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 S-O 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.