JEE Advanced levelSingle correct optionDifficulty 3 of 3el032
A messenger molecule M activates receptor R. Drug Y has a structure partly like M, binds R more tightly than M does, and does not change the shape of R in the way M does. Which prediction is correct?
AY alone gives a larger response than M, so Y is an agonist
BY alone gives no response, and Y reduces the response to M, so Y is an antagonist
CY alone gives no response and increases the response to M
DY is an enzyme inhibitor and has no effect on the receptor
Answer: B
Because Y occupies R without producing the shape change that triggers the response, it gives no signal. Its tight binding keeps M out, so the response to M falls. This behaviour defines an antagonist.