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

1. 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. 2. Free Mg(2+) binds to the enzyme in an equilibrium fashion and remains bound during all further catalytic cycles. MgATP(2-) binds next, followed by HCO(3) (-) and then pyruvate. Oxaloacetate is released before the random release, at equilibrium, of P(i) and MgADP(-). 3. 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[unk][(32)P]P(i) equilibrium exchange reaction. 4. Values are presented for the stability constants of the magnesium complexes of ATP, ADP, acetyl-CoA, P(i), pyruvate and oxaloacetate.