Storey, K.B. and Hochachka, P.W. 1974. Enzymes of energy metabolism in a vertebrate facultative anaerobe, Pseudemys scripta: turtle heart phosphofructokinase. J. Biol. Chem. 249, 1417-1422.

Enzymes of Energy Metabolism from a Vertebrate Facultative Anaerobe, Pseudemys scripta


TURTLE HEART PHOSPHOFRUCTOKINASE

K. B. STOREY AND P. W. HOCHACHKA


SUMMARY
Phosphofructokinase (EC 2.7.1.11) occurs in turtle heart at a specific activity of about 19 units per g wet weight; Mg2+ or Mn2+ can satisfy the divalent cation requirement; ATP, GTP, or ITP all can serve as phosphate donors for the reaction. Although the pH optimum for the enzyme is pH 7.5, the intracellular pH of the anoxic turtle heart, pH 6.8, was chosen for characterizing the enzyme. At this pH, turtle heart phosphofructokinase displays hyperbolic saturation kinetics with respect to both substrates. The Michaelis constant (Km) for fructose-6-P is 0.034 mM with Mn2+ as the divalent cation and is 0.06 mM with Mg2+ as the cation cofactor. The maximum velocity in the presence of Mn2+ is only 75% that obtained with Mg2+. In the presence of citrate, a potent inhibitor of the enzyme, fructose-6-P saturation curves become sigmoidal, but the effects of citrate on the apparent enzyme-substrate affinity and upon the maximum velocity can be reversed with adenosine 3',5'monophosphate (cyclic AMP), AMP, or ADP. High concentrations of ATP also inhibit the enzyme, 8.5 mM ATP reducing the maximum velocity by half. The effects of ATP could be reversed with cyclic AMP, AMP, ADP, and fructose-l ,6-P2. Of other compounds tested, only creatine phosphate, within the expected physiological concentration range, was found to potently inhibit turtle heart phosphofructokinase. At 1 mM concentrations of creatine phosphate, substrate saturation curves become highly sigmoidal with a large reduction in enzyme-substrate affinity. All creatine phosphate effects can be reversed by fructose-1,6-P2, 3-phosphoglycerate, AMP, and cyclic AMP.