Ferredoxins are a group of iron-sulphur 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-sulphur cluster(s). One of these subgroups are the 4Fe-4S ferredoxins, which are found in bacteria and which are thus often referred as 'bacterial-type' ferredoxins. The structure of these proteins [1] consists of the duplication of a domain of twenty six amino acid residues; each of these domains contains four cysteine residues that bind to a 4Fe-4S centre.

Several structures of the 4Fe-4S ferredoxin domain have been determined [2]. The clusters consist of two interleaved 4Fe- and 4S-tetrahedra forming a cubane-like structure, in such a way that the four iron occupy the eight corners of a distorted cube. Each 4Fe-4S is attached to the polypeptide chain by four covalent Fe-S bonds involving cysteine residues.

A number of proteins have been found [3] that include one or more 4Fe-4S binding domains similar to those of bacterial-type ferredoxins.

The pattern of cysteine residues in the iron-sulphur region is sufficient to detect this class of 4Fe-4S binding proteins. This entry represents the whole domain.

Note:In some bacterial ferredoxins, one of the two duplicated domains has lost one or more of the four conserved cysteines. The consequence of such variations is that these domains have either lost their iron-sulphur binding property or bind to a 3Fe-3S centre instead of a 4Fe-4S centre.

1. Examination of protein sequence homologies: IV. Twenty-seven bacterial ferredoxins. J. Mol. Evol. 26, 257-67
2. Refined crystal structure of the 2[4Fe-4S] ferredoxin from Clostridium acidurici at 1.84 A resolution. J. Mol. Biol. 243, 683-95
3. Recent developments in the field of iron-sulfur proteins. FASEB J. 4, 2483-91

Ferredoxins are a group of iron-sulphur proteins which mediate electron transfer in a wide variet
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