glycogen

Glycogen is a highly branched polysaccharide that serves as the primary storage form of glucose in animals and some fungi. In humans it is mainly stored in liver and skeletal muscle, where it serves distinct metabolic roles: maintaining blood glucose during fasting and providing rapid glucose for muscle contraction.

Chemically, glycogen is a polymer of glucose residues linked primarily by α-1,4-glycosidic bonds with α-1,6-branches that occur roughly every 8 to 12 residues. The molecule is assembled by glycogen synthase using UDP-glucose as the glucose donor; glycogenin acts as a protein primer, and the branching enzyme introduces α-1,6 linkages to form a branched structure.

Glycogen is degraded by glycogen phosphorylase, which cleaves α-1,4 bonds to produce glucose-1-phosphate, and a debranching system that removes branches. In the liver, glucose-6-phosphatase converts glucose-6-phosphate to free glucose for release into the bloodstream, enabling maintenance of blood glucose during fasting; in muscle, glucose-6-phosphatase is absent, so glycogenolysis powers locally available energy.

Glycogenesis and glycogenolysis are tightly regulated by hormones and energy status. Insulin promotes synthesis, while glucagon and epinephrine activate breakdown via cAMP-dependent pathways. Allosteric factors such as glucose-6-phosphate, ATP, and AMP modulate enzyme activities, differing between liver and muscle.

Pathways of glycogen storage disorders include McArdle disease (myophosphorylase deficiency), Hers disease (liver phosphorylase deficiency), Andersen disease (branching enzyme deficiency), and Pompe disease (acid alpha-glucosidase deficiency).