<p>Ferredoxins are iron-sulphur proteins that mediate electron transfer in a range of metabolic reactions [<cite idref="PUB00000122"/>,<cite idref="PUB00003406"/>]; they fall into several subgroups according to the nature of their iron-sulphur cluster(s). One group,originally found in chloroplast membranes, has been termed 'chloroplast-type' or 'plant-type', and includes ferredoxinsfrom plants, algae, archaea, rhodobacter and a toluene degrading pseudomonas. Here, the active centre is a2Fe-2S cluster, where the irons are tetrahedrally coordinated by both inorganic sulphurs and sulphurs provided by 4conserved Cys residues [<cite idref="PUB00000345"/>]. In chloroplasts, 2Fe-2S ferredoxins function as electron carriers in thephotosynthetic electron transport chain and as electron donors to various cellular proteins [<cite idref="PUB00003104"/>]. Inhydroxylating bacterial dioxygenase systems, they serve as intermediate electron-transfer carriers betweenreductase flavoproteins and oxygenase [<cite idref="PUB00000122"/>].</p><p>Several oxidoreductases contain redox domains similar to 2Fe-2S ferredoxins, including ferredoxin/ferredoxin reductase components of several bacterial aromatic di- and monooxygenases, phenol hydroxylase, methane monooxygenase, vanillate demethylase oxidoreductase, phthalate dioxygenase reductase, bacterial fumarate reductase iron-sulphur protein, eukaryotic succinate dehydrogenase and xanthine dehydrogenase. 3D structures are known for a number of 2Fe-2S ferredoxins [<cite idref="PUB00000345"/>] and for the ferredoxin reductase/ferredoxin fusion protein phthalate dioxygenase reductase [<cite idref="PUB00005157"/>]. 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.</p><p>In the 2Fe-2S ferredoxins, four cysteine residues bind the iron-sulphur cluster. Three of these cysteines are clustered together in the same region of the protein. This sequence cover the three cysteine residues involved in iron-sulphur binding.</p>