Glycogenolysis is the process of breaking down stored glycogen in the liver so that glucose may be produced for use in energy metabolism. The process makes use of an inorganic phosphate. Two key enzymes in glycogenolysis are glycogen phosphorylase and debranching enzyme. Glycogen phosphorylase catalyzes the release of glucose-1-phosphate from the terminal alpha-1,4-glycosidic bond. Debranching enzyme lops off the branches.1
Glycogenolysis occurs in the hepatocytes. Glycogen in the liver is broken down to provide a source of blood glucose especially during in between meals when blood glucose level is low. The opposite of glycogenolysis is glycogenesis, which is the process wherein glucose molecules are added to chains of glycogen for storage. Glycogenolysis and glycogenesis are regulated by the effects of hormones, glycagon and insulin. Glucagon stimulates glycogenolysis; insulin inhibits it and favors glycogenesis. Thus, one of the main functions of glycogenolysis is for the regulation of glucose concentration in the bloodstream, however, in the short term only (see gluconeogenesis for long term. The pancreatic alpha cells secrete glucagon in response to low blood glucose, for instance as a result of moderate exercise or fasting.
Glycogenolysis also takes place in myocytes, particularly, during the fight-or-flight response. The hormone that regulates it is epinephrine. Glucogenolysis in myocytes differ from that in hepatocytes in a way that the former generally does not lead to the release of glucose into the bloodstream but stays within the myocyte for glycolysis.
Other regulators of glycogenolysis include cortisol, human growth hormone, and intracellular enzymes.1
1 Naish, J. & Court, D. S. (2014). Medical Sciences. Elsevier Health Sciences. p. 75 -76.