ResolvingApparent Conflicts between Theoretical andExperimental Models of Phosphate Monoester Hydrolysis

摘要

Understanding phosphoryl and sulfuryl transfer is central to many biochemical processes. However, despite decades of experimental and computational studies, a consensus concerning the precise mechanistic details of these reactions has yet to be reached. In this work we perform a detailed comparative theoretical study of the hydrolysis of p-nitrophenyl phosphate, methyl phosphate and p-nitrophenyl sulfate, all of which have served as key model systems for understanding phosphoryl and sulfuryl transfer reactions, respectively. We demonstrate the existence of energetically similar but mechanistically distinct possibilities for phosphate monoester hydrolysis. The calculated kinetic isotope effects for p-nitrophenyl phosphate provide a means to discriminate between substrate- and solvent-assisted pathways of phosphate monoester hydrolysis, and show that the solvent-assisted pathway dominates in solution. This preferred mechanism for p-nitrophenyl phosphate hydrolysis is difficult to find computationally due to the limitations of compressing multiple bonding changes onto a 2-dimensional energy surface. This problem is compounded by the need to include implicit solvation toat least microsolvate the system and stabilize the highly chargedspecies. In contrast, methyl phosphate hydrolysis shows a preferencefor a substrate-assisted mechanism. For p-nitrophenylsulfate hydrolysis there is only one viable reaction pathway, whichis similar to the solvent-assisted pathway for phosphate hydrolysis,and the substrate-assisted pathway is not accessible. Overall, ourresults provide a unifying mechanistic framework that is consistentwith the experimentally measured kinetic isotope effects and reconcilesthe discrepancies between theoretical and experimental models forthese biochemically ubiquitous classes of reaction.