Phosphofructokinase, in glycolysis

Allosteric interactions. The flow of molecules in most metabolic pathways is determined primarily by the activities of certain enzymes rather than by the amount of substrate available. Enzymes that catalyze essentially irreversible reactions are likely control sites, and the first irreversible reaction in a pathway (the committed step) is nearly always tightly controlled. Enzymes catalyzing committed steps are allosterically regulated, as exemplified by phosphofructokinase in glycolysis and acetyl CoA carboxylase in fatty acid synthesis. Allosteric interactions enable such enzymes to rapidly detect diverse signals and to adjust their activity accordingly. 

The accumulation of lactate is often cited as a cause of fatigue during intense muscle contraction but the effect is actually due to a low pH in muscles. Protons dissociate in a reversible manner from lactic acid, glycolytic intermediates, and the purine nucleotides, thus contributing to a net increase of protons in solution. A marked drop in pH is not conducive to efficient operation of the contractile machinery, as well as inhibiting hexokinase and phosphofructokinase in glycolysis. 

The reaction catalysed by phosphofructokinase in glycolysis, the phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate (see Figure 5.10), is essentially... 

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