Regulation of Maize (Zea mays L.) Phosphoenolpyruvate Carboxylase Isoenzymes by pH

Abstract

Summary The activity of phosphoenolpyruvate carboxylase (PEPC) isoenzymes from maize leaves is regulated by pH. The S0.5 values for phosphoenolpyruvate (PEP) and Mg2+ ions decrease for both isoenzymes at pH below 7.5 and above 8.5. The Hill coefficient values for PEP are about 1.0 at pH above 8.0 for both isoenzymes, however, they increase to values higher than 1.0 with the decrease of pH below 7.5 (mainly with the PEPC I isoenzyme). The Hill coefficient values for Mg 2+ are pH-independent. The pK values of the amino acid groups present in the active site of the protein molecule of both PEPC isoenzymes are determined from the dependence of the enzyme's activity on the concentration of hydrogen ions. The pK values of about 7.3, 8.5 and 9.0 indicate that histidine, cysteine and lysine residues, respectively, are present in the active centre of both isoenzymes.