Does the bifunctional uridylate synthase channel orotidine 5'-phosphate? Kinetics of orotate phosphoribosyltransferase and orotidylate decarboxylase activities fit a noninteracting sites model

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Abstract
Uridylate synthase is a bifunctional protein that first forms orotidine 5''-phosphate (OMP) from orotate via its orotate phosphoribosyltransferase activity (EC 2.4.2.10) and then converts OMP to uridine 5''-phosphate (UMP) via the OMP decarboxylase activity (EC 4.1.1.23). A computer modeling analysis of the experiments that led to the proposal [Traut, T. W., and Jones, M. E. (1977) J. Biol. Chem. 252, 8374-8381] that uridylate synthase channels intermediate OMP suggests that the experimental results do not demonstrate preferential use of OMP generated in the bifunctional complex as against exogenous OMP. This analysis shows that the experimentally observed amounts of [6-14C]UMP from [6-14C]orotate in the presence of various amounts of exogenous [7-14C]OMP agree well with the amounts predicted by the computer simulations. Thus we conclude that uridylate synthase does not channel OMP. Additionally, the subsequent suggestion that channeling of OMP occurs to protect the intermediate from degradation by a nucleotidase [Traut, T. W., (1980) Arch. Biochem. Biophys. 200, 590-594] seems unlikely. The appropriate computer simulation demonstrates that low transient levels of OMP and protection of the intermediate are provided for strictly by the kinetic parameters of orotate phosphoribosyltransferase, OMP decarboxylase, and the nucleotidase. Additionally, calculations show that, in both sets of published experiments, the concentration of transient OMP greatly exceeded the concentration of OMP decarboxylase active sites. Thus, channeling of OMP by the bifunctional complex cannot be involved to explain the evolution of uridylate synthase, and that event must be the result of some other selective pressure.