fosforylazy

Fosforylazy, or phosphorylases in English, are enzymes that catalyze the phosphorolysis of polysaccharides containing α-1,4-glycosidic bonds, principally glycogen in animals and starch in plants. In the presence of inorganic phosphate, they cleave a glucose residue from the non-reducing end to yield glucose-1-phosphate and a shortened glucan chain. This reaction allows rapid mobilization of stored carbohydrate reserves without the direct use of ATP.

Several forms exist. Vertebrates express muscle glycogen phosphorylase (myophosphorylase) and liver glycogen phosphorylase, while plants possess plastidial and cytosolic starch phosphorylases. Bacteria and some fungi also have glucan phosphorylases with specialized substrate specificities.

Mechanism and regulation: Phosphorylases operate via phosphorolysis using inorganic phosphate; the reaction is reversible under cellular conditions. In mammals, the enzyme is regulated by covalent modification: phosphorylation by phosphorylase kinase converts an inactive b-form to an active a-form; Ca2+ ions and hormonal signals influence this pathway. Allosteric effectors such as AMP activate the b-form, whereas glucose-6-phosphate and other metabolites can provide feedback inhibition and stabilization of the less active states.

Physiological role: In animals, glycogen phosphorylases liberate glucose-1-phosphate for glycolysis during energy demand; in liver this helps maintain blood glucose levels; in plants, starch phosphorylases initiate starch breakdown during germination or stress.

Clinical and research notes: Deficiencies in glycogen phosphorylase cause glycogen storage diseases (e.g., McArdle disease for muscle, Hers disease for liver). Phosphorylases are studied for agricultural and industrial applications, including starch processing and bioenergy.