The nature of the transition state for enzyme-catalyzed phosphoryl transfer. Hydrolysis of O-aryl phosphorothioates by alkaline phosphatase.

Abstract

There has been much speculation that enzymes change the nature of the transition state for phosphoryl transfer from the dissociative transition state observed in solution reactions to an associative transition state at the enzyme's active site. This proposal can be tested by comparing linear free energy relationships (LFERs) for nonenzymatic and enzymatic reactions, provided that the specificity of the enzyme's binding site does not perturb the dependence of rate on the intrinsic reactivity of a series of substrates. The shallow binding groove of Escherichia coli alkaline phosphatase (AP) and its wide specificity suggest that this enzyme may be suited for such an approach. A second requirement of this approach is that the actual chemical step is rate-limiting. Comparisons of the reactions of aryl phosphorothioates and aryl phosphates support the previous conclusion that a nonchemical step limits kcat/KM for reactions of aryl phosphates, but suggest that the chemical cleavage step is rate-limiting for the aryl phosphorothioates. We therefore determined the dependence of the rate of AP-catalyzed cleavage of a series of aryl phosphorothioates on the intrinsic reactivity of the substrates. The large negative values of beta leaving group = -0.8 for the enzymatic reaction (kcat/KM) and -1.1 for the nonenzymatic hydrolysis reaction suggest that there is considerable dissociative character in both the enzymatic and nonenzymatic transition states. Despite the wide specificity of AP, certain substrates deviate from the LFER, underscoring that extreme care is required in applying LFERs to enzymatic reactions. The large negative value of beta leaving group suggests that AP can achieve substantial catalysis via a transition state with dissociative character.