Final answer:
ATP is an allosteric inhibitor of phosphofructokinase, signaling that the cell has sufficient energy and slowing down the glycolysis process to conserve resources.
Step-by-step explanation:
Phosphofructokinase is a key regulatory enzyme in glycolysis. It is allosterically regulated by various compounds. ATP functions as an allosteric inhibitor, high levels of ATP signal that the cell has a sufficient energy supply and thus downregulate glycolysis to conserve resources. The decrease in phosphofructokinase activity in the presence of ATP is an example of end product inhibition. This slows down the glycolytic pathway when there is enough ATP, preventing the unnecessary conversion of glucose into more ATP.