The transient-state kinetics of L-glutamate dehydrogenase. pH-dependence of the burst rate parameters

The pH dependence of the initial transient velocity of NADPH production during the burst phase of the oxidative deamination of L-glutamate by L-glutamate dehydrogenase (L-glutamate : NAD(P)+ oxidoreductase (deaminating), EC 1.4.1.3) and NADP+ has been measured by stopped-flow spectrophotometry. These studies provide evidence that the entire pH dependence below pH 8.26 arises from reaction steps contributing to V of the burst with an apparent pKa of 8.1 +/- 0.1. The data are consistent with a model in which the formation of the first enzyme-coenzyme-substrate ternary complex on the reaction path equilibrates rapidly and in which the pH-dependent steps are mechanistically close to and may include the catalytic hydrogen transfer itself. At pH 8.87, there is evidence that L-glutamate binds less tightly to the enzyme and to the enzyme-NADP+ complex than at lower pH values.