Cyanide exerts its toxic effect primarily by inhibiting which cytochrome in the electron transport chain?

Explanation

## Mechanism of Cyanide Toxicity **Key Point:** Cyanide binds irreversibly to the ferric iron (Fe³⁺) in cytochrome c oxidase (Complex IV), the final enzyme in the electron transport chain, preventing the transfer of electrons to oxygen. ### Biochemical Basis Cyanide's lethal action occurs through: 1. **Binding site:** Fe³⁺ in the heme a₃ group of cytochrome c oxidase 2. **Effect:** Blocks electron transfer to molecular oxygen 3. **Consequence:** Halts ATP production via oxidative phosphorylation 4. **Result:** Rapid cellular hypoxia despite normal oxygen availability (histotoxic hypoxia) ### Why This Is Lethal Unlike other forms of hypoxia (anemic, circulatory, hypoxic), cells cannot extract oxygen from blood because the final step of aerobic respiration is blocked. The brain and heart—organs with the highest oxygen demand—are affected first, leading to rapid loss of consciousness and cardiac arrhythmias. **High-Yield:** Cyanide poisoning produces a clinical paradox: **normal arterial oxygen saturation (SpO₂) but profound cellular dysfunction**—this is the hallmark of histotoxic hypoxia. **Clinical Pearl:** Victims may have a "healthy" pink complexion due to high venous oxygen saturation (blood cannot unload oxygen at the tissue level), which can mislead initial assessment. [cite:Parikh's Forensic Medicine 7e Ch 15]