Glycogen degradation (glycogenolysis) in humans primarily requires three key enzymes that act in sequence on the branched glycogen molecule.

Core Enzymes

  • Glycogen phosphorylase
    • Cleaves glycogen’s α-1,4 glycosidic bonds from non‑reducing ends to release glucose 1‑phosphate.
* Stops when it is about four residues away from a branch point (cannot handle α-1,6 bonds).
  • Debranching enzyme (bifunctional)
    This is a single polypeptide with two activities:
* **4-α-D-glucanotransferase (transferase activity)** : transfers a short oligosaccharide (usually three glucosyl units) from a branch to a nearby chain, exposing the branch point.

* **Amylo-α-1,6-glucosidase (α-1,6-glucosidase activity)** : hydrolyzes the remaining α-1,6-linked glucosyl residue, releasing free glucose.

Fate Of The Product

  • The main product of glycogen phosphorylase is glucose 1‑phosphate , which is converted by phosphoglucomutase to glucose 6‑phosphate for entry into glycolysis or further processing.
  • In liver, glucose 6-phosphatase can then convert glucose 6‑phosphate to free glucose for release into the blood, but this enzyme is part of glucose export, not the core glycogenolysis steps.

One-Line “Quick Scoop”

For exam-style wording:

Glycogen degradation requires glycogen phosphorylase, the bifunctional debranching enzyme (transferase and α-1,6-glucosidase activities), and phosphoglucomutase for conversion of glucose 1‑phosphate to glucose 6‑phosphate.

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