Cyanide poisoning causes cellular asphyxia despite normal blood oxygen levels. Cyanide inhibits which complex of the mitochondrial electron transport chain?
- A Complex I (NADH dehydrogenase)
- B Complex II (succinate dehydrogenase)
- C Complex IV (cytochrome c oxidase) ✓
- D Complex III (cytochrome bc1 complex)
Correct answer: C. Complex IV (cytochrome c oxidase)
Explanation
Cyanide (CN-) binds with very high affinity to the ferric (Fe3+) form of cytochrome a3 in Complex IV (cytochrome c oxidase), the terminal electron acceptor of the ETC that normally donates electrons to molecular oxygen to form water. By blocking electron transfer to oxygen, cyanide halts the entire ETC, collapses the proton gradient, and stops ATP synthase. Treatment uses methemoglobin inducers (amyl nitrite, sodium nitrite) to create Fe3+ hemoglobin that competes for cyanide, followed by sodium thiosulfate to convert cyanide to thiocyanate.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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