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.

Alpha-amylase/alpha-mannosidase, GH57 family [Carbohydrate transport and metabolism].

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 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 4-alpha-glucanotransferase; glycoside hydrolase family 57 (GH57). 4-alpha-glucanotransferase (TLGT, EC 2.4.1.25) plays a key role in the maltose metabolism. It catalyzes the disproportionation of amylose and the formation of large cyclic alpha-1,4-glucan (cycloamylose) from linear amylose. TLGT functions as a homodimer. Each monomer is composed of two domains, an N-terminal catalytic domain with a (beta/alpha)7 barrel fold and a C-terminal domain with a twisted beta-sandwich fold. Some family members have been designated as alpha-amylases, such as the heat-stable eubacterial amylase from Dictyoglomus thermophilum (DtAmyA) and the extremely thermostable archaeal amylase from Pyrococcus furiosus(PfAmyA). However, both of these proteins are 4-alpha-glucanotransferases. DtAmyA was shown to have transglycosylating activity and PfAmyA exhibits 4-alpha-glucanotransferase activity.