Electron Transfer Proteins

Rubredoxin, [1Fe0S]
Rubredoxins (such as that from clostridium pasteurianum, shown) are the simplest FeS proteins, consisting of a single [1Fe0S] unit, with one tetrahedral iron atom bound by four cysS groups. The geometry changes very little between the Fe(III) and Fe(II) forms.

Ferredoxin, [2Fe2S]
This field horsetail ferredoxin contains a [2Fe2S] unit, with a tetrahedral coordination at each Fe completed by two cysS groups. The FeS cluster contains 2 Fe(III) ions and zero unpaired electrons when oxidized, 1 Fe(III) and 1 Fe(II) with a single unpaired electron when reduced.

Ferredoxin, [4Fe4S]
This ferredoxin from bacillus thermoproteolyticus contains a [4Fe4S] unit, with tetrahedral coordination at each Fe being completed by one cysS group. The FeS cluster contains 2 Fe(III) and 2 Fe(II) ions when oxidized, 1 Fe(III) and 3 Fe(II) when reduced.

Cytochrome c
Cytochrome c is typical of heme-containing electron transfer proteins. In addition to the porphyrin ring, the iron centre is bound by two axial ligands, one NHis and one SMet, giving octahedral coordination. As in iron-sulfur proteins, the two iron oxidation states are Fe(III) d5 and Fe(II) d6, both low spin. The transfer of a non-bonding t2g electron doesn't dramatically affect the coordination geometry of the iron atom, making for kinetically facile electron transfer. Different cytochromes have different axial ligands (often replacing the SMet with a second NHis, and this tunes the redox potential in a range of about 0 to +400 mV.

Poplar plastocyanin exhibits a copper ion geometry described alternatively as a "distorted" or "flattened" tetrahedron, or as a trigonal plane with a long axial group. The coordination environment is formed by two hisN, a short bond to cysS, and a long bond to metS. The same geometry is maintained for both the oxidized Cu(II) d9 ion (which prefers square planar coordination) and the reduced Cu(I) d10 ion (which prefers tetrahedral), thereby facilitating the electron transfer. Plastocyanins have redox potentials near +370 mV

Azurins have an even longer Cu-SMet interaction, and an additional weak axial interaction to the carbonyl oxygen of a glycine residue - the two "axial" groups are highly distorted from the perpendicular. The glycine interaction is principally electrostatic, with little true covalent bond character. The additional interaction helps stabilize the Cu(II) form, lowering the redox potential to around +300 mV.

Plantacyanins (aka "basic blue proteins") and Pseudoazurins
Plantacyanins and pseudoazurins have the same copper ligation as plastocyanins, but the SMet bond is shorter and skewed from a pefectly axial position relative to the trigonal plane of the other groups. Redox potentials for planatacyanins range from +300 to 350 mV; those of pseudoazurins are near +265 mV.

The unusual axial ligand in cucumber stellacyanin is a glutamine oxygen, promoting a low redox potential of +260 mV.

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