Phosphorylase

Phosphorylase is the name given to a group of enzymes that catalyze the phosphorolytic cleavage of polysaccharides, most notably glycogen and starch. In these reactions, inorganic phosphate is added to the glucose residue at the non-reducing end of the polymer, producing glucose-1-phosphate and a shortened polysaccharide chain. This activity is distinct from kinases, which transfer phosphate from ATP to substrates. The best-known example is glycogen phosphorylase, which mobilizes glycogen stores in liver and muscle.

In glycogen phosphorylase, the substrate is glycogen or related α-1,4-glycosidic linkages, and the main product is glucose-1-phosphate. The enzyme uses pyridoxal phosphate (PLP) as a cofactor and typically functions as a dimer. In plants, starch phosphorylases perform a similar reaction on starch. These enzymes can also act on related polysaccharides and are integrated into larger pathways of carbohydrate metabolism, providing glucose-1-phosphate that can enter glycolysis or be converted to glucose-6-phosphate for other uses.

Regulation of phosphorylases is complex and involves both allosteric control and covalent modification. In mammals, phosphorylase activity is increased when phosphorylated by phosphorylase kinase, a process stimulated by hormones such as adrenaline and by Ca2+ signaling in muscle. Allosterically, AMP activates, while ATP and glucose-6-phosphate inhibit the enzyme. Tissue differences exist; liver and muscle isozymes show distinct regulatory responses, reflecting their roles in maintaining blood glucose and providing energy to muscle, respectively.

Deficiencies or dysregulation of glycogen phosphorylase can contribute to glycogen storage disorders, such as Hers disease (glycogen storage disease type VI), which affects hepatic glycogen metabolism. Phosphorylases thus play essential roles in energy balance and carbohydrate management across organisms.