Phosphoenolpyruvate carboxykinase from Fasciola hepatica

Behm C. A. and Bryant C. 1982. Phosphoenolpyruvate carboxykinase from Fasciola hepatica. International Journal for Parasitology12: 271–278. The kinetic properties of a partially purified preparation of phosphoenolpyruvate carboxykinase (PEPCK) from F. hepatica were examined. The pH optimum for the carboxylation reaction is 5.8–6.2. The enzyme is more active with Mn²⁺ than Mg²⁺ and the Mn²⁺ saturation curve was sigmoid. Apparent K_m values for the substrates GDP, IDP, PEP and HCO₃^? were determined and found to be in the same range as those reported for other helminths except that the enzyme is less sensitive to low PEP concentrations. GTP and ATP at 0.5 and 1.0 mM inhibit the enzyme; the GTP inhibition was greater in the presence of Mg²⁺ than Mn²⁺ and was competitive with GDP. It was concluded that the activity of PEPCK from F. hepatica is controlled by the concentration of reactants and the ambient pH, that the accumulation of GTP is a sensitive mechanism for inhibiting the carboxylation reaction and that PEPCK activity in the cytosol is likely to be favoured over that of pyruvate kinase except when pH is high and PEP concentration low.