glikojen

Glycogen, or glikogen, is a highly branched polysaccharide of glucose and the principal storage form of carbohydrates in animals and fungi. In humans it is stored mainly in the liver and in skeletal muscle. The molecule consists of glucose units linked predominantly by α(1→4) glycosidic bonds in linear chains, with about one branch every 8–12 residues connected by α(1→6) bonds, forming a branched, compact structure.

Reservoir sizes: a typical adult stores about 100 g of glycogen in the liver and about 300–400 g in skeletal muscle, though the exact amount depends on body mass, training, and dietary state.

Metabolism: glycogenesis is the synthesis of glycogen from glucose-1-phosphate, catalyzed by UDP-glucose pyrophosphorylase and glycogen synthase with a branching enzyme; glycogenolysis is the breakdown, initiated by glycogen phosphorylase, which releases glucose-1-phosphate, followed by debranching enzyme action. In the liver, glucose-6-phosphatase converts glucose-6-phosphate to free glucose for release into the bloodstream; in muscle, G6P is used for energy via glycolysis.

Regulation: insulin promotes glycogenesis; glucagon (liver) and epinephrine (liver and muscle) activate glycogenolysis via cAMP-dependent signaling. Calcium signaling also participates in muscle glycogen breakdown during contraction.

Role and significance: glycogen provides rapid, readily mobilizable energy during short-term fasting and physical activity; hepatic glycogen helps maintain blood glucose between meals; muscle glycogen supports intense exercise but cannot directly raise blood glucose.

Pathology: defects in glycogen metabolism lead to glycogen storage diseases, such as von Gierke disease (G6Pase deficiency) and Pompe disease (acid maltase).