N-terminal putative catalytic domain of mainly uncharacterized prokaryotic proteins similar to archaeal thermoactive amylopullulanases; glycoside hydrolase family 57 (GH57). This subfamily of mainly uncharacterized bacterial proteins, shows high sequence homology to GH57 archaeal thermoactive amylopullulanases (APU, E.C 3.2.1.1/41). Thermoactive APUs are type II pullulanases with both pullulanolytic and amylolytic activities. They have an acid pH optimum and the presence of Ca2+ might increase their activity, thermostability, and substrate affinity.

Domain of unknown function (DUF3536). This presumed domain is functionally uncharacterized. This domain is found in bacteria and archaea. This domain is typically between 274 to 285 amino acids in length. This domain is found associated with pfam03065.

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.

Glycosyl hydrolase family 57. This family includes alpha-amylase (EC:3.2.1.1), 4--glucanotransferase (EC:2.4.1.-) and amylopullulanase enzymes.

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.