Q.20 Cyanide is probably the most potent inhibitor of electron transport chain. It binds to:

1. NADH dehydrogenase
2. ATP synthase
3. Succinate dehydrogenase
4. Fe³⁺ of cytochrome oxidase

   Cyanide is the most potent inhibitor of the electron transport chain (ETC), binding specifically to the Fe3+ iron in cytochrome oxidase (Complex IV), halting electron flow to oxygen and ATP production.

   Correct Answer

   Fe3+ of cytochrome oxidase

   Option Analysis

   • NADH dehydrogenase: Incorrect. This is Complex I; cyanide does not bind here—rotenone or amytal are its inhibitors, blocking NADH electron entry early in the chain.

   • ATP synthase: Incorrect. This is Complex V, using the proton gradient for ATP synthesis; cyanide indirectly stops it by collapsing the gradient but does not bind it (oligomycin is the direct inhibitor).

   • Succinate dehydrogenase: Incorrect. This is Complex II, oxidizing succinate to fumarate; cyanide has no effect here (malonate inhibits it), as Complex II feeds electrons downstream.

   • Fe3+ of cytochrome oxidase: Correct. Cyanide (CN-) binds irreversibly to the ferric (Fe3+) heme a3-CuB binuclear center in Complex IV (cytochrome c oxidase), preventing O2 reduction and backing up electrons across the ETC.

   Cyanide electron transport chain binding to Fe3+ of cytochrome oxidase makes it a deadly poison by stopping cellular respiration at Complex IV. This halts electron flow from cytochrome c to O2, preventing proton gradient formation and ATP synthesis, leading to rapid cell death.

   Mechanism of Inhibition

   Cyanide’s CN- ion coordinates with the Fe3+ in cytochrome oxidase’s heme a3, mimicking O2 and locking the site. Upstream complexes (NADH dehydrogenase/Complex I, succinate dehydrogenase/Complex II) accumulate reduced carriers, but no oxidation occurs, causing lactic acidosis. ATP synthase fails without protons.

   Effects on ETC Components

   Component	Role	Cyanide Effect
   NADH dehydrogenase	Complex I entry	Backed up electrons; no direct binding
   ATP synthase	ATP production	Indirect halt via no gradient
   Succinate dehydrogenase	Complex II entry	Unaffected directly; flow stops downstream
   Cytochrome oxidase	O2 reduction	Direct Fe3+ binding blocks respiration

   Clinical Implications

   Cyanide poisoning causes hypoxia despite O2 presence, treated with hydroxocobalamin or thiosulfate. Understanding cyanide electron transport chain binding aids toxicology and mitochondrial research.