Oxidative decarboxylation of 6-phosphogluconate by 6-phosphogluconate dehydrogenase proceeds by a stepwise mechanism with NADP and APADP as oxidants

Primary kinetic deuterium, C-13, and multiple deuterium/C-13-isotope effects on V/K-6PG have been measured for the Candida utilis (cu) and sheep liver (sl) 6-phosphogluconate dehydrogenases (6PGDH). With NADP as the dinucleotide substrate, the following values of (D)(V/K-6PG), (13)(V/K-6PG)(H), and (13)(V/K-6PG)(D) were measured at pH 8 for cu6PGDH (sl6PGDH): 1.57 +/- 0.08 (1.87 +/- 0.10), 1.0209 +/- 0.0005 (1.0059 +/- 0.000 10), 1.0158 +/- 0.0001 (1.0036 +/- 0.0008). With APADP as the dinucleotide substrate, values for the above isotope effects at pH 8 are as follows: 2.98 +/- 0.08 (2.47 +/- 0.06), 1.0106 +/- 0.0002 (1.0086 +/- 0.000 09), and 0.9934 +/- 0.0003 (0.9950 +/- 0.0003). Results indicate the oxidative decarboxylation of 6PG to the 1,2-enediol of ribulose 5-phosphate proceeds via a stepwise mechanism with hydride transfer preceding decarboxylation in all cases. The inverse C-13-isotope effect observed with APADP and 6PG-3d may reflect a preequlibrium isotope effect on the binding of 6PG preceding hydride transfer. Deuterium-isotope effects on V, V/K-NADP, and V/K-6PG are identical at all pHs and for both enzymes. The primary deuterium-isotope effect on V/K-6PG for both enzymes is constant at pH values below the pK in the pH profile for V/K-6PG, and decreases as the pH increases. Data suggest the development of rate limitation by a step or steps other than the hydride-transfer step as the pH is increased.