A-Level Biology October/November 2024 Q10(b): Glycogen phosphorylase catalyses the conversion of glycogen to glucose in liver cells.…

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  Glucagon binds to specific receptors on the surface of liver cells, which activates an enzyme cascade. This cascade is intended to activate the enzyme glycogen phosphorylase.

  However, in an individual with GSDVI, the mutation in the PYGL gene results in the production of a non-functioning or less-functioning version of glycogen phosphorylase. This is because the mutation alters the primary structure, leading to an incorrect folding of the polypeptide and a change in the tertiary structure and the shape of the enzyme's active site.

  Consequently, even though the initial signalling pathway involving glucagon is functioning, the final step is inhibited. The faulty enzyme cannot effectively catalyse the breakdown of glycogen to glucose, meaning less or no glycogenolysis occurs. The overall cell signalling process is therefore affected because the intended response (glucose release) does not happen.

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  How the marks are awarded

  • B1 — Describing the initial part of the glucagon signalling pathway, such as stating that glucagon binding activates an enzyme cascade.
  • B1 — Explaining that the mutation results in a non-functioning or less-functioning glycogen phosphorylase enzyme, for example due to a change in its tertiary structure or active site.
  • B1 — Stating the final outcome, which is that less or no glycogen is converted to glucose (glycogenolysis), thus affecting the cell's response to the signal.

  Common mistakes

  • Stating that the mutation prevents glucagon from binding to its receptor, which is incorrect as the mutation affects a downstream enzyme.
  • Confusing the roles of glucagon and insulin, and describing the insulin pathway instead.
  • Being too vague about the effect of the mutation, e.g., 'the enzyme doesn't work', without linking it to a change in the tertiary structure or active site.
  • Confusing glycogenolysis (breakdown of glycogen) with glycogenesis (synthesis of glycogen), and stating that glycogen cannot be made.

  Examiner tip: When a question links a gene mutation to a metabolic pathway, always trace the effect from the specific faulty protein to its impact on the pathway's final product or response.

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