The enzyme phosphofructokinase (PFK) from E. coli is a tetrameric enzyme that allosterically binds AMP. The binding of AMP increases the affinity of the substrates for the active site of PFK. Which of the following is true concerning this system?

A. AMP is a competitive inhibitor. B. AMP is an allosteric activator and will stabilize PFK in a conformation that will require less substrate to achieve 1/2 Vmax. C. AMP is an allosteric inhibitor and will stabilize PFK in a conformation that will require more substrate to achieve 1/2 Vmax. D. AMP is a cofactor/coenzyme for PFK and is absolutely required for PFK activity.