Hysteretic behaviour of phosphoenolpyruvate carboxylase from Panicum maximum

Phospoenolpyruvate carboxylase isolated from Panicum maximum Jacq. leaf presented a hysteretic behcaviour that resulted in a kinetical lag in the reaction progress curve.  This lag depended on the concentration of enzyme, total phospoenolpyruvate (PEP) and total Mg²⁺.  Through an analysis of these dependencies it is suggested that the hysteresis is due to an association-dissociation process, influenced by union, to one form of the enzyme, of more than one of the three possible ligands: free Mg²⁺, free PEP or PEP-Mg²⁺ complex.  The partially purified enzyme showed, during steady state, a Michaelis-Mentin kinetics for PEP and Mg2+ (total concentration) and pH optima between 7.8 and 8.2.