fosforylaza

Fosforylaza, commonly referred to as phosphorylase, is a family of enzymes that catalyze the phosphorolysis of glycosidic bonds in polysaccharides such as glycogen and starch. In glycogen metabolism, these enzymes cleave glucose units from the glycogen molecule by adding inorganic phosphate, producing glucose-1-phosphate that can enter glycolysis or be converted to glucose in the liver.

Two main classes occur in nature: glycogen phosphorylase, which has muscle and liver isoforms in animals, and starch phosphorylase, found in plants. The mammalian muscle enzyme primarily supplies glucose for energy during contraction, while the liver enzyme helps regulate blood glucose levels by releasing glucose into the bloodstream as needed.

The general reaction is glycogen(n) + Pi ⇌ glycogen(n-1) + glucose-1-phosphate. Phosphorylases typically require pyridoxal phosphate (PLP) as a cofactor and operate through a phosphorolysis mechanism that preserves the glucose residue as a phosphate ester. The enzymes are regulated both allosterically and by covalent modification.

Regulation and physiological role are tightly linked to energy status. Phosphorylation by phosphorylase kinase converts the enzyme to a more active form. Hormonal signals such as glucagon (liver) and epinephrine (liver and muscle) promote phosphorylation during fasting or stress. In muscle, AMP activates the enzyme, while ATP and glucose-6-phosphate inhibit it. Calcium ions activate phosphorylase kinase in muscle during contraction, enhancing glycogen breakdown.

Clinical relevance includes glycogen storage diseases caused by deficient phosphorylase activity, such as Hers disease (liver) and McArdle disease (muscle). Phosphorylases also play a role in plant starch metabolism, mobilizing sugars during night and germination.