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. Homodimer. Homotetramer; to form the enzymatically active phosphorylase A. Phosphorylation of Ser-15 converts phosphorylase B (unphosphorylated) to phosphorylase A. Defects in PYGM are the cause of glycogen storage disease V (GSD-V); also known as McArdle disease. Belongs to the glycogen phosphorylase family.