Glycogen Metabolism
1. What is Glycogen?
- Glycogen is the storage form of glucose in animals.
- It is a highly branched polymer of α-D-glucose.
- Stored mainly in liver (~10% of weight) and skeletal muscle (1–2%), but muscle contains more total glycogen because of larger mass.
- Its branched structure allows rapid release of glucose when required.
2. Importance of Glycogen
A. Liver Glycogen
- Maintains blood glucose levels between meals.
- Liver releases glucose into blood because it has Glucose-6-phosphatase.
B. Muscle Glycogen
- Used only inside muscle for ATP production during contraction.
- Muscle lacks Glucose-6-phosphatase, so cannot release glucose into blood.
- Function is local energy supply, especially during exercise.
3. Glycogenesis (Glycogen Synthesis)
Goal: Convert glucose → glycogen (storage).
Steps:
- Glucose → Glucose-6-phosphate (by hexokinase/glucokinase).
- Glucose-6-phosphate → Glucose-1-phosphate (via phosphoglucomutase).
- Glucose-1-phosphate + UTP → UDP-glucose (activated glucose donor).
- Glycogen synthase adds glucose units to a pre-existing glycogen primer (glycogenin).
- Branching enzyme introduces α-1,6 branches, increasing solubility and synthesis rate.
Regulation:
- Insulin activates glycogen synthase → promotes storage after meals.
- High glucose & ATP stimulate synthesis.
4. Glycogenolysis (Glycogen Breakdown)
Goal: Convert glycogen → glucose (energy release).
Steps:
- Glycogen phosphorylase cleaves α-1,4 bonds → forms Glucose-1-phosphate.
- Debranching enzyme removes α-1,6 branches.
- Glucose-1-phosphate → Glucose-6-phosphate (via phosphoglucomutase).
- In liver: Glucose-6-phosphate → Free glucose → released into blood.
- In muscle: Glucose-6-phosphate → enters glycolysis → ATP for contraction.
Regulation:
- Glucagon (in fasting) activates glycogen phosphorylase in liver.
- Epinephrine activates glycogen breakdown in both liver and muscle.
- Ca²⁺ and AMP activate glycogen phosphorylase in muscle during exercise.