Product Deuterium Isotope Effect for Orotidine 5-Monophosphate Decarboxylase: Evidence for the Existence of a Short-Lived Carbanion Intermediate

摘要

We report that equal yields of [6-~1H]-uridine 5'-monophosphate (50%) and [6-~2H]-uridine 5'-monophosphate (50%) are obtained from the decarboxylation of orotidine 5'-monophosphate (OMP) catalyzed by orotidine 5'-monophosphate decarboxylase in a solvent of 50/50 (v/v) H_2O/D_2O. This observation of an unusually small product isotope effect of unity eliminates a proposed mechanism in which proton transfer from Lys-93 to C-6 provides electrophilic push to the loss of CO_2 from OMP in a conceited reaction. It provides evidence that proton transfer from the ammonium cation side chain of Lys-93 to a vinyl carbanion intermediate is faster than the bond rotation that exchanges the positions of the acidic N-L~+ hydrons of this side chain.