Rubredoxin active site contains an iron ion which is coordinated by the sulfurs of four conserved cysteine residues forming an almost regular tetrahedron.

The electron exchange rate is accurately determined by standard kinetics measurements of visible absorption (490 nm) spectra.

The electron transfer reaction of rubredoxin is carried out by a reversible Fe3+/Fe2+ redox coupling by the reduction of Fe3+ to Fe2+ and a gating mechanism caused by the conformational changes of Leu41.

The reduced Fe2+ structure of rubredoxin results in a small increase in electrostatic stabilization of the amide-NH H-bonding to the S-Cys, leading to a lower reorganizational energy that allows faster electron transfer.

[1] The lower Fe2+ cation change of the reduced state leaves a higher negative charge on the Cys 9 Sγ-donor which attracts water strongly.

The proximity of water to the [Fe(S-Cys)4] 2- active site stabilizes the higher net negative charge of the Fe2+ oxidation state.