α-Amylase: an enzyme specificity found in various families of glycoside hydrolases

alpha-Amylase (EC 3.2.1.1) represents the best known amylolytic enzyme. It catalyzes the hydrolysis of alpha-1,4-glucosidic bonds in starch and related alpha-glucans. In general, the alpha-amylase is an enzyme with a broad substrate preference and product specificity. In the sequence-based classification system of all carbohydrate-active enzymes, it is one of the most frequently occurring glycoside hydrolases (GH). alpha-Amylase is the main representative of family GH13, but it is probably also present in the families GH57 and GH119, and possibly even in GH126. Family GH13, known generally as the main alpha-amylase family, forms clan GH-H together with families GH70 and GH77 that, however, contain no alpha-amylase. Within the family GH13, the alpha-amylase specificity is currently present in several subfamilies, such as GH13_1, 5, 6, 7, 15, 24, 27, 28, 36, 37, and, possibly in a few more that are not yet defined. The alpha-amylases classified in family GH13 employ a reaction mechanism giving retention of configuration, share 4-7 conserved sequence regions (CSRs) and catalytic machinery, and adopt the (beta/alpha)(8)-barrel catalytic domain. Although the family GH57 alpha-amylases also employ the retaining reaction mechanism, they possess their own five CSRs and catalytic machinery, and adopt a (beta/alpha)(7)-barrel fold. These family GH57 attributes are likely to be characteristic of alpha-amylases from the family GH119, too. With regard to family GH126, confirmation of the unambiguous presence of the alpha-amylase specificity may need more biochemical investigation because of an obvious, but unexpected, homology with inverting beta-glucan-active hydrolases.