Glycogen phosphorylase that regulates glycogen mobilization. Phosphorylase is an important allosteric enzyme in carbohydrate metabolism. Enzymes from different sources differ in their regulatory mechanisms and in their natural substrates. However, all known phosphorylases share catalytic and structural properties. [(1->4)-alpha-D-glucosyl](n) + phosphate = [(1->4)-alpha-D-glucosyl](n-1) + alpha-D-glucose 1-phosphate Activity of phosphorylase is controlled both by allosteric means (through the non-covalent binding of metabolites) and by covalent modification. Thus AMP allosterically activates, whereas ATP, ADP, and glucose-6-phosphate allosterically inhibit, phosphorylase B (By similarity). Homodimer. Dimers associate into a tetramer to form the enzymatically active phosphorylase A (By similarity). Phosphorylation of Ser-15 converts phosphorylase B (unphosphorylated) to phosphorylase A. Belongs to the glycogen phosphorylase family.