Fractionation of Isozymes and Determination of the Subsite Structure of Glucoamylase from Rhizopus niveus

Abstract

Four isozymes of glucoamylase were isolated from a commercial preparation of the enzyme from Rhizopus niveus by ion-exchange chromatography on CM-Sephadex C-50 and/or the preparative isoelectric focusing method on Sephadex IEF.
The kinetic parameters, Michaelis constant Km and molecular activity k₀, for the hydrolysis of maltooligosaccharides (degree of polymerization DP from 2 to 7) were determined on the major component enzyme at pH 4.5 and 25°C. It was found that Km decreases and k₀ increases with increasing DP. From the DP-dependences of Km and k₀, the subsite affinities of the seven subsites of the enzyme active site were evaluated on the basis of subsite theory. These values are very similar to those obtained for an unfractionated preparation of Rhizopus delemar glucoamylase [Hiromi et al, B. B. A., 302, 362 (1973)].