Are All Metabolic Reactions at Equilibrium? (a) Phosphoenolpyruvate (PEP) is one of the two phosphoryl group donors in the synthesis of ATP during glycolysis. In human erythrocytes, the steady-state concentration of ATP is 2.24 mm, that of ADP is 0.25 mm, and that of pyruvate is 0.051 mm. Calculate the concentration of PEP at 25 C, assuming that the pyruvate kinase reaction (see Fig. 13-13) is at equilibrium in the cell. (b) The physiological concentration of PEP in human erythrocytes is 0.023 mm. Compare this with the value obtained in (a). Explain the significance of this difference.