A student claims that competitive inhibitors always reduce the rate of an enzyme-controlled reaction to the same extent, regardless of substrate concentration. Explain why this claim is incorrect.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Competitive inhibitors bind to the active site of an enzyme. They are structurally similar to the substrate and compete with it for access to the active site.
Model answer (4 marks)
Competitive inhibitors bind to the same active site as the substrate, so they compete for access.
If the substrate concentration is increased, the probability that a substrate molecule, rather than an inhibitor, occupies the active site rises.
Consequently the extent of inhibition is reduced as substrate concentration increases.
At sufficiently high substrate concentration the enzyme is saturated with substrate and the reaction rate approaches that of the uninhibited enzyme – Vmax is unchanged.
If the substrate concentration is increased, the probability that a substrate molecule, rather than an inhibitor, occupies the active site rises.
Consequently the extent of inhibition is reduced as substrate concentration increases.
At sufficiently high substrate concentration the enzyme is saturated with substrate and the reaction rate approaches that of the uninhibited enzyme – Vmax is unchanged.
Examiner tips
- Use the word ‘compete’ to link inhibitor and substrate. Show the effect of increasing substrate on binding probability. Explain that Vmax remains unchanged. Keep answer to 4 points, each worth one mark.
Common mistakes
- Saying the inhibitor always reduces the rate, ignoring substrate concentration. Claiming Vmax is lowered. Using vague terms like ‘always’ or ‘always the same’ without explanation.
Mark scheme (4 marks)
- Competitive inhibitor and substrate molecules compete for the same active site
- Increasing substrate concentration increases the probability of substrate (rather than inhibitor) binding to the active site
- Therefore the degree of inhibition decreases as substrate concentration increases
- At sufficiently high substrate concentration, the rate approaches that of the uninhibited enzyme (Vmax is unchanged)
Key terms in this question
competitive inhibitor · substrate concentration
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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