Pig liver pyruvate carboxylase. The reaction pathway for the carboxylation of pyruvate

Abstract

The reaction pathway for the carboxylation of pyruvate, catalysed by pig liver pyruvate carboxylase, was studied in the presence of saturating concentrations of K+ and acetyl CoA. Free Mg2+ binds to the enzyme in an equilibrium fashion and remains bound during all further catalytic cycles. MgATP2- binds next, followed by HCO3- and then pyruvate. Oxaloacetate is released before the random release, at equilibrium of P(i) and MgADP-. This reaction pathway is compared with the double displacement (Ping Pong) mechanisms that have previously been described for pyruvate carboxylases from other sources. The reaction pathway proposed for the pig liver enzyme is superior in that it shows no kinetic inconsistencies and satisfactorily explains the low rate of the ATP⇆[32P]P(i) equilibrium exchange reaction. Values are presented for the stability constants of the magnesium complexes of ATP, ADP, acetyl CoA, P(i), pyruvate and oxaloacetate.