Complex allosteric regulatory interactions control the kinetic behaviour of phosphoenolpyruvate carboxylase (PEPC) in E. coli.
(A) PEPC regulates anaplerotic metabolism in E. coli through several allosteric interactions. This enzyme carries out the conversion of phosphoenolpyruvate and bicarbonate into oxaloacetate and inorganic phosphate. All reactants are highlighted in black. The red and green dashed lines denote the inhibition and activation exerted by different effectors, respectively. (B) Ordered Bi-Bi mechanism for PEPC catalysis. Phosphoenolpyruvate, bicarbonate, oxaloacetate and orthophosphate are denoted by the abbreviations pep, hco3−, oaa and pi, respectively. (C) Mechanism of synergistic activation of PEPC transitions proposed by Smith et al. . This mechanism considers two separate binding sites for the two types of activators (allosteric sites) and another for the substrates (catalytic site), each of them being capable of independently interacting with different enzymatic complexes. Notably, this model assumes the existence of the relaxed enzyme (active) only in the presence of activators, one of which may be the substrate pep.