In yeast cells, the interaction of yeast pyruvate kinase (YPK) with PEP and its allosteric effector Fructose 1,6-bisphosphate (FBP,) was found to be enhanced by the presence of Mg2+. Therefore, Mg2+ was concluded to be an important cofactor in the catalysis of PEP into pyruvate by pyruvate kinase. Furthermore, the metal ion Mn2+ was shown to have a similar, but stronger effect on YPK than Mg2+. The binding of metal ions to the metal binding sites on pyruvate kinase enhances the rate of this reaction.