Przemyslaw G. Czyryca

Transition State of the Sulfuryl Transfer Reaction of Estrogen Sulfotransferase

Kinetic isotope effects have been measured for the estrogen sulfotransferase-catalyzed sulfuryl (SO3) transfer from p-nitrophenyl sulfate to the 5′-phosphoryl group of 3′-phosphoadenosine 5′-phosphate. 18(V/K)nonbridge = 1.0016 ± 0.0005, 18(V/K)bridge = 1.0280 ± 0.0006, and 15(V/K) = 1.0014 ± 0.0004. (15(V/K) refers to the nitro group in p-nitrophenyl sulfate). The kinetic isotope effects indicate substantial SO bond fission in the transition state, with partial charge neutralization of the leaving group. The small kinetic isotope effect in the nonbridging sulfuryl oxygen atoms suggests no significant change in bond orders of these atoms occurs, consistent with modest nucleophilic involvement. A comparison of the data for enzymatic and uncatalyzed sulfuryl transfer reactions suggests that both proceed through very similar transition states.

The mechanism of the phosphoryl transfer catalyzed by Yersinia protein-tyrosine phosphatase: a computational and isotope effect study

In order to evaluate various mechanistic proposals that have been made regarding the mechanism of the first step of the reaction catalyzed by protein-tyrosine phosphatases, new experimental data have been obtained, and some existing data have been carefully reevaluated. New kinetic isotope effect data for the uncatalyzed hydrolysis of p-nitrophenyl phosphate allow a better evaluation of previously reported data from enzymatic reactions with this substrate. The interpretation, and misinterpretation, of pH rate studies is considered. The pathway of this reaction has been modeled computationally and is found to be generally consistent with experimental studies, except for the extent of proton transfer to the leaving group.