Phosphofructokinase is a four-subunit protein with four active sites. phosphofructokinase catalyzes step 3 of glycolysis, converting fructose-6-phosphate to fructose-1,6-bisphosphate. phosphoenolpyruvate (pep) is the product of step 9 of glycolysis. the pep concentration in the cell affects phosphofructokinase activity. select the true statements about pep regulation of phosphofructokinase. a. pep is a feedback inhibitor of phosphofructokinase. b. the apparent affinity of phosphofructokinase for its substrate increases when pep binds. c. pep is a positive effector of phosphofructokinase. d. pep inhibition of phosphofructokinase yields a sigmoidal velocity versus substrate curve. e. pep competes with fructose-6-phosphate for the active site of phosphofructokinase. f. the binding of pep to one phosphofructokinase subunit causes a conformation change that affects the ability of the substrate to bind to the other subunits.