 In glycolysis , hexokinase is directly inhibited by its product , glucose @-@ 6 @-@ phosphate , and pyruvate kinase is inhibited by ATP itself . The main control point for the glycolytic pathway is phosphofructokinase ( PFK ) , which is allosterically inhibited by high concentrations of ATP and activated by high concentrations of AMP . The inhibition of PFK by ATP is unusual , since ATP is also a substrate in the reaction catalyzed by PFK ; the biologically active form of the enzyme is a tetramer that exists in two possible conformations , only one of which binds the second substrate fructose @-@ 6 @-@ phosphate ( F6P ) . The protein has two binding sites for ATP – the active site is accessible in either protein conformation , but ATP binding to the inhibitor site stabilizes the conformation that binds F6P poorly . A number of other small molecules can compensate for the ATP @-@ induced shift in equilibrium conformation and reactivate PFK , including cyclic AMP , ammonium ions , inorganic phosphate , and fructose @-@ 1,6- and -2,6 @-@ biphosphate . 
