Glucógeno

Glucógeno (glycogen) is a branched polysaccharide of glucose that serves as the primary storage form of glucose in animals and fungi. Its chains consist of glucose units linked by α-1,4-glycosidic bonds with α-1,6-glycosidic branches about every 8–12 residues, creating a highly branched, water-soluble molecule. Glycogen is stored in cytosolic granules that contain enzymes for synthesis and degradation.

Sites and function: In humans, glycogen is mainly stored in liver and skeletal muscle. Liver glycogen maintains blood glucose during fasting by releasing glucose-6-phosphate that is converted to free glucose by glucose-6-phosphatase. Muscle glycogen provides a rapid glucose-6-phosphate supply for glycolysis to support muscle contraction.

Biochemistry: Glycogenesis converts UDP-glucose into glycogen; key enzymes include glycogen synthase (rate-limiting for synthesis) and branching enzyme (4:6 transferase). Glycogenolysis begins with glycogen phosphorylase removing glucose units as glucose-1-phosphate, with debranching enzymes resolving α-1,6 branches. In liver, glucose-6-phosphatase produces free glucose. In muscle, glucose-6-phosphate enters glycolysis.

Regulation: Hormonal control by insulin (promotes synthesis) and glucagon/epinephrine (promote breakdown). Allosteric and energetic signals modulate enzyme activities: glucose-6-phosphate activates synthase; AMP activates phosphorylase; high ATP inhibits breakdown. Substrate availability includes UDP-glucose.

Clinical relevance: Disorders of glycogen metabolism, or glycogen storage diseases, include GSD I (glucose-6-phosphatase deficiency), GSD V (muscle glycogen phosphorylase deficiency), and Pompe disease (lysosomal α-glucosidase deficiency).