The specificity of the synthetic reaction of two yeast alpha-glucosidases.

The specificity of the hydrolytic reaction has been compared to that of the synthetic reaction for maltase and isomaltase (alpha-methyl-D-glucosidase) from Saccharomyces oviformis. Maltase which hydrolyzes the alpha-1,4-disaccharide, maltose, and the alpha-1,6-disaccharide, isomaltose, catalyzes the formation of both maltose and isomaltose from free glucose. Isomaltase, which hydrolyzes isomaltose but not maltose, catalyzes the formation only of isomaltose from glucose. Both enzymes hydrolyze p-nitrophenyl-alpha-D-glucoside releasing the alpha-anomer of glucose. The enzymes utilize the alpha-anomer but not the beta-anomer for the synthesis of the disaccharides. These results are consistent with the double displacement mechanism for glycosidases and with the proposal that the glucosyl-enzyme complex is an intermediate in the reaction. The competitive inhibition by D-glucose is independent of its anomeric form for both enzymes.