Alpha amylase catalytic domain found in Amylosucrase
Amylosucrase is a glucosyltransferase that catalyzes the transfer of a D-glucopyranosyl moiety from sucrose onto an acceptor molecule. When the acceptor is another saccharide, only alpha-1,4 linkages are produced. Unlike most amylopolysaccharide synthases, it does not require any alpha-D-glucosyl nucleoside diphosphate substrate. In the presence of glycogen it catalyzes the transfer of a D-glucose moiety onto a glycogen branch, but in its absence, it hydrolyzes sucrose and synthesizes polymers, smaller maltosaccharides, and sucrose isoforms. The Alpha-amylase family comprises the largest family of glycoside hydrolases (GH), with the majority of enzymes acting on starch, glycogen, and related oligo- and polysaccharides. These proteins catalyze the transformation of alpha-1,4 and alpha-1,6 glucosidic linkages with retention of the anomeric center. The protein is described as having 3 domains: A, B, C. A is a (beta/alpha) 8-barrel; B is a loop between the beta 3 strand and alpha 3 helix of A; C is the C-terminal extension characterized by a Greek key. The majority of the enzymes have an active site cleft found between domains A and B where a triad of catalytic residues (Asp, Glu and Asp) performs catalysis. Other members of this family have lost the catalytic activity as in the case of the human 4F2hc, or only have 2 residues that serve as the catalytic nucleophile and the acid/base, such as Thermus A4 beta-galactosidase with 2 Glu residues (GH42) and human alpha-galactosidase with 2 Asp residues (GH31). The family members are quite extensive and include: alpha amylase, maltosyltransferase, cyclodextrin glycotransferase, maltogenic amylase, neopullulanase, isoamylase, 1,4-alpha-D-glucan maltotetrahydrolase, 4-alpha-glucotransferase, oligo-1,6-glucosidase, amylosucrase, sucrose phosphorylase, and amylomaltase.
Jiyao W et al.(2023). "The conserved domain database in 2023.",