Which poison gas inhibits cytochrome c oxidase in mitochondria, producing effects similar to carbon monoxide?

• A. Cyanide
• B. Hydrogen sulfide
• C. Arsenic
• D. Nitric oxide

Answer: (A)

This item tests how certain poisonous gases shut down cellular energy production by blocking the mitochondrial electron transport chain. Cyanide binds to cytochrome c oxidase (Complex IV) in mitochondria, preventing electrons from being transferred to oxygen. Since oxidative phosphorylation can’t proceed, cells can’t make ATP even though oxygen is available, forcing a switch to anaerobic glycolysis and causing lactic acidosis and histotoxic hypoxia. The resulting tissue energy failure mirrors the effects seen with carbon monoxide, which also impairs oxygen utilization, though by a different primary mechanism (CO mainly reduces oxygen delivery by binding hemoglobin). Cyanide is the classic gas that directly inhibits this enzyme, making it the best answer here. (Hydrogen sulfide can have a similar mitochondrial effect, but cyanide is the prototypical example.)