Kinetic analysis of effectors of phosphoenolpyruvate carboxylase from Bryophyllum fedtschenkoi.

The activity of phosphoenolpyruvate carboxylase (orthophosphate:oxaloacetate carboxy-lyase (phosporylating) EC 4.1.1.31) purified from Bryophyllum fedtschenkoi has been measured in the presence of various concentrations of phosphoenolpyruvate, L-malate and glucose 6-phosphate. At high pH, the enzyme is competitively inhibited by L-malate and activated by glucose 6-phosphate. A reaction scheme describing the interaction of enzyme, substrate and effectors is proposed. Values for the appropriate equilibrium constants have been calculated for the enzyme acting at pH 7.8, which is one of its two pH optima. The kinetics are more complicated at low pH, partly because of non-linear reaction rates and partly because inhibition by L-malate is not competitive. Activation by glucose 6-phosphate is similar at high and low pH values. The behaviour of a wide range of other possible effectors is described briefly.