The Mechanism of End Product Inhibition of Serine Biosynthesis

Phosphoglycerate dehydrogenase was shown to consist of four identical subunits. This conclusion was based on data obtained from disc electrophoresis of extensively reduced and [14C]carboxymethylated enzyme, tryptophan determinations, and peptide maps. These studies eliminated the possibility that the enzyme contained regulatory subunits. The enzyme was crystallized with both oxidized and reduced coenzyme bound to the protein. Kinetic studies of phosphoglycerate oxidation have been made possible by substitution of 3-acetylpyridine DPN (3-AcPy-DPN) for DPN in the assay system. The enzyme gave biphasic kinetic and derived Hill plots for phosphoglycerate oxidation at 25°, indicative of negative cooperative interactions. At low phosphoglycerate concentrations, serine was a noncompetitive inhibitor. Subunit interactions mediated by phosphoglycerate and serine were influenced by temperature and pH.