A Levels Biology (9700)•9700/11/M/J/18

Explanation
Distinguishing enzyme inhibitors by effects on Km and Vmax
Steps:
- Competitive inhibitors increase Km (apparent lower substrate affinity) but Vmax unchanged, as they compete for the active site.
- Non-competitive inhibitors decrease Vmax (reduced maximum rate) but Km unchanged, as they bind elsewhere and alter enzyme function.
- Graph line X shows same Km (x-intercept) but lower Vmax (y-intercept), indicating non-competitive inhibition.
- Non-competitive inhibitors bind to an allosteric site, not the active site, confirming X's identity.
Why D is correct:
- Non-competitive inhibition decreases Vmax without affecting Km, and these inhibitors bind to a site other than the active site by definition.
Why the others are wrong:
- A: X shows unchanged Km and decreased Vmax, characteristic of non-competitive, not competitive inhibition.
- B: Y shows increased Km and unchanged Vmax, indicating competitive inhibition that mimics substrate shape for active site binding.
- C: Y exhibits competitive inhibition traits (increased Km, same Vmax), not non-competitive.
Final answer: D
Topic: Factors that affect enzyme action
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