Concerted Regulation in Vitro of Phosphoenolpyruvate Carboxylase from Escherichia coli

Fructose 1,6-diphosphate, in low concentration, activates phosphoenolpyruvate carboxylase from Escherichia coli synergistically, with either acetyl coenzyme A or laurate. The same combinations, in high concentration, do not show this effect, but synergistically relieve aspartate inhibition. Acetyl-CoA and laurate, together, show neither effect. These findings are consistent with a changing need for the enzyme in response to metabolic conditions. Also reported are the kinetic parameters of these activators, corrected for nonactivated velocities. Maximum activations by laurate and acetyl-CoA, between pH 7.0 and 8.5, are about the same in magnitude and independent of pH. Activation by acetyl-CoA shows positive cooperativity that is quantitatively independent of pH. Laurate curves are not cooperative. Fructose 1,6-diphosphate shows positive cooperativity at pH 7.0, but not at pH 7.3 or above. Maximum activation velocity by fructose 1,6-diphosphate decreases at higher pH. Aspartate is a competitive inhibitor to each of the three activators, with positive cooperativity profiles obtained in each instance.