Formation of pyruvate producing ATP The conversion of PEP to pyruvate is catalyzed by pyruvate kinase , the third irreversible reaction of glycolysis. The equilibrium of the pyru- vate kinase reaction favors the formation of ATP (see Figure 8.18). [Note: This is another example of substrate-level phosphorylation.] 1. Feed-forward regulation: In liver, pyruvate kinase is activated by fructose 1,6-bisphosphate, the product of the phosphofructo kinase reaction. This feed-forward (instead of the more usual feedback) regulation has the effect of linking the two kinase activities: increased phosphofructokinase activity results in elevated levels of fructose 1,6-bisphosphate, which activates pyruvate kinase . 2. Covalent modulation of pyruvate kinase: Phosphorylation by a cAMP-dependent protein kinase leads to inactivation of pyruvate kinase in the liver (Figure 8.19). When blood glucose levels are low, elevated glucagon increases the intracellular level of cAMP, which causes the phosphorylation and inactivation of pyruvate kinase . Therefore, PEP is unable to continue in glycoly- sis, but instead enters the gluconeogenesis pathway. This, in part, explains the observed inhibition of hepatic glycolysis and stimulation of gluconeogenesis by glucagon. Dephos phorylation of pyruvate kinase by a phosphoprotein phosphatase results in re activation of the enzyme. 3. Pyruvate kinase deficiency: The normal, mature erythrocyte lacks mitochondria and is, therefore, completely dependent on glycoly- sis for production of ATP. This high-energy compound is required to meet the metabolic needs of the red blood cell, and also to fuel the pumps necessary for the maintenance of the biconcave, flexi-