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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, [Fe₂S₂] 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, [Fe₄S₄] cluster, consists of four iron ions, two chloride ions sulfide ions placed at the vertices of a cubane-type structure, coordinated by four cysteinyl ligands. The [Fe₄S₄] electron-transfer proteins ([Fe₄S₄] 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:

The formal oxidation numbers of the iron ions can be [2Fe³⁺, 2Fe²⁺] or [1Fe³⁺, 3Fe²⁺] in low-potential ferredoxins. The oxidation numbers of the iron ions in high-potential ferredoxins can be [3Fe³⁺, 1Fe²⁺] or [2Fe³⁺, 2Fe²⁺].

There also are proteins containing an [Fe₃S₄] centre, in which one iron is missing from the [Fe₄S₄] core. Three sulfide ions bridge two iron ions each, while the fourth sulfide bridges three iron ions. Their formal oxidation states may vary from [Fe₃S₄]⁺ (all-Fe³⁺ form) to [Fe₃S₄]^2- (all-Fe²⁺ form). In a number of iron-sulfur proteins, the [Fe₄S₄] cluster can be reversibly converted by oxidation and loss of one iron ion to a [Fe₃S₄] cluster. E.g., the inactive form of aconitase possesses an [Fe₃S₄] and is activated by addition of Fe²⁺ and reductant.

More complex polymetallic systems are found in nitrogenase and hydrogenase.

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• MeSH Iron-Sulfur+Proteins