2Fe-2S ferredoxin core, molecular model. Ferredoxin electron transfer proteins are widespread among organisms and versatile in their redox chemistry. The active sites of these enzymes are clusters of iron (orange) and sulphur (yellow) atoms, usually either [2Fe-2S] or [4Fe-4S]. In either case, the cluster is bonded to four cysteine residues resulting in FeS4 tetrahedra. Shown here is the [2Fe-2S] cluster structure, paired with putidaredoxin, a small bacterial ferredoxin.