IRON-SULFUR PROTEIN

'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 and four 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:

	''High-potential ferredoxins''
oxidized [Fe4S4]3+	e- <=>	reduced [Fe4S4]2+ oxidized	e- <=>	[Fe4S4]+ reduced
			''Low-potential ferredoxins''

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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References

★ Iron-sulfur proteins: ancient structures, still full of surprises, Beinert, H., , , J. Biol. Inorg. Chem., 2000

★ Fe-S proteins in sensing and regulatory functions, Beinert, H. and Kiley, P.J., , , Curr. Opin. Chem. Biol., 1999

★ Iron-sulfur proteins: new roles for old clusters, Johnson, M.K., , , Curr. Opin. Chem. Biol., 1998

★ Nomenclature of iron-sulfur proteins. Recommendations 1978, Nomenclature Committee of the International Union of Biochemistry (NC-IUB), , , Eur. J. Biochem., 1979

★ Insights into properties and energetics of iron-sulfur proteins from simple clusters to nitrogenase, Noodleman, L., Lovell, T., Liu, T., Himo, F. and Torres, R.A., , , Curr. Opin. Chem. Biol., 2002

★ Iron-sulfur proteins, Spiro, T.G., Ed., , , Wiley, 1982, ISBN 0-471-07738-0

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