The third step is the phosphorylation of fructose-6-phosphate, catalyzed by the enzyme phosphofructokinase. a second atp molecule donates a high-energy phosphate to fructose-6-phosphate, producing fructose-1,6-bisphosphate. in this pathway, phosphofructokinase is a rate-limiting enzyme. it is active when the concentration of adp is high; it is less active when adp levels are low and the concentration of atp is high. thus, if there is "sufficient" atp in the system, the pathway slows down. this is an example of