Activation of phosphofructokinase by fructose 2,6-bisphosphate. (A) The sigmoidal dependence of velocity on substrate concentration becomes hyperbolic in the presence of 1 μM 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 reversed by fructose 2,6-bisphosphate.

[Data from E. Van Schaftingen, M. F. Jett, L. Hue, and H. G. Hers, Proc. Natl. Acad. Sci. U. S. A. 78:3483–3486, 1981.]