Close evolutionary relatedness among functionally distantly related members of the (α/β)8‐barrel glycosyl hydrolases suggested by the similarity of their fifth conserved sequence region

Abstract

A short conserved sequence equivalent to the fifth conserved sequence region of α‐amylases (173_LPDLD, Aspergillus oryzae α‐amylase) comprising the calcium‐ligand aspartate, Asp‐175, was identified in the amino acid sequences of several members of the family of (α/β)₈‐barrel glycosyl hydrolases. Despite the fact that the aspartate is not invariantly conserved, the stretch can be easily recognised in all sequences to be positioned 26–28 amino acid residues in front of the well‐known catalytic aspartate (Asp‐206, A. oryzae α‐amylase) located in the β4‐strand of the barrel. The identification of this region revealed remarkable similarities between some α‐amylases (those from Bacillus megaterium, Bacillus subtilis and Dictyoglomus thermophilum) on the one hand and several different enzyme specificities (such as oligo‐1,6‐glucosidase, amylomaltase and neopullulanase, respectively) on the other hand. The most interesting example was offered by B. subtilis α‐amylase and potato amylomaltase with the regions LYDWN and LYDWK, respectively. These observations support the idea that all members of the family of glycosyl hydrolases adopting the structure of the α‐amylase‐type (α/β)₈‐barrel are mutually closely related and the strict evolutionary borders separating the individual enzyme specificities can be hardly defined.