Human Phosphoribosylpyrophosphate Synthetase

Human PP-ribose-P synthetase, partially purified from erythrocytes, was found to have Michaelis constants for ribose-5-P, MgATP, and magnesium of 33 µm, 14 µm, and 0.2 mm, respectively. Based on studies of initial velocity and product inhibition, the kinetic mechanism is postulated to be an ordered Bi Bi reaction with ribose-5-P binding first and PP-ribose-P being released last. Magnesium binds sequentially with the substrates, but its exact role is unclear. A large number of compounds including ADP, GMP, GDP, GTP, IDP, ITP, 2,3-DPG, XDP, CDP, CTP, TDP, TTP, NADH, NADPH, and FAD are inhibitors of the enzyme. Inhibition by ADP, which is competitive with respect to ATP, may reflect regulation of the enzyme by cellular energy levels as previously suggested by Atkinson and Fall ((1967) J. Biol. Chem. 242, 3241). Inhibition by 2,3-DPG and PP-ribose-P, which is competitive with respect to ribose-5-P, may provide a link between oxygenation of hemoglobin and regulation of PP-ribose-P synthesis in the erythrocyte. The other inhibitors studied had a high Ki for the enzyme and inhibition appeared to be noncompetitive which is consistent with the hypothesis that PP-ribose-P synthetase activity is also regulated by a mechanism of heterogeneous metabolic pool inhibition.