Nucleophilic Competition in Some β‐Galactosidase‐Catalyzed Reactions

Abstract

A study of nucleophilic competition in β‐galactosidase‐catalyzed reactions has been performed with different substrates: o, m and p ‐nitrophenyl β‐ d ‐galactosides, phenyl β‐ d ‐galactoside, o ‐aminophenyl, β‐ d ‐galactoside, p ‐aminophenyl, β‐ d ‐galactoside, o ‐nitrophenyl α‐ l ‐arabinoside. For each substrate the limiting step and the individual rate constants have been determined. The rate of formation of the galactosyl‐enzyme intermediate, k 2 , does not correlate with the Hammett values, suggesting that this step remains more complex than a single chemical reaction. A preliminary investigation of the effect of pH and Mg 2+ concentration on each individual rate constant is presented. Both k 2 and k 3 seem to exhibit the same pH profile. The action of various nucleophilic compounds on the galactosyl‐enzyme has been tested. A peculiar effect of 2‐mercaptoethanol has to be emphasized; the behaviour of this nucleophile has been analyzed and discussed.