Chapter 26: Formation and Degradation of Glycogen

Glycogen consists of glucose units linked through α-1,4 glycosidic bonds with α-1,6 branch points, a structural arrangement that permits both rapid glucose release during energy demand and efficient storage in hepatic and skeletal muscle tissue. The synthetic pathway begins when glucose enters cells and undergoes phosphorylation to glucose 6-phosphate, which is then converted to glucose 1-phosphate and activated by coupling to uridine diphosphate to form UDP-glucose. Glycogen synthase catalyzes the extension of α-1,4 chains, while the branching enzyme introduces α-1,6 linkages that increase the number of branch termini available for rapid mobilization. Glycogenolysis, the breakdown process, is initiated by glycogen phosphorylase, which cleaves α-1,4 bonds to release glucose 1-phosphate, complemented by the debranching enzyme that removes and hydrolyzes branch points. Liver glycogen serves a critical role in systemic glucose homeostasis, particularly during fasting states, whereas muscle glycogen functions as a localized energy reserve driving ATP production through glycolytic pathways during physical exertion. Hormonal regulation differs significantly between tissues: hepatic glycogen synthesis is promoted by insulin while glucagon and epinephrine trigger phosphorylation cascades involving cyclic adenosine monophosphate and protein kinase A to stimulate breakdown. Muscle glycogen catabolism responds directly to adenosine monophosphate, calcium influx, and epinephrine independent of glucagon signaling. The chapter integrates clinical applications through case studies illustrating neonatal hypoglycemia from depleted maternal glycogen stores, profound glucose depletion from inappropriate insulin administration during exercise, and inherited glycogen storage diseases including McArdle disease, Hers disease, and von Gierke disease that result from enzymatic deficiencies. Additionally, glycogen synthase kinase-3 is presented as a regulatory molecule with implications extending beyond glycogen metabolism into glucose metabolism disorders and neurodegenerative conditions.