Glycogen metabolism
Glycogen metabolism is the set of pathways that build and break down glycogen, the branched storage form of glucose found mainly in liver and skeletal muscle. Glycogenesis stores excess glucose after a meal, and glycogenolysis releases it during fasting or exercise.
Glycogen is a highly branched polymer of glucose. Straight chains are joined by α-1,4 glycosidic bonds, and branch points occur roughly every 8 to 12 residues through α-1,6 bonds. The branching multiplies the number of non-reducing ends available for simultaneous attack, so glucose can be mobilized quickly. Liver glycogen exists to maintain blood glucose for the whole body; muscle glycogen is used locally, because muscle lacks glucose-6-phosphatase and therefore cannot export free glucose.
In glycogenesis, glucose is phosphorylated to glucose-6-phosphate, converted to glucose-1-phosphate, and activated as UDP-glucose. Glycogen synthase, the rate-limiting enzyme, adds these units in α-1,4 linkage, while branching enzyme transfers blocks of residues to create α-1,6 branches. In glycogenolysis, glycogen phosphorylase — the rate-limiting enzyme — cleaves α-1,4 bonds to release glucose-1-phosphate, and debranching enzyme handles the α-1,6 branch points with its transferase and α-1,6-glucosidase activities.
The two pathways are reciprocally regulated so they never run at full speed simultaneously. Insulin promotes glycogen synthesis by activating glycogen synthase and inhibiting phosphorylase. Glucagon acts on the liver and epinephrine on both liver and muscle to trigger a cAMP and protein kinase A cascade that activates phosphorylase and inactivates synthase. Muscle adds a distinct control layer: calcium released during contraction and rising AMP both stimulate glycogen breakdown independent of hormonal signals.
These pathways underlie the glycogen storage diseases, which are a favorite USMLE Step 1 topic — von Gierke disease from glucose-6-phosphatase deficiency, Pompe disease from lysosomal acid maltase deficiency, Cori disease from debranching enzyme deficiency, and McArdle disease from muscle phosphorylase deficiency. The MCAT covers the same pathways within bioenergetics and metabolic regulation, emphasizing how glycogen handling integrates with glycolysis and gluconeogenesis in the fed and fasted states.
Key takeaways
- Glycogen is a branched glucose polymer with α-1,4 linear bonds and α-1,6 branch points.
- Glycogen synthase is rate-limiting for synthesis; glycogen phosphorylase is rate-limiting for breakdown.
- Liver glycogen maintains blood glucose; muscle glycogen serves only the muscle, which lacks glucose-6-phosphatase.
- Insulin favors glycogenesis while glucagon and epinephrine favor glycogenolysis through a cAMP cascade.
- Enzyme defects in these pathways cause the glycogen storage diseases tested on USMLE Step 1.