Question:
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
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The Correct Option is B
Solution and Explanation
Step 1: Cyanide is a classic inhibitor of the mitochondrial electron transport chain. Its target is the terminal enzyme that passes electrons to molecular oxygen.
Step 2: That terminal enzyme is cytochrome c oxidase, which is Complex IV. Cyanide binds to the ferric (Fe3+) form of the haem a3 in cytochrome c oxidase, halting electron flow to oxygen and blocking oxidative phosphorylation. Other Complex IV poisons are carbon monoxide, hydrogen sulphide and azide.
Step 3: Note that cytochrome c oxidase and Complex IV are the same structure, so options (b) and (c) are effectively identical. The exam key marks (b) cytochrome c oxidase as the answer. Complex I (a) is blocked by rotenone and amobarbital, and Complex III (d) by antimycin A, so neither is the cyanide target.
Step 4: Hence cyanide inhibits cytochrome c oxidase (Complex IV), option (b).
Step 2: That terminal enzyme is cytochrome c oxidase, which is Complex IV. Cyanide binds to the ferric (Fe3+) form of the haem a3 in cytochrome c oxidase, halting electron flow to oxygen and blocking oxidative phosphorylation. Other Complex IV poisons are carbon monoxide, hydrogen sulphide and azide.
Step 3: Note that cytochrome c oxidase and Complex IV are the same structure, so options (b) and (c) are effectively identical. The exam key marks (b) cytochrome c oxidase as the answer. Complex I (a) is blocked by rotenone and amobarbital, and Complex III (d) by antimycin A, so neither is the cyanide target.
Step 4: Hence cyanide inhibits cytochrome c oxidase (Complex IV), option (b).
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