Final answer:
ATP is an allosteric inhibitor of phosphofructokinase. It signals that the cell already has sufficient energy, thereby reducing the enzyme's activity to regulate energy production.
Step-by-step explanation:
The molecule that is an allosteric inhibitor of phosphofructokinase, a key regulatory enzyme in glycolysis, is ATP. High levels of ATP indicate that the cell has sufficient energy, leading to a decrease in the activity of phosphofructokinase to slow down glycolysis. This type of regulation is beneficial because it prevents the cell from making more ATP than it needs, avoiding wasteful consumption of glucose.