Abstract
Phosphoenolpyruvate carboxykinase (GTP/oxaloacetate-carboxylyase (transphosphorylating) EC 4. 1. 1. 32), a key enzyme in gluconeogenesis, is found inboth the cytosol and mitochondrial compartments of the liver, although the distribution of the enzyme in the two cell compartments depends upon the species. The cytosol enzyme activity in bullfrog liver shows seasonal variation, but the mitochondrial enzyme activity does not.
The cytosol phosphoenolpyruvate carboxykinase has been purified to electrophoretical homogeneity from bullfrog liver homogenate. The enzyme is a single polypeptide chain of molecular weight approximately 72, 000-75, 000. The purified enzyme catalyzes pyruvate formation from oxaloacetate, oxaloacetate decarboxylation (nucleoside triphosphate supported), phosphoenolpyruvate carboxylation and an exchange reaction between oxaloacetate and [14C] HCO3- in the presence of ITP or GTP.
Manganese ion is absolutely required for the enzyme catalyzed phosphoenolpyruvate carboxylation, whereas it can be replaced by Mg2+ for the oxaloacetate decarboxylation and the exchange reaction. The optimal pH of each reaction is dependent on the divalent metal ion used. The Km value for Mn2+ is markedly different for the phosphoenolpyruvate carboxylation and oxaloacetate decarboxylation reaction.
