starch to form glucose syrups: insoluble starch granules are dispersed in water by heating, and the starch is partially hydrolysed with thermostable α-amylases from B. amyloliquefaciens and/or B. licheniformis. Further hydrolysis is achieved using e.g. γ-amylases (see below).
β-Amylases ((1 → 4)-α-D-glucan maltohydrolases, EC 22.214.171.124) are exoenzymes which cleave alternate bonds from the nonreducing end of a linear (1 → 4)-α-D-glucan; thus, e.g. amylose is degraded to maltose. β-Amylase action is halted at (1 → 6)-α branch points; thus maltose and β-limit dextrins are formed from glycogen and amylopectin. β-Amylases are common in plants and are produced e.g. by Bacillus and Streptomyces spp. They may be used, together with DEBRANCHING ENZYMES, in the manufacture of maltose syrups from starch. β-Amylases are inhibited by SCHARDINGER DEXTRINS.
γ-Amylases (amyloglucosidases, glucoamylases, (1 → 4)-α-D-glucan glucohydrolases, exo-(1 → 4)-α-glucosidases, EC 126.96.36.199) are exoenzymes which cleave (1 → 4)-α-bonds consecutively from the non-reducing end of a (1 → 4)-α-D-glucan to yield β-D-glucose; they can also cleave (1 → 6)-α- and (1 → 3)-α-bonds, although at a much lower rate. γ-Amylases are found mainly in fungi and are obtained commercially e.g. from Aspergillus niger and Rhizopus spp. They are used to convert malto-oligosaccharides (e.g. limit dextrins) to D-glucose (see above).
(See also BREAD-MAKING and ENZYMES.)
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