Ferredoxins [<cite idref="PUB00003397"/>] are iron-sulphur proteins that mediate electron transfer in a range of metabolic reactions. The proteins fall into several subgroups according to the nature of their iron-sulphur cluster(s). The 7Fe ferredoxins [<cite idref="PUB00000597"/>, <cite idref="PUB00003240"/>] contain both 4Fe-4S and 3Fe-4S centres. The 4Fe-4S domain is similar to those found in other bacterial-type ferredoxins.The 3D structure of the 7Fe ferredoxin from <taxon tax_id="354">Azotobacter vinelandii</taxon> has been determined to 1.9 A resolution [<cite idref="PUB00003240"/>]. The fold belongs to the alpha + beta class, with 3 helices and 4 strands forming a barrel-like structure, and an extruded loop containing 3 of the 4 cysteinyl residues of the iron-sulphur cluster. 7Fe ferredoxin