fosforylaz

Fosforylaz, commonly referred to in English as phosphorylases, are enzymes that catalyze the phosphorolysis of polysaccharides. In these reactions, an inorganic phosphate (Pi) is added to a sugar residue at the non-reducing end of a polymer, releasing a sugar-1-phosphate and shortening the polymer. This mechanism uses Pi rather than ATP, distinguishing phosphorylases from many kinases.

The most studied members are glycogen phosphorylases and starch phosphorylases. Glycogen phosphorylase acts in animals to degrade glycogen, providing glucose-1-phosphate for glycolysis or other pathways during energy demand. In humans, the liver and muscle isoforms fulfill tissue-specific roles in maintaining blood glucose and fueling muscle activity, respectively. Plant and some bacterial systems contain starch phosphorylases that initiate and regulate starch breakdown during germination or leaf metabolism.

Regulation of fosforylaz is a balance of allosteric control and covalent modification. For glycogen phosphorylase, AMP stimulates activity while ATP and glucose-6-phosphate inhibit it, reflecting cellular energy status. Activity is also modulated by phosphorylation: phosphorylase kinase activates the enzyme by phosphorylation, and phosphoprotein phosphatases reverse this modification. Hormonal signals such as epinephrine or glucagon raise cyclic AMP, promoting phosphorylation and activation in liver and muscle under stress or fasting conditions.

Structurally, many phosphorylases function as dimers and often require the cofactor pyridoxal phosphate (PLP) for catalysis, particularly in glycogen phosphorylases. Their action plays a central role in carbohydrate metabolism, enabling rapid mobilization of glucose units from storage polysaccharides.

Clinical relevance includes glycogen storage diseases caused by deficiencies in glycogen phosphorylase, such as impaired glycogen breakdown leading to exercise intolerance or hepatomegaly in different tissues.