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JEE Advanced levelSingle correct optionDifficulty 3 of 3el023

Three inhibitors were tested on one enzyme. P: the inhibition falls steadily as the substrate concentration is raised 50-fold, and P is structurally similar to the substrate. Q: the inhibition is unchanged by raising the substrate, and Q does not resemble the substrate. R: inhibition persists after 24 h of dialysis, and analysis shows R covalently bonded to a serine in the active site. Choose the correct assignment.

  1. AP competitive reversible; Q allosteric; R irreversible, active-site
  2. BP allosteric; Q competitive; R reversible, active-site
  3. CP irreversible, active-site; Q competitive; R allosteric
  4. DP competitive reversible; Q irreversible, active-site; R allosteric

Answer: A

P is overcome by substrate and resembles it, so it is a competitive reversible inhibitor. Q is not overcome and does not resemble the substrate, so it acts at an allosteric site. R survives dialysis and is covalently attached in the active site, so it is an irreversible active-site inhibitor. This matches the first option.
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