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A-Level Biology October/November 2024 Q2(e): P. jirovecii produces an enzyme known as 1,3-β-D-glucan synthase. The enzyme catalyses…
A-Level Biology · Paper 9700/22 · October/November 2024 · Question 2(e) · [5 marks]
P. jirovecii produces an enzyme known as 1,3-β-D-glucan synthase. The enzyme catalyses the synthesis of 1,3-β-D-glucan. The therapeutic drug caspofungin is a non-competitive inhibitor of 1,3-β-D-glucan synthase. With reference to the mechanism of action of caspofungin, explain how the drug may be useful to treat cases of pneumonia caused by P. jirovecii.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
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Caspofungin is a non-competitive inhibitor. This means it binds to the 1,3-β-D-glucan synthase enzyme at an allosteric site, which is a site other than the active site.
This binding causes a change in the tertiary structure of the enzyme, which in turn alters the specific three-dimensional shape of the active site. As a result, the active site is no longer complementary to the substrate molecules. This prevents the formation of enzyme-substrate complexes.
Consequently, the synthesis of the product, 1,3-β-D-glucan, is prevented or significantly reduced. Since 1,3-β-D-glucan is an essential structural component of the cell wall of P. jirovecii, its absence hinders the formation of a new cell wall or weakens the existing one.
The weakened cell wall cannot withstand the high internal hydrostatic pressure. Water enters the fungal cell by osmosis, down a water potential gradient, causing the cell to swell and eventually undergo osmotic lysis (burst). This kills the fungal cells, reducing the population of P. jirovecii in the lungs and allowing the patient's immune system to clear the infection, thereby treating the pneumonia.
How the marks are awarded
- M1 — Correctly stating that caspofungin, as a non-competitive inhibitor, binds to a site on the enzyme other than the active site (an allosteric site).
- M1 — Explaining that this binding changes the shape of the enzyme's active site.
- M1 — Linking the changed active site shape to the inability of the substrate to bind, thus preventing enzyme-substrate complex formation.
- M1 — Stating that the inhibition of the enzyme prevents the synthesis of 1,3-β-D-glucan, which is required for the cell wall.
- M1 — Explaining that the weakened cell wall leads to the osmotic lysis (bursting) of the fungal cell, reducing the pathogen's population.
Common mistakes
- Confusing non-competitive with competitive inhibition, for example by stating that the drug binds to the active site and competes with the substrate.
- Failing to link the inhibition of the enzyme to the specific consequence for the fungus; for example, describing enzyme inhibition correctly but not mentioning the cell wall or cell lysis.
- Giving a vague description of the effect on the cell, such as 'it kills the fungus', without explaining the mechanism of cell wall weakening and subsequent osmotic lysis.
- Forgetting to mention the effect on the population of P. jirovecii, which is the ultimate reason the drug is an effective treatment for the disease.
Examiner tip: For mechanism questions, always build a logical chain of cause and effect, linking the molecular level (e.g., enzyme inhibition) to the cellular level (e.g., cell lysis) and finally to the organismal level (e.g., treatment of disease).
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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