Alpha amylase catalytic domain found in maltosyltransferase
Maltosyltransferase (MTase), a maltodextrin glycosyltransferase, acts on starch and maltooligosaccharides. It catalyzes the transfer of maltosyl units from alpha-1,4-linked glucans or maltooligosaccharides to other alpha-1,4-linked glucans, maltooligosaccharides or glucose. MTase is a homodimer. The catalytic core domain has the (beta/alpha) 8 barrel fold with the active-site cleft formed at the C-terminal end of the barrel. Substrate binding experiments have led to the location of two distinct maltose-binding sites: one lies in the active-site cleft and the other is located in a pocket adjacent to the active-site cleft. It is a member of the alpha-amylase family, but unlike typical alpha-amylases, MTase does not require calcium for activity and lacks two histidine residues which are predicted to be critical for binding the glucose residue adjacent to the scissile bond in the substrates. The common reaction chemistry of the alpha-amylase family of enzymes is based on a two-step acid catalytic mechanism that requires two critical carboxylates: one acting as a general acid/base (Glu) and the other as a nucleophile (Asp). Both hydrolysis and transglycosylation proceed via the nucleophilic substitution reaction between the anomeric carbon, C1 and a nucleophile. 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.",