A student investigates the activity of the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen. The student adds a competitive inhibitor to separate reaction mixtures, keeping all other variables constant. Explain how the presence of a competitive inhibitor reduces the rate of the catalase-catalysed reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Catalase is found in almost all living cells and has one of the highest turnover numbers of any known enzyme. A competitive inhibitor has a molecular shape complementary to the active site of catalase.
Model answer (4 marks)
The inhibitor has a shape complementary to the active site of catalase, so it can bind to the same site as hydrogen peroxide. It therefore competes with the substrate for binding. Because fewer enzyme–substrate complexes are formed, fewer molecules of water and oxygen are produced per unit time. Consequently the overall rate of the catalase‑catalysed reaction is reduced.
Examiner tips
- Mention the complementary shape of the inhibitor
- Explain the competition for the active site
- State that fewer ES complexes form
- Link this to a lower product formation rate
Common mistakes
- Confusing competitive inhibition with non‑competitive inhibition
- Failing to mention the shape complementarity
- Saying the inhibitor simply blocks the enzyme without explaining the competition
Mark scheme (4 marks)
- The competitive inhibitor has a complementary shape to the active site of catalase
- The competitive inhibitor competes with hydrogen peroxide (substrate) for binding to the active site
- Fewer enzyme–substrate complexes are formed per unit time
- Therefore the rate of reaction is reduced because less product is formed per unit time
Key terms in this question
competitive inhibitor · catalase
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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