Explain how competitive and non-competitive inhibition differ in their effects on the kinetics of an enzyme-catalysed reaction, including reference to how increasing substrate concentration affects each type of inhibitor.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Competitive inhibitors bind to the active site, so they compete with substrate for the same site. Increasing substrate concentration can out‑compete the inhibitor, restoring the maximum rate (V\u00b5max) but increasing the apparent Km.
Non‑competitive inhibitors bind to an allosteric or other site on the enzyme. They do not compete with substrate; increasing substrate concentration cannot displace the inhibitor. The inhibitor reduces the catalytic activity, lowering V\u00b5max, while the apparent Km remains unchanged.
Non‑competitive inhibitors bind to an allosteric or other site on the enzyme. They do not compete with substrate; increasing substrate concentration cannot displace the inhibitor. The inhibitor reduces the catalytic activity, lowering V\u00b5max, while the apparent Km remains unchanged.
Examiner tips
- Use the terms ‘active site’ and ‘allosteric site’ to show understanding of binding sites. Use the Michaelis–Menten parameters (V\u00b5max, Km) to explain the kinetic effects. Show that competitive inhibition can be overcome by high [S] but non‑competitive cannot.
- common_mistakes
- :
- Confusing the effects on V\u00b5max and Km for each type of inhibition. Saying that non‑competitive inhibition increases Km. Not mentioning that competitive inhibition can be overcome by high substrate concentration.
Mark scheme (4 marks)
- Competitive inhibitors bind to the active site, whereas non-competitive inhibitors bind to an allosteric/different site on the enzyme.
- Increasing substrate concentration can overcome competitive inhibition because substrate outcompetes the inhibitor for the active site, restoring Vmax.
- Non-competitive inhibition cannot be overcome by increasing substrate concentration because the inhibitor binds at a site other than the active site and is not displaced by substrate.
- Non-competitive inhibition reduces Vmax (maximum rate of reaction) by reducing the catalytic activity/changing the shape of the active site, whereas apparent Km is increased by competitive inhibition but Vmax is ultimately unchanged.
Key terms in this question
competitive inhibition · non-competitive inhibition
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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