The influence of the substrates fructose 6-phosphate and ATP and the effect of potassium, ammonium and magnesium ions on the phosphofructokinase from rat erythrocytes have been investigated. The results are expressed in terms of the Monod-Wyman-Changeux model. Potassium and ammonium ions are equally bound to the active R-state and the inactive T-state (type-I binding site) the allosteric equilibrium between R and T is not affected. Both ions compete for this site, the dissociation constants being 0.2 mM for ammonium and 10 mM for potassium. Ammonium ions additionally occupy a specific site (type-I1 binding site, dissociation constant 10 mM) of the R-state thus shifting the sigmoidal fructose 6-phosphate saturation curve to the left. Magnesium does not activate the phosphofructokinase. It removes the inhibition by uncomplexed ATP which is strongly bound to the T-state
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