Allosteric enzyme that catalyzes the rate-limiting step in glycogen catabolism, the phosphorolytic cleavage of glycogen to produce glucose-1-phosphate, and plays a central role in maintaining cellular and organismal glucose homeostasis. [(1->4)-alpha-D-glucosyl](n) + phosphate = [(1->4)-alpha-D-glucosyl](n-1) + alpha-D-glucose 1-phosphate Allosterically regulated through the non-covalent binding of metabolites, being activated by AMP and inhibited by ATP, ADP, and glucose-6-phosphate. The activity is also controlled by post-translational modifications including phosphorylation and acetylation. Homodimer; enzymatically active (PubMed:10980448, PubMed:10949035). Interacts with PPP1R3B; recruits the phosphatase PP1 which dephosphorylates and inactivates PYGL/glycogen phosphorylase (PubMed:22225877). Acetylation, which is up-regulated by glucose and insulin and down-regulated by glucagon, inhibits the glycogen phosphorylase activity by promoting PPP1R3B-mediated recruitment of phosphatase PP1 and Ser-15 dephosphorylation. Phosphorylation at Ser-15 converts inactive phosphorylase b into active phosphorylase a (PubMed:10949035). Dephosphorylation of Ser-15 by phosphatase PP1 inactivates the enzyme (PubMed:22225877). The disease is caused by variants affecting the gene represented in this entry. Belongs to the glycogen phosphorylase family.