Question

Potassium cyanide is a potent poison that kills, because it inhibits electron transport by binding Fe3+ in the heme of cytochromes (including cytochrome c). Why would this stop electron transport? Briefly explain.

Answer 1

Ans : Cyanide acts as an inhibitor of electron transport chain, as it binds with cytochrome c oxidase / complex IV & inhibits the terminal transfer of electrons to molecular oxygen.

* Cyanide binds with Fe3+ form (oxidized form) of cytochrome a3 present before molecular oxygen , accepting the electrons & thus, preventing their further transfer to molecular oxygen.

* After cyanide poisoning, the electron transport chain will not be able to transfer electrons to intermembrane space, increasing the pH of intermembrane space but reducing the pH gradient & thus stopping the synthesis of ATP by ATP synthase, which uses chemiosmotic mechanism for ATP synthesis.

* So, some oragnisms have alternative pathway for flow of electrons from NADH directly to oxygen. In this pathway , complex I passes electrons from NADH to ubiquinone, ubiquionone transfers electrons directly to oxygen through a special mediator, alternative oxidase. This pathway does not involve H+ pumping by complex III & IV , as transfer of electrons does not involve complex IV , NADH oxidation by this pathway is cyanide resistant.

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