PHOSPHOFRUCTOKINASE .2. ROLE OF LIGANDS IN PH-DEPENDENT STRUCTURAL-CHANGES OF RABBIT MUSCLE ENZYME

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  • 1 January 1976
    • journal article
    • research article
    • Vol. 251  (18) , 5637-5643
Abstract
The effect of ligands, including substrates and allosteric effectors, on the pH-dependent inactivation and reactivation of rabbit muscle phosphofructokinase was examined in terms of the mechanism proposed previously. Many ligands may exert their effect by binding preferentially to either protonated or unprotonated forms of the enzyme and thus shifting an apparent pK for the inactivation or reactivation process. ATP and fructose 6-phosphate influence the apparent pK to different extents and different directions, with ATP binding preferentially to the protonated forms and fructose 6-phosphate to the unprotonated forms. Enzyme inactivated by ATP can be reactivated by the addition of fructose 6-phosphate. The experiments indicate that inactivation and reactivation in the presence of these ligands can occur by kinetically different pathways as has been found for these processes in the absence of ligands. The results are discussed in relation to what might be expected for ligand binding properties of the enzyme as a function of pH, temperature and enzyme concentration. The effect of ATP and MgATP is complex, perhaps representing more than 1 site of binding. Citrate appears to bind preferentially to protonated forms of the enzyme, while fructose 1,6-bisphosphate and AMP bind preferentially to the unprotonated forms. ADP, K+ and NH4+ appear to have little or no preference in binding to different enzyme forms.