Forms oxaloacetate, a four-carbon dicarboxylic acid source for the tricarboxylic acid cycle. oxaloacetate + phosphate = hydrogencarbonate + phosphoenolpyruvate Activity not stimulated by acetyl-CoA in the absence of any allosteric inhibitor, while the corresponding protein from E.coli is strongly stimulated. Belongs to the PEPCase type 1 family.