Glycogen: The Liver's Storage Form of Glucose

Glycogen is a highly branched polysaccharide of glucose, serving as the primary short-term energy reserve in animals. In humans, it is stored mainly in the liver (100–120 g) and skeletal muscle (400–500 g).

Structure

• α-1,4-glycosidic bonds in linear chains; α-1,6 branches every 8–12 residues.
• Compact, soluble, rapidly mobilizable.

Hepatic Glycogen Metabolism

State	Hormonal Signal	Pathway	Outcome
Fed (high glucose/insulin)	Insulin ↑, glucagon ↓	Glycogenesis: glucose → G6P → G1P → UDP-glucose → glycogen (glycogen synthase)	Glucose stored
Fasting (low glucose/insulin)	Glucagon ↑, insulin ↓, epinephrine	Glycogenolysis: glycogen → G1P → G6P → glucose (glycogen phosphorylase, debranching enzyme, glucose-6-phosphatase)	Glucose released to blood

Key: Liver expresses glucose-6-phosphatase → releases free glucose into bloodstream for other organs (brain, RBCs). Muscle lacks this enzyme → uses glycogen locally (G6P → glycolysis).

Clinical Relevance

• Glycogen storage diseases (GSD): Enzyme defects (e.g., Von Gierke GSD Ia – G6Pase deficiency → hypoglycemia, hepatomegaly, lactic acidosis).
• Hypoglycemia management: Glucagon IM mobilizes hepatic glycogen (ineffective if stores depleted – starvation, adrenal insufficiency, GSD).
• Exercise: Muscle glycogen fuels contraction; liver glycogen maintains blood glucose.
• Diabetes: Impaired glycogen synthesis (insulin resistance) + excessive glycogenolysis (glucagon dysregulation) → hyperglycemia.

Glycogen is the critical buffer maintaining euglycemia between meals.