Kinetic Studies on Glucoamylase

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Abstract
1) Evidence was obtained that glucoamylase*6 [EC 3.2.1.3] from Rhizopus delemar catalyzes the hydrolysis of phenyl α-glucoside, although the rate is much lower than that for phenyl α-maltoside. 2) The Michaelis constant (Km) and the molecular activity (k0) were determined at pH 4.5 and 25°C for glucoamylase-catalyzed hydrolyses of phenyl α-glucosides with the substituents p-NO2, H, p-CH3, and p-C(CH3)3, and phenyl α-maltosides with p-NO2, H, p-CH3, p-C2H5, and p-C(CH3)3. 3) The effect of substituents upon Km was marked for both the glucosides and the maltosides, and there was a slight effect upon k0 for glucosides. No appreciable effect on k0 was observed for maltosides, as was expected. 4) When the values of Km and k0 for a particular glucoside were compared with those of the corresponding maltoside having the same substituent, Km was about 10 times larger and k0 was 500–1, 000 times smaller for glucosides than for maltosides. 5) The remarkable difference in k0 between the glucosides and maltosides, which cannot be explained on the basis of the chemical nature of the bond to be hydrolyzed, was reasonably accounted for in terms of the statistical weight of productive and nonproductive complexes as determined by the subsite affinity of this enzyme (l).

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