Final answer:
High levels of ATP or citrate and a lower, more acidic pH decrease phosphofructokinase activity, acting as a regulatory mechanism for glycolysis to conserve energy when the cell has sufficient supply.
Step-by-step explanation:
The statement about phosphofructokinase and the regulation of glycolysis that is true is that high levels of ATP, citrate, or a lower pH (more acidic) decrease the enzyme's activity. Phosphofructokinase is a rate-limiting enzyme in the glycolytic pathway. It catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate using ATP. When ATP levels are high, this indicates that the cell has ample energy, and the pathway is slowed down to avoid unnecessary energy expenditure. This phenomenon is known as end product inhibition, as ATP is the main end product of glycolysis.
Similarly, high levels of citrate, also indicative of sufficient energy supply, inhibit the enzyme. A lower pH, which can be caused by increased production of lactic acid during fermentation, also inhibits phosphofructokinase, helping to prevent an acidic environment that could damage cellular components.