Iron-sulfur proteins are proteins characterized by the presence of polymetallic systems (iron-sulfur clusters) containing sulfide ions, in which the iron ions have variable oxidation states.

The simplest polymetallic system, ^* cluster, is constituted by two iron ions bridged by two sulfide ions and coordinated by four cysteinyl ligands (in Fe₂S₂ ferredoxins) or by two cysteines and two histidines (in Rieske proteins). The oxidized proteins contain two Fe³⁺ ions, whereas the reduced proteins contain one Fe³⁺ and one Fe²⁺ ion.

Another common polymetallic system, cluster, consists of four iron ions and four sulfide ions placed at the vertices of a cubane-type structure, coordinated by four cysteinyl ligands. The ^* ferredoxins) may be further subdivided into low-potential (bacterial-type) and high-potential (HiPIP) ferredoxins.

Low- and high-potential ferredoxins are related by the following redox scheme:

	High-potential ferredoxins
oxidized *3+	e- <=>	reduced *2+ oxidized	e- <=>	*+ reduced
			Low-potential ferredoxins

The formal oxidation numbers of the iron ions can be 2Fe²⁺ or 3Fe²⁺ in low-potential ferredoxins. The oxidation numbers of the iron ions in high-potential ferredoxins can be 1Fe²⁺ or 2Fe²⁺.

There also are proteins containing an centre, in which one iron is missing from the 3S₄" target="_blank" >^*+" target="_blank" >(all-Fe³⁺ form) to 4S₄" target="_blank" >^*" target="_blank" >cluster can be reversibly converted by oxidation and loss of one iron ion to a aconitase possesses an [Fe₃S₄" target="_blank" >^* and is activated by addition of Fe²⁺ and reductant.

More complex polymetallic systems are found in nitrogenase and hydrogenase.

References

Cluster chemistry | Iron-sulfur proteins