Question:medium
In the electron transport chain, cyanide inhibits?
In the electron transport chain, cyanide inhibits?
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Cyanide blocks the last ETC enzyme that gives electrons to oxygen: cytochrome c oxidase.
Updated On: Jun 24, 2026
- Complex I
- Cytochrome C oxidase
- Complex IV
- Complex III
Show Solution
The Correct Option is B
Solution and Explanation
Step 1: Recognise cyanide as a potent blocker of cellular respiration that produces histotoxic hypoxia, meaning oxygen is present but cannot be used.
Step 2: This effect arises because cyanide acts at the very end of the respiratory chain. The last complex, cytochrome c oxidase (Complex IV), normally donates electrons to oxygen; cyanide binds its haem-a3 iron in the ferric state and stops this final transfer.
Step 3: With electron flow arrested, the proton gradient collapses and ATP synthesis fails. Because Complex IV and cytochrome c oxidase denote one and the same enzyme, the two matching options are equivalent and the printed key selects the named enzyme. Rotenone hits Complex I and antimycin A hits Complex III, so those choices are incorrect for cyanide.
Step 4: Therefore cyanide inhibits cytochrome c oxidase, the Complex IV of the chain.
\[\boxed{\text{Cytochrome C oxidase (Complex IV)}}\]
Step 2: This effect arises because cyanide acts at the very end of the respiratory chain. The last complex, cytochrome c oxidase (Complex IV), normally donates electrons to oxygen; cyanide binds its haem-a3 iron in the ferric state and stops this final transfer.
Step 3: With electron flow arrested, the proton gradient collapses and ATP synthesis fails. Because Complex IV and cytochrome c oxidase denote one and the same enzyme, the two matching options are equivalent and the printed key selects the named enzyme. Rotenone hits Complex I and antimycin A hits Complex III, so those choices are incorrect for cyanide.
Step 4: Therefore cyanide inhibits cytochrome c oxidase, the Complex IV of the chain.
\[\boxed{\text{Cytochrome C oxidase (Complex IV)}}\]
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