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This is a display of rubredoxin. Rubredoxins are small redox proteins which occur in certain bacteria and contain just one iron centre. The colour of this protein in its oxidized form is an intense red, which is the result of a sulfur to iron charge transfer transition (LMCT).
The α-helices are coloured in blue and the β-sheets are coloured in red. The iron ion is situated close to the surface of the protein in order to make electron transport fast. In the oxidized and reduced forms, the Fe ion has a charge of 3+ and 2+, respectively.
Four cysteinate ligands from two amino acid sequences -Cys-X2-Cys- ligate the iron centre in a distorted tetrahedral fashion. Unlike other Fe-S centres, rubredoxin contains just one iron centre.
This is a representation of the oxidized centre in rubredoxin. The Fe-S bond distances are labelled. The average distance is 2.28 Å This distance is shorter than that of the reduced centre, which will be displayed next.
This is a representation of the reduced centre in rubredoxin. The average Fe-S bond distance in the reduced centre is 2.31 Å This is approximately 0.03 Å longer than the average distance of the oxidized centre. The small changes in bond lengths and angles on going from Fe(III) to Fe(II) helps to make electron transfer reactions fast. These X-ray structures are obtained with a resolution of 1.8 Å Therefore the differences in bond lengths may not be significant.
This is the oxidized form of rubredoxin for you to explore further.