Question:medium
Which of the following is true about a non-competitive antagonist (inhibitor)?
Which of the following is true about a non-competitive antagonist (inhibitor)?
Show Hint
The inhibitor binds away from the active site, so substrate affinity does not change but maximal velocity falls.
Updated On: Jun 23, 2026
- Km remains same, Vmax decreases
- Km increases, Vmax remains same
- Km decreases, Vmax increases
- Km increases, Vmax increases
Show Solution
The Correct Option is A
Solution and Explanation
Compare it against competitive inhibition and the answer falls out. A competitive inhibitor fights for the active site, so more substrate wins it back: Km rises, Vmax is unchanged. A non-competitive inhibitor does the opposite. It sits on a separate allosteric site and cripples catalysis whether or not substrate is bound.
Because the active site is still free to bind substrate normally, affinity (Km) is untouched. But no amount of extra substrate can undo the block, so the maximum reaction rate (Vmax) drops. Reading the options that means Km same and Vmax decreased, which is option 1. On the double-reciprocal plot this shows up as a higher 1/Vmax intercept with the same x-intercept, the classic crossing-on-the-x-axis pattern. The remaining options describe activation or competitive-type changes and are wrong.
Ref: Lippincott's Biochemistry, 5th edn.
Because the active site is still free to bind substrate normally, affinity (Km) is untouched. But no amount of extra substrate can undo the block, so the maximum reaction rate (Vmax) drops. Reading the options that means Km same and Vmax decreased, which is option 1. On the double-reciprocal plot this shows up as a higher 1/Vmax intercept with the same x-intercept, the classic crossing-on-the-x-axis pattern. The remaining options describe activation or competitive-type changes and are wrong.
Ref: Lippincott's Biochemistry, 5th edn.
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