Deciphering the Molecular Basis of the Broad Substrate Specificity of α-Glucosidase from Bacillus sp. SAM1606

The α-glucosidase of Bacillus sp. strain SAM1606 is a member of glycosyl hydrolase family 13, and shows an extraordinarily broad substrate specificity and is one of very few α-glucosidases that can efficiently hydrolyze the α-1,1-glucosidic linkage of α,α′ -trehalose (trehalose). Phylogenetic analysis of family-13 enzymes suggests that SAM1606 α-glucosidase may be evolutionally derived from an α-1,6-specific ancestor, oligo-1,6-glucosidase (O16G). Indeed, replacement of Pro^273* and Thr^342* of B. cereus O16G by glycine and asparagine (the corresponding residues in the SAM1606 enzyme), respectively, was found to cause 192-fold enhancement of the relative catalytic efficiency for trehalose, suggesting that O16G may easily "evolved" into an enzyme with an extended substrate specificity by substitution of a limited number of amino acids, including that at position 273* (an asterisk indicates the amino-acid numbering of the SAM1606 sequence). To probe the role of the amino acid at position 273* of α-glucosidase in determination of the substrate specificity, the amino acid at position 273 of SAM1606 α-glucosidase was replaced by all other naturally occurring amino acids, and the resultant mutants were kinetically characterized. The results showed that substitution of bulky residues (e.g., isoleucine and methionine) for glycine at this position resulted in large increases in the K_m values for trehalose and maltose, whereas the affinity to isomaltose was only minimally affected by such an amino-acid substitution at this position. Three-dimensional structural models of the enzyme-substrate complexes of the wild-type and mutant SAM1606 α-glucosidases were built to explore the mechanism responsible for these observations. It is proposed that substitution by glycine at position 273* could eliminate steric hindrance around subsite +1 that originally occurred in parental O16G and is, at least in part, responsible for the acquired broad substrate specificity of SAM1606 α-glucosidase.