Chemical mapping of the active site of the glucoamylase of Aspergillus niger

Abstract

A recently developed technique for the probing of the combining sites of lectins and antibodies, to establish the structure of the epitope that is involved in the binding of an oligosaccharide, is used to study the binding of methyl α-isomaltoside by the enzyme glucoamylase. The procedure involved the determination of the effects on the kinetics of hydrolysis of both monodeoxygenation and mono-O-methylation at each of the seven hydroxyl groups in order to gain an estimate of the differential changes in the free energies of activation. A model has been developed which provides a network of hydrogen bonds that appears to well represent the activated complex formed by the glucoamylase with both maltose and isomaltose because the structures appear to provide a sound rationale for both the specificity and catalysis provided by the enzyme.