Glycogen is a highly branched polymer of glucose that serves as the primary short-term energy reserve in animals. It is stored mainly in the liver and skeletal muscle, where it can be rapidly mobilized to maintain blood glucose levels or fuel muscle contraction.
Glycogen Structure
Glycogen is a homopolymer of glucose linked by alpha-1,4-glycosidic bonds with alpha-1,6-glycosidic bonds at branch points occurring every 8 to 14 residues. The high degree of branching provides multiple non-reducing ends for rapid glucose release. The glycogen particle is organized around a core protein called glycogenin, which initiates glycogen synthesis.
Glycogenesis
Glycogenesis is the process of glycogen synthesis. It begins with glucose-6-phosphate being converted to glucose-1-phosphate by phosphoglucomutase. Glucose-1-phosphate then reacts with UTP to form UDP-glucose, catalyzed by UDP-glucose pyrophosphorylase. UDP-glucose serves as the activated glucose donor.
Glycogen synthase adds glucose units from UDP-glucose to the non-reducing end of a growing glycogen chain, forming alpha-1,4 linkages. When the chain reaches about 11 residues, branching enzyme transfers a segment of 6 to 8 glucose units to an interior position, creating an alpha-1,6 branch point. This cycle of elongation and branching builds the characteristic tree-like glycogen structure.
Glycogenolysis
Glycogenolysis is the breakdown of glycogen to release glucose. Glycogen phosphorylase cleaves alpha-1,4 linkages using inorganic phosphate, producing glucose-1-phosphate. This is the rate-limiting enzyme of glycogenolysis and exists in active and inactive forms. The debranching enzyme has two activities: it transfers a trisaccharide unit from a branch to a nearby non-reducing end, and cleaves the alpha-1,6 bond to release free glucose. The combined action of glycogen phosphorylase and debranching enzyme yields about 90% glucose-1-phosphate and 10% free glucose.
Glucose-1-phosphate is converted to glucose-6-phosphate by phosphoglucomutase. In the liver, glucose-6-phosphatase converts glucose-6-phosphate to free glucose for release into the bloodstream. Muscle lacks glucose-6-phosphatase, so glycogen-derived glucose-6-phosphate enters glycolysis directly.
Hormonal Regulation
Glycogen metabolism is tightly regulated by hormones responding to blood glucose levels. Insulin stimulates glycogenesis by activating glycogen synthase through dephosphorylation and inhibiting glycogen phosphorylase. Glucagon (in the liver) and epinephrine (in muscle and liver) stimulate glycogenolysis by activating glycogen phosphorylase through a cAMP-dependent phosphorylation cascade, providing glucose for tissues and substrate for gluconeogenesis.
Glycogen Storage Diseases
Defects in glycogen metabolism enzymes cause glycogen storage diseases. Von Gierke disease results from glucose-6-phosphatase deficiency, causing severe hypoglycemia and glycogen accumulation in the liver. McArdle disease involves muscle glycogen phosphorylase deficiency, leading to exercise intolerance and muscle cramps. Pompe disease is caused by lysosomal alpha-1,4-glucosidase deficiency, resulting in fatal cardiomyopathy in infants.
