Glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides: revised kinetic mechanism and kinetics of ATP inhibition.

The kinetic mechanisms of the NAD- and NADP-linked reactions catalyzed by glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides were examined using product inhibition, dead-end inhibition and alternate substrate experiments. The results are consistent with a steady-state random mechanism for the NAD-linked and an ordered, sequential mechanism with NADP+ binding first for the NADP-linked reaction. Thus, the enzyme can bind NADP+, NAD+, and glucose 6-phosphate, but the enzyme-glucose 6-phosphate complex can react only with NAD+, not with NADP+. This affects the rate equation for the NADP-linked reaction by introducing a term for a dead-end enzyme-glucose 6-phosphate complex. The kinetic mechanisms represent revisions of those proposed previously (C. Olive, M.E. Geroch, and H.R. Levy, 1971, J. Biol. Chem. 246, 2047-2057) and provide a kinetic basis for the regulation of coenzyme utilization of the enzyme by glucose 6-phosphate concentration (H.R. Levy, and G.H. Daouk, 1979, J. Biol. Chem. 254, 4843-4847) and NADPH/NADP+ concentration ratios (H.R. Levy, G.H. Daouk, and M.A. Katopes, 1979, Arch, Biochem. Biophys. 198, 406-413). The kinetic mechanisms were found to be the same at pH 6.2 and pH 7.8. The kinetics of ATP inhibition of the NAD- and NADP-linked reactions were examined at pH 6.2 and pH 7.8. The results are interpreted in terms of ATP addition to binary enzyme-coenzyme and enzyme-glucose 6-phosphate complexes.