Effect of Substituents on the Hydrolysis of Substituted Phenyl β-Acetylglucosaminides by Bovine Liver β-Acetylglucosaminidase

Abstract

Michaelis constant, Km and relative catalytic constant, k^rel_catwere determined for the β-acetylglucosaminidase-catalyzed hydrolysis of a series of substituted phenyl β-acetylglucosaminides ; the substituents are 2, 4-dinitro, p-nitro, 4-nitro-3-methyl, m-nitro, p-chloro, m-chloro, p-methyl, m-methyl, p-methoxy and p-hydroxyl groups. The values were evaluated in terms of the ρ-σ relationship of Hammett. Plots of log Km with respect to Hammett substituent constant, σ and Hansch substituent constant, π showed the enzyme-substrate affinity to be dependent on the electronic nature of the substituents (ρ=-0.42, correlation coefficient γ=0.943) but not on the hydrophobic nature. The log k^rel_cat value was only slightly dependent on σ value (ρ=+0.16, γ=0.626). An experiment on nucleophilic competition with methanol was carried out in an attempt to explain the small ρ value for k^rel_cat. Methanol competes with water for the glycosyl enzyme to some extent but does not increase k^rel_cat for the glycosides examined, indicating that the deglycosylation is not rate-limiting and hence the ρ value for k^rel_cat pertains to the reaction of glycoside bond cleavage.