HYDROLYSIS OF PHOSPHOENOLPYRUVATE CATALYZED BY PHOSPHOENOLPYRUVATE CARBOXYLASE FROM ZEA-MAYS

In addition to the normal carboxylation reaction, phosphoenolpyruvate carboxylase from Zea mays catalyzes a HCO3--dependent hydrolysis of phosphoenolpyruvate to pyruvate and P(i). Two independent methods were used to establish this reaction. First, the formation of pyruvate was coupled to lactate dehydrogenase in assay solutions containing high concentrations of L-glutamate and aspartate amino-transferase. Under these conditions, oxalacetic acid produced in the carboxylation reaction was efficiently transaminated, and decarboxylation to form spurious pyruvate was negligible. Second, sequential reduction of oxalacetate and pyruvate was achieved by initially running the reaction in the presence of malate dehydrogenase with NADH in excess over phosphoenolpyruvate. After the reaction was complete, lactate dehydrogenase was added, thus giving a measure of pyruvate concentration. At pH 8.0 in the presence of Mg2+, the rate of phosphoenolpyruvate hydrolysis was 3-7% of the total reaction rate. The hydrolysis reaction catalyzed by phosphoenolpyruvate carboxylase was strongly metal dependent, with rates decreasing in the order Ni2+ > Co2+ > Mn2+ > Mg2+ > Ca2+. These results suggest that the active site metal ion binds to the enolate oxygen, thus stabilizing the proposed enolate intermediate. The more stable the enolate, the less reactive it is toward carboxylation and the greater the opportunity for hydrolysis.