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