Oxamate Is an Alternative Substrate for Pyruvate Carboxylase from Rhizobium etli

Oxamate, an isosteric and isoelectronic inhibitory analogue of pyruvate, enhances the rate of enzymatic decarboxylation of oxaloacetate in the carboxyl transferase domain of pyruvate carboxylase (PC). It is unclear, though, how oxamate exerts a stimulatory effect on the enzymatic reaction. Herein, we report direct 13C nuclear magnetic resonance (NMR) evidence that oxamate acts as a carboxyl acceptor, forming a carbamylated oxamate product and thereby accelerating the enzymatic decarboxylation reaction. 13C NMR was used to monitor the PC-catalyzed formation of [4-13C]oxaloacetate and subsequent transfer of 13CO2 from oxaloacetate to oxamate. In the presence of oxamate, the apparent Km for oxaloacetate is artificially suppressed (from 15 to 4–5 μM). Interestingly, the steady-state kinetic analysis of the initial rates determined at varying concentrations of oxaloacetate and fixed concentrations of oxamate revealed initial velocity patterns inconsistent with a simple ping-pong-type mechanism. Rather, the patterns suggest the existence of an alternate decarboxylation pathway in which an unstable intermediate is formed. The steady-state kinetic analysis coupled with the normal 13(V/K) kinetic isotope effect observed on C-4 of oxaloacetate [13(V/K) = 1.0117 ± 0.0005] indicates that the transfer of CO2 from carboxybiotin to oxamate is the partially rate-limiting step of the enzymatic reaction. The catalytic mechanism proposed for the carboxylation of oxamate is similar to that proposed for the carboxylation of pyruvate, which occurs via the formation of an enol intermediate.