Q7(b). Active sites of [2Fe–2S] and [4Fe–4S] ferredoxins

Structures

1. Ferredoxins are non-haem iron–sulfur proteins. Every Fe sits in a tetrahedron of S atoms: cysteine thiolate (Cys-S⁻) from the protein and inorganic sulfide (S²⁻, released as H₂S by acid, “acid-labile sulfur”).
2. [2Fe–2S]: Fe₂S₂(S-Cys)₄. Two Fe and two bridging μ-S²⁻ form a planar rhomb; each Fe carries two terminal cysteine S. Found in plant (spinach) ferredoxins.
3. [4Fe–4S]: Fe₄S₄(S-Cys)₄. Four Fe and four μ₃-S²⁻ occupy alternate corners of a distorted cube (two interpenetrating tetrahedra, a cubane); each Fe has one terminal cysteine. Found in bacterial ferredoxins, often two clusters per protein.

Oxidation states in [2Fe–2S]

1. With sulfide as -2 and the cysteines counted separately, the core [Fe₂S₂] carries the charge. Oxidised [Fe₂S₂]²⁺: 2x+2(-2)=+2 ⇒ x=+3: two Fe(III)
2. Reduced [Fe₂S₂]⁺ (one electron added): 2x-4=+1, average +2.5: one Fe(III) + one Fe(II)
3. In the reduced plant ferredoxin the extra electron is localised on one iron (trapped valence: distinct Fe(III) and Fe(II) in the Mössbauer spectrum).
4. Both Fe are high spin (weak-field, tetrahedral S donors): Fe(III) d⁵, S=52; Fe(II) d⁶, S=2. They are coupled antiferromagnetically through the sulfide bridges: oxidised form S=0 (diamagnetic, no EPR signal); reduced form S=52-2=12, EPR active.
5. The couple works at a strongly negative potential, about -0.4 V. [4Fe–4S] ferredoxins use the [Fe₄S₄]²⁺/[Fe₄S₄]⁺ couple (2Fe(III) 2Fe(II) / Fe(III) 3Fe(II)), with the electron delocalised.

Application. ferredoxins carry electrons from photosystem I to NADP⁺ in photosynthesis and supply electrons to nitrogenase in nitrogen fixation; iron–sulfur clusters also carry electrons in complexes I–III of the respiratory chain.