The kinetic properties of phosphoenolpyruvate carboxykinase [ATP: oxaloacetate carboxy-lyase (transphosphorylating), EC 220.127.116.11] from leaves of the C4 grass Panicum maximum have been re-evaluated due to conflicting reports concerning pH optima, Mn2+ requirement of activity and Km for different substrates among C4 and CAM plants. The native enzyme has a Mr of 360 000 and a strict requirement of Mn2+ for activity. The optimum pH for the carboxylation reaction is 6.8 and the Km's for PEP, ADP and HCO3 - are 1.46 mM, 60 μM and 3.7 mM, respectively; for the decarboxylation process the pH optimum is 8.4 and the respective Km for ATP and oxaloacetate are 21 and 61 μM. At pH 7.6 the maximum rate of the decarboxylation reaction is three-fold higher than that of the carboxylation process. Steady-state kinetic studies of the decarboxylation reaction under optimal conditions and the evaluation of product inhibition effects show that the kinetic mechanism of this enzyme is a sequential ordered bi-tri process, ATP being the first substrate entering the active site, followed by oxaloacetate, while CO2 is the first substrate to emerge, followed by PEP and ADP.
- C4 metabolism
- kinetic mechanism.
- Panicum maximum
- phosphoenolpyruvate carboxykinase