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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2016 · Paper I

Q7(b). Ferredoxins: structure and role in photosynthesis

  • 15 marks
  • Answered
Question

Describe the important structural features of “Ferredoxin molecule”. What is its role in photosynthesis?

Answer in brief

Ferredoxins are small non-haem iron–sulfur proteins whose iron atoms are each tetrahedrally bound by cysteine thiolate S and bridging, acid-labile sulfide, in [2Fe–2S] (plant), [4Fe–4S] or [3Fe–4S] clusters. They carry one electron at a time at very negative potential. In photosynthesis reduced ferredoxin takes electrons from photosystem I and, through ferredoxin–NADP⁺ reductase, reduces NADP⁺ to NADPH.

Key points

  • Small proteins with non-haem iron; no porphyrin. The cluster lies near the surface, which helps fast electron transfer.
  • cysteine thiolate groups of the protein and inorganic sulfide ions that bridge the Fe atoms. Dilute acid releases the sulfide as H₂S (“acid-labile sulfur”).
  • [2Fe–2S] (chloroplasts, plant type): Fe₂S₂(S-Cys)₄, two Fe bridged by two sulfides, each Fe with two Cys. [4Fe–4S] (bacterial type): Fe₄S₄(S-Cys)₄, a distorted cubane with Fe and S at alternate corners; some bacterial…
  • Thiolate and sulfide are weak-field ligands in a tetrahedral field, so each Fe is high spin; the Fe atoms are coupled antiferromagnetically through the sulfide bridges (oxidised [2Fe–2S]²⁺ is diamagnetic, S=0; the reduced…

The full model answer

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

Structural features

  1. Small proteins with non-haem iron; no porphyrin. The cluster lies near the surface, which helps fast electron transfer.
  2. Every Fe is tetrahedral, bound by S only: cysteine thiolate groups of the protein and inorganic sulfide ions that bridge the Fe atoms. Dilute acid releases the sulfide as H₂S (“acid-labile sulfur”).
  3. [2Fe–2S] (chloroplasts, plant type): Fe₂S₂(S-Cys)₄, two Fe bridged by two sulfides, each Fe with two Cys. [4Fe–4S] (bacterial type): Fe₄S₄(S-Cys)₄, a distorted cubane with Fe and S at alternate corners; some bacterial ferredoxins carry two such clusters. [3Fe–4S]: a cube with one Fe missing.
  4. Thiolate and sulfide are weak-field ligands in a tetrahedral field, so each Fe is high spin; the Fe atoms are coupled antiferromagnetically through the sulfide bridges (oxidised [2Fe–2S]²⁺ is diamagnetic, S=0; the reduced form, S=12, gives an EPR signal).
  5. Each cluster transfers one electron at a time, although it contains several Fe:
ClusterOxidisedReducedE°
[2Fe–2S][2Fe–2S]²⁺: 2 Fe(III)[2Fe–2S]⁺: Fe(III) + Fe(II)0 to -0.4 V
[4Fe–4S][4Fe–4S]²⁺: 2 Fe(III) + 2 Fe(II)[4Fe–4S]⁺: Fe(III) + 3 Fe(II)-0.2 to -0.7 V
[3Fe–4S][3Fe–4S]⁺: 3 Fe(III)[3Fe–4S]⁰: 2 Fe(III) + Fe(II)-0.1 to -0.4 V
  1. The added electron is delocalised over the cluster, so bond lengths hardly change and the reorganisation energy is small: electron transfer is fast. The protein (hydrogen bonds to S, burial depth) tunes the potential; the same cubane works at +0.35 V in HiPIP (3+/2+ couple).
  2. Ferredoxins have negative reduction potentials, more negative than most cytochromes: reduced ferredoxin is a strong biological reducing agent.

Role in photosynthesis

CHAINH₂O → PS II → PQ → cyt b₆f → PC → PS I → Fd → FNR → NADP⁺

  1. Light excites P700 of photosystem I; the electron passes through Fe–S centres of PS I to soluble ferredoxin (a [2Fe–2S] protein) in the stroma.
  2. Reduced ferredoxin hands its electron to ferredoxin–NADP⁺ reductase (FNR, a flavoprotein). FNR collects two electrons and reduces NADP⁺ to NADPH, which then reduces CO₂ to carbohydrate in the Calvin cycle.
  3. Cyclic electron flow: ferredoxin can instead return electrons to the cytochrome b₆f complex, pumping protons and making extra ATP without NADPH.

Application. reduced ferredoxin also supplies electrons to nitrite reductase, sulfite reductase and, in nitrogen-fixing bacteria, nitrogenase.

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Model answer to Q7(b), 2016 Paper I: solution page 38
Solution page 38 of 46
Read the whole 2016 solution2016 question paper

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