Final answer:
Glycogen phosphorylase is inhibited by insulin, as insulin promotes the storage of glucose, opposite to glycogen breakdown.
Step-by-step explanation:
Glycogen phosphorylase is an enzyme involved in the breakdown of glycogen into glucose, a process known as glycogenolysis. Its activity is regulated by various factors within the cell. One of the inhibitors of glycogen phosphorylase is insulin, a hormone that signals the uptake and storage of glucose, and therefore counters the breakdown of glycogen. In contrast, glucagon and epinephrine activate glycogen phosphorylase to promote glycogenolysis and increase glucose availability. ATP and acetyl-CoA are indicators of high energy levels in the cell and inhibit enzymes like pyruvate kinase; however, they are not the direct inhibitors of glycogen phosphorylase. AMP, on the other hand, indicates low energy levels and thus would not inhibit glycogen phosphorlyase as its accumulation signals the need for more glucose, thereby activating glycogen breakdown. Therefore, the correct inhibitor of glycogen phosphorylase in the options provided is insulin.