N-terminal catalytic domain of thermoactive amylopullulanases; glycoside hydrolase family 57 (GH57). Pullulanases (EC 3.2.1.41) are capable of hydrolyzing the alpha-1,6 glucosidic bonds of pullulan, producing maltotriose. Amylopullulanases (APU, E.C 3.2.1.1/41) are type II pullulanases which can also degrade both the alpha-1,6 and alpha-1,4 glucosidic bonds of starch, producing oligosaccharides. This subfamily includes GH57 archaeal thermoactive APUs, which show both pullulanolytic and amylolytic activities. They have an acid pH optimum and the presence of Ca2+ might increase their activity, thermostability, and substrate affinity. Besides GH57 thermoactive APUs, all mesophilic and some thermoactive APUs belong to glycoside hydrolase family 13 with catalytic features distinct from GH57. This subfamily also includes many uncharacterized proteins found in bacteria and archaea.
N-terminal catalytic domain of heat stable retaining glycoside hydrolase family 57. Glycoside hydrolase family 57(GH57) is a chiefly prokaryotic family with the majority of thermostable enzymes coming from extremophiles (many of these are archaeal hyperthermophiles), which exhibit the enzyme specificities of alpha-amylase (EC 3.2.1.1), 4-alpha-glucanotransferase (EC 2.4.1.25), amylopullulanase (EC 3.2.1.1/41), and alpha-galactosidase (EC 3.2.1.22). This family also includes many hypothetical proteins with uncharacterized activity and specificity. GH57s cleave alpha-glycosidic bonds by employing a retaining mechanism, which involves a glycosyl-enzyme intermediate, allowing transglycosylation.
N-terminal catalytic domain of alpha-amylase ( AmyC ) and similar proteins. Alpha-amylases (alpha-1,4-glucan-4-glucanohydrolases, EC 3.2.1.1) play essential roles in alpha-glucan metabolism by catalyzing the hydrolysis of polysaccharides such as amylose starch, and beta-limit dextrin. This subfamily is represented by a novel alpha-amylase (AmyC) encoded by hyperthermophilic organism Thermotoga maritime ORF tm1438, and its prokaryotic homologs. AmyC functions as a homotetramer and shows thermostable amylolytic activity. It is strongly inhibited by acarbose. AmyC is composed of a N-terminal catalytic domain, containing a distorted TIM-barrel structure with a characteristic (beta/alpha)7 fold motif, and two additional less conserved domains. There are other two canonical alpha-amylases encoded from T. maritime that lack the sequence similarity to AmyC, and belong to a different superfamily.