Rubredoxin-type Fe(Cys)4 protein Rubredoxin is a low molecular weight iron-containing bacterial protein involved in electron transfer [<cite idref="PUB00000489"/>, <cite idref="PUB00003266"/>], sometimesreplacing ferredoxin as an electron carrier [<cite idref="PUB00008022"/>].<p>The 3-D structures of a number of rubredoxins have been solved [<cite idref="PUB00008023"/>, <cite idref="PUB00008024"/>]. The fold belongs to the alpha+beta class, with 2 alpha-helices and 2-3 beta-strands. Its active site contains an iron ion which is co-ordinated by the sulphurs of four conserved cysteine residues forming analmost regular tetrahedron. The conserved cysteines reside on two loops, which are the most conserved regions of the protein. In addition, a ring of acidic residues in the proximity of the [Fe(Cys)4] centre is also well-conserved [<cite idref="PUB00008024"/>]. </p>