Kinetic properties of phosphoenolpyruvate carboxylase from lupin nodules and roots

The kinetic properties of two forms of phosphoenolpyruvate carboxylase (PEPC I and PEPC II, EC 4.1, 1.31) from lupin ( Lupinus luteus L. cv. Ventus) nodules and one enzyme form (PEPC III) from roots were studied. The Michaelis constant (K m ) values for PEP, Mg 2+ and especially HCO 3 − were lower for PEPC I. Kinetic studies showed that aspartate is a competitive inhibitor at pH 7.2 and inhibitor constant (K i ) values are different for the three forms of PEPC. Malate is a competitive inhibitor for PEPC I and PEPC III and shows mixed‐type inhibition for PEPC II. Malate inhibition is dependent upon the pH of the assay. Different effect of several metabolites was also observed. The temperature optimum was near 39°C for PEPC I and around 43°C for PEPC II and PEPC III. PEPC I appeared to be the most thermolabile. It is suggested that PEPC I from lupin nodules is closely associated with N 2 fixation.