Protein engineering in the ?-amylase family: catalytic mechanism, substrate specificity, and stability

Abstract

Most starch hydrolases and related enzymes belong to the α-amylase family which contains a characteristic catalytic (β/α)₈-barrel domain. Currently known primary structures that have sequence similarities represent 18 different specificities, including starch branching enzyme. Crystal structures have been reported in three of these enzyme classes: the α-amylases, the cyclodextrin glucanotransferases, and the oligo-1,6-glucosidases. Throughout the α-amylase family, only eight amino acid residues are invariant, seven at the active site and a glycine in a short turn. However, comparison of three-dimensional models with a multiple sequence alignment suggests that the diversity in specificity arises by variation in substrate binding at the β→α loops. Designed mutations thus have enhanced transferase activity and altered the oligosaccharide product patterns of α-amylases, changed the distribution of α-, β- and γ-cyclodextrin production by cyclodextrin glucanotransferases, and shifted the relative α-1,4:α-1,6 dual-bond specificity of neopullulanase. Barley α-amylase isozyme hybrids and Bacillus α-amylases demonstrate the impact of a small domain B protruding from the (β/α)₈-scaffold on the function and stability. Prospects for rational engineering in this family include important members of plant origin, such as α-amylase, starch branching and debranching enzymes, and amylomaltase.