A conformational change in phosphoglycerate dehydrogenase induced by a shift in pH

The fluorescence of NADH bound to phosphoglycerate dehydrogenase (3-phosphoglycerate: NAD+ oxidoreductase, EC 1.1.1.95) decreased by 42% between pH 8.5 and 7.0 Serine, an allosteric inhibitor, quenched the fluorescence of enzyme-bound NADH by 29% at pH 8.5, but not at all at pH 7.0. The kinetics of the fluorescence change which occurred when the pH of an enzyme-NADH solution was rapidly shifted from 8.5 to 7.0 was measured using stopped-flow fluorimetry. The kinetics were first order, with a rate constant of 2.83 s-1. This rate constant was similar in magnitude to the rate constants for fluorescence quenching at pH 8.5 by saturating concentrations of serine and glycine, another allosteric inhibitor (Dubrow, R. and Pizer, L.I. (1977) J. Biol. Chem. 252, 1527-1538). These results indicate that the conformation of phosphoglycerate dehydrogenase at pH 7.0 is similar to, but not identical with, the serine-induced conformation at pH 8.5.