Figure 16.15 The activation of phosphofructokinase by fructose 2,6-bisphosphate. (A) The sigmoidal dependence of velocity on substrate concentration becomes hyperbolic in the presence of 1 mM fructose 2,6-bisphosphate, indicating that more of the enzyme is active at lower substrate concentrations in the presence of fructose 2,6-bisphosphate. (B) ATP, acting as a substrate, initially stimulates the reaction. As the concentration of ATP increases, it acts as an allosteric inhibitor. The inhibitory effect of ATP is reduced by fructose 2,6-bisphosphate, which renders the enzyme less sensitive to ATP inhibition.