Phosphofructokinase catalyzes the first irreversible step in glycolysis after the paths from glucose and glycogen converge (Fig. 13.3). The enzyme is allosterically regulated by ATP (Fig. 13.14). When ATP concentrations are high, ATP binds to regulatory sites on phosphofructokinase and locks it into an inactive (low affinity for fructose phosphate) conformation. When the concentration of ATP is reduced, levels of AMP increase (because ADP is converted to ATP and AMP, page 259). AMP will compete with ATP so an increasing number of phosphofructokinase molecules come to have AMP in the regulatory sites, which causes the enzyme to switch to the active, high-affinity conformation. Fructose-1,6-bisphosphate is produced, feeding the glycolytic pathway that in turn feeds the mitochondria with pyru-vate for the production of ATP. This process is negative feedback because changes in the concentration of ATP act, through its allosteric action on phosphofructokinase, to reverse the change in ATP concentration.
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