Biochemistry · Enzyme
Glycogen phosphorylase !
Glycogen phosphorylase releases glucose 1-phosphate from glycogen; it controls glycogen breakdown in liver and muscle.
Updated 2026-11-01
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Key facts
- Glycogen (n) + Pi → glycogen (n − 1) + glucose 1-phosphate: it cleaves with phosphate (phosphorolysis), not water, saving one ATP.
- It needs pyridoxal phosphate (B6); muscle keeps much of the body's B6 in this enzyme.
- It only cuts α-1,4 bonds and stops 4 residues from a branch.
- Phosphorylated it is active (form a); phosphorylase kinase phosphorylates it.
- Structure 1FA9 (human liver) has glucose in the active site, PLP, and AMP in the allosteric site.
Regulation
- Liver: glucagon (cAMP → PKA → phosphorylase kinase) activates it; free glucose and insulin inactivate it.
- Muscle: adrenaline activates it, as do Ca²⁺ from contraction (through phosphorylase kinase) and AMP directly.
- ATP and glucose 6-phosphate inhibit it in muscle.
Inhibitors and toxins
- Glucose (liver)
- Caffeine (nucleoside site, weak effect)
- Synthetic inhibitors studied for type 2 diabetes
Clinical notes
- Muscle deficiency (PYGM): McArdle disease, with cramps, myoglobinuria and the second-wind phenomenon.
- Liver deficiency (PYGL): Hers disease, with hepatomegaly and mild hypoglycaemia.
- Phosphorylase kinase deficiency (GSD IX) gives a liver picture similar to Hers disease.
Pathways
Related conditions
Related
Test yourself
Think of the answer, then tap the question to see it.
Which vitamin cofactor does glycogen phosphorylase carry?
Pyridoxal phosphate, derived from vitamin B6.
Why is it better to cleave glycogen with phosphate rather than water?
Glucose 1-phosphate comes out already phosphorylated: the hexokinase ATP is saved and the glucose stays trapped in the cell.
What activates muscle phosphorylase during exercise?
Ca²⁺ from contraction (through phosphorylase kinase), AMP and adrenaline.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: PYGL: Glycogen phosphorylase, liver form (human) (UniProt, 2026); PYGM: Glycogen phosphorylase, muscle form (human) (UniProt, 2026); PYGM gene: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); 1FA9: HUMAN LIVER GLYCOGEN PHOSPHORYLASE A COMPLEXED WITH AMP (RCSB Protein Data Bank, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.