Ferredoxins [1] are a group of iron-sulfur proteins which mediate electron transfer in a wide variety of metabolic reactions. Ferredoxins can be divided into several subgroups depending upon the physiological nature of the iron sulfur cluster(s) and according to sequence similarities. One family of ferredoxins groups together the following proteins that all bind a single 2Fe-2S iron-sulfur cluster:

• Adrenodoxin (ADX) (adrenal ferredoxin), a vertebrate mitochondrial protein which transfers electrons from adrenodoxin reductase to cytochrome P450scc, which is involved in cholesterol side chain cleavage.
• Putidaredoxin (PTX), a Pseudomonas putida protein which transfers electrons from putidaredoxin reductase to cytochrome P450-cam, which is involved in the oxidation of camphor.
• Terpredoxin [2], a Pseudomonas protein which transfers electrons from terpredoxin reductase to cytochrome P450-terp, which is involved in the oxidation of α-terpineol.
• Rhodocoxin [3], a Rhodococcus protein which transfers electrons from rhodocoxin reductase to cytochrome CYP116 (thcB), which is involved in the degradation of thiocarbamate herbicides.
• Escherichia coli ferredoxin (gene fdx) [4] whose exact function is not yet known.
• Rhodobacter capsulatus ferredoxin VI [5], which may transfer electrons to a yet uncharacterized oxygenase.
• Caulobacter crescentus ferredoxin (gene fdxB) [6].

In these proteins, four cysteine residues bind the iron-sulfur cluster. Three of these cysteines are clustered together in the same region of the protein. Our signature pattern spans that iron-sulfur binding region.