UPSC Civil Services (Mains), Chemistry optional · 2026 · Paper I

Q5(e). Ferredoxins and their classification

Question

What is ferredoxin? How is it classified?

Small iron–sulfur proteins that carry one electron at a time at negative potential; classified by their cluster: [2Fe–2S], [4Fe–4S], [3Fe–4S].

an electron-transfer protein with non-haem iron, each Fe tetrahedral, held by cysteine S and bridging sulfide. The sulfide is acid-labile (dilute acid gives H₂S). Each cluster transfers one electron at a time; in reduced…
[2Fe–2S] (plant type, chloroplasts): Fe₂S₂(S-Cys)₄, two Fe bridged by two sulfides; 2+/1+ couple.
[4Fe–4S] (bacterial type): Fe₄S₄(S-Cys)₄, a distorted cubane with Fe and S at alternate corners. Some carry two clusters (“8Fe” ferredoxins).
[3Fe–4S]: a cubane with one Fe missing. Related: HiPIP, [4Fe–4S] working at the 3+/2+ level (+0.35 V). Rubredoxin, Fe(S-Cys)₄, has no sulfide and is not a ferredoxin.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

What it is: an electron-transfer protein with non-haem iron, each Fe tetrahedral, held by cysteine S and bridging sulfide. The sulfide is acid-labile (dilute acid gives H₂S). Each cluster transfers one electron at a time; in reduced [4Fe–4S] the added electron is delocalised over the cube, in reduced [2Fe–2S]⁺ it sits on one Fe (one Fe(II), one Fe(III)).
[2Fe–2S] (plant type, chloroplasts): Fe₂S₂(S-Cys)₄, two Fe bridged by two sulfides; 2+/1+ couple.
[4Fe–4S] (bacterial type): Fe₄S₄(S-Cys)₄, a distorted cubane with Fe and S at alternate corners. Some carry two clusters (“8Fe” ferredoxins).
[3Fe–4S]: a cubane with one Fe missing. Related: HiPIP, [4Fe–4S] working at the 3+/2+ level (+0.35 V). Rubredoxin, Fe(S-Cys)₄, has no sulfide and is not a ferredoxin.

Function: electron carriers in photosynthesis (photosystem I → NADP⁺) and nitrogen fixation (to nitrogenase).

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