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. Upon the reduction of Fe3+ to Fe2+, the four Fe-S bond lengths increase and the amide-NH H-bonding to the S(Cys) become shortened. 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. A gating mechanism involving the conformational change of the Leu41’s non-polar sidechain further stabilizes the Fe2+ oxidation state. A site-directed mutagenesis of Leu41 to Alanine shows a 50mV shift of the Fe3+/2+redox potential. The substitution of the smaller CH3 shows that the Leu41 side chain stabilizes the Fe2+ oxidation state more than the Fe3+ oxidation state. The X-ray structure in the reduced Fe2+ state shows the Leu41 side chain adopting two different conformations with 40% in a "open conformation" and 60% in a "closed conformation". The Leu41’s non-polar side chain controls access to the redox site by adopting either an open or closed conformation. In the reduced Fe2+ state, the Leu41 side-chain faces away from Cys 9 Sγ, exposing the Cys 9 Sγ and increasing the polarity of the Fe3+ /Fe2+ center. [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. As a result, water is able to penetrate and form H-bonds with the Cys 9 Sγ thiolate that blocks the gate from closing, resulting in an open conformation. In contrast, the oxidized Fe3+ state produces a less negatively charged Cys 9 Sγ-donor that does not attract the water strongly. Without H-bonding of the water to the Cys 9 Sγ, the gate remains closed. Thus, the conformation of Leu41 is determined by the presence of water and the oxidation state of rubredoxin. The proximity of water to the [Fe(S-Cys)4] 2- active site stabilizes the higher net negative charge of the Fe2+ oxidation state. The stabilization of the Fe2+ oxidation state shifts the reduction potential to a more positive E0 value. See also Bioinorganic chemistry Iron-sulfur protein Ferredoxin Cytochrome Rieske protein References Further reading External links - X-ray structure of rubredoxin from Clostridium pasteurianum - neutron diffraction structure of rubredoxin from Pyrococcus furiosus - InterPro entry for rubredoxin A Little Iron-Sulfur Protein Iron–sulfur proteins Metalloproteins Cofactors