Step by Step
Str
Glycogen structure
Glycogen is a highly branched polymer of glucose linked by α-1,4 bonds (straight chains) and α-1,6 bonds at branch points (every 8-12 residues). Branching increases solubility and the number of free ends available for rapid degradation. Glycogen synthase (adds α-1,4) and branching enzyme (creates α-1,6 branches) build it. Glycogenin is the protein primer.
Memory trick: Glycogen = a tree with many branches. More branches = more places to add or remove glucose simultaneously.
Syn
Glycogen synthesis
Steps: Glucose → G6P (hexokinase) → G1P (phosphoglucomutase) → UDP-glucose (UDP-glucose pyrophosphorylase) → added to glycogen chain by glycogen synthase (α-1,4). Branching enzyme creates α-1,6 branch points. Requires energy: 1 UTP per glucose added.
Bk
Glycogen breakdown (glycogenolysis)
Glycogen phosphorylase cleaves α-1,4 bonds → releases glucose-1-phosphate (G1P). Debranching enzyme removes α-1,6 branch points (releases free glucose — only unphosphorylated glucose from glycogen). G1P → G6P (phosphoglucomutase). Liver: G6Pase converts G6P → free glucose → exported to blood. Muscle: no G6Pase → G6P enters glycolysis directly.
Memory trick: Phosphorylase = the shredder. Debranching enzyme = the branch cutter.
Hor
Hormonal control
Epinephrine (muscle and liver) and glucagon (liver only): activate glycogen phosphorylase kinase → phosphorylates glycogen phosphorylase → ACTIVE (breakdown). Simultaneously phosphorylate glycogen synthase → INACTIVE (synthesis off). Insulin: activates protein phosphatase → dephosphorylates → synthase ON, phosphorylase OFF. Classic reciprocal regulation.
Applied Walkthrough
1
During a sprint: epinephrine floods the body → activates phosphorylase in muscle → rapid glycogen breakdown → G6P → glycolysis → ATP for muscle contraction.
2
Simultaneously: glycogen synthase is phosphorylated → inactivated → no synthesis. All glycogen mobilization, no storage.
3
After a meal: insulin rises → protein phosphatase activated → glycogen synthase dephosphorylated → ACTIVE. Glucose from blood is stored as glycogen. Phosphorylase is inactivated.
4
Von Gierke disease (G6Pase deficiency): glycogen accumulates in liver and kidney — cannot export glucose → severe fasting hypoglycemia. Classic glycogen storage disease exam question.
Exam Application
Exams test glycogen synthesis (UDP-glucose, glycogen synthase, branching enzyme), glycogenolysis (phosphorylase, debranching enzyme), why liver glycogen maintains blood glucose but muscle glycogen doesn't (G6Pase), and hormonal control (epinephrine/glucagon = breakdown; insulin = synthesis). Glycogen storage diseases (Von Gierke, McArdle, Pompe) are frequently tested.
⚠ Common Trap
Students confuse glycogen phosphorylase with glycogen synthase — phosphorylase BREAKS down glycogen; synthase BUILDS it. Also: glycogen phosphorylase releases glucose-1-phosphate, not free glucose (except at branch points where debranching enzyme releases free glucose). Only the liver can export free glucose because muscle lacks G6Pase.
✓ Quick Self-Check
1. What bonds are found in glycogen?
α-1,4 bonds in straight chains and α-1,6 bonds at branch points.
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2. What enzyme breaks down glycogen?
Glycogen phosphorylase — cleaves α-1,4 bonds, releasing glucose-1-phosphate.
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3. Why can muscle glycogen not maintain blood glucose?
Muscle lacks glucose-6-phosphatase — it cannot convert G6P to free glucose for export to blood. Muscle glycogen is used only for local energy needs.
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4. How do epinephrine and glucagon regulate glycogen metabolism?
They activate glycogen phosphorylase (breakdown) and inactivate glycogen synthase (stops synthesis) — via a phosphorylation cascade.
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5. What is Von Gierke disease?
Glucose-6-phosphatase deficiency — glycogen accumulates in liver and kidney; patients cannot export glucose → severe fasting hypoglycemia.
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