Amylolytic families of glycoside hydrolases: focus on the family GH-57

The amylolytic and related enzymes have been classified into the families of glycoside hydrolases (GHs). At present there are almost one hundred GH families. The main α-amylase family is the family GH-13 that forms the clan GH-H together with the families GH-70 and GH-77. β-Amylases and glucoamylases have their own families GH-14 and GH15, respectively. Some amylolytic enzymes are grouped also in the family GH-31. The main topic of this review, the family GH-57, was established as the novel α-amylase family in 1996, based on the two sequences of amylases that were obviously dissimilar to the α-amylases of the family GH-13. The family GH-57 contains also the amylopullulanase, α-galactosidase and 4-α-glucanotransferase specificities in addition to the specificity of α-amylase. The family is remarkable by several facts: (i) it consists of only 10% of real enzymes (90% is formed by the putative proteins); (ii) the protein members originate exclusively from prokaryotic sources of extremophilic nature (many of them are archaeal hyperthermophiles); (iii) the amino acid sequences vary from less than 400 to more than 1,300 residues and are extremely diversified. There are five conserved sequence regions characteristic of the family GH-57. The catalytic domain of a GH-57 member is the (β/α)7-barrel with the Glu (nucleophile) and the Asp (proton donor) residues at the β-strands β4 and β7 of the barrel, respectively, employing the retaining mechanism. From the evolutionary point of view, the family GH-57 could be divided into several subfamilies. Importantly, the individual enzyme specificities form their own independent clusters.