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Rat liver pyruvate carboxylase. II. Kinetic studies of the forward reaction.

Abstract The kinetics of the pyruvate carboxylase reaction have been examined at saturating concentrations of acetyl-CoA, Mg++, and K+. The results of substrate kinetics and inhibition studies suggest a two-site "ping-pong" mechanism in which the enzyme-biotin-CO2 intermediate is formed at one site with the hydrolysis of MgATP. Because of its attachment to the enzyme by a carbon chain, the biotin-CO2 can swing to the second site where pyruvate is carboxylated to form oxalacetate. However, the initial velocity patterns indicate that the affinity for bicarbonate at the first site is dramatically increased by higher levels of pyruvate without any effect on the maximum velocity. The two-site mechanism was confirmed by inhibition patterns which indicated that competitive inhibitors of MgATP had only a small uncompetitive effect when pyruvate was varied at high MgATP. Similarly a competitive inhibitor of pyruvate had only a negligible effect on MgATP saturation at its site.