Formation and Degradation of Glycogen

• Glycogen is the storage form of glucose, composed of glucosyl units linked by α-1,4 glycosidic bonds with α-1,6 branches occurring about every 8 to 10 glucosyl units.
• Glycogen synthesis requires energy.
• Glycogen synthase transfers a glucosyl residue from the activated intermediate UDPglucose to the ends of existing glycogen chains during glycogen synthesis. The branching enzyme creates α-1,6 linkages in the glycogen chain.
• Glycogenolysis is the degradation of glycogen. Glycogen phosphorylase catalyzes a phosphorolysis reaction, utilizing exogenous inorganic phosphate to break α-1,4 linkages at the ends of glycogen chains, releasing glucose 1-phosphate. The debranching enzyme hydrolyzes the α-1,6 linkages in glycogen, releasing free glucose.
• Liver glycogen supplies blood glucose.
• Glycogen synthesis and degradation are regulated in the liver by hormonal changes which signify the need for or excess of blood glucose.
• Lack of dietary glucose, signaled by a decrease of the insulin/glucagon ratio, activates liver glycogenolysis and inhibits glycogen synthesis. Epinephrine also activates liver glycogenolysis.
• Glucagon and epinephrine release lead to phosphorylation of glycogen synthase (inactivating it) and glycogen phosphorylase (activating it).
• Glycogenolysis in muscle supplies glucose 6-phosphate for adenosine triphosphate synthesis in the glycolytic pathway.
• Muscle glycogen phosphorylase is allosterically activated by AMP, as well as by phosphorylation.
• Increases in sarcoplasmic Ca²⁺ stimulate phosphorylation of muscle glycogen phosphorylase.