Iron-sulfur protein

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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 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.

References

• Beinert, H. (2000). "Iron-sulfur proteins: ancient structures, still full of surprises". J. Biol. Inorg. Chem. 5: 2–15. PMID 10766431.
• Beinert, H. and Kiley, P.J. (1999). "Fe-S proteins in sensing and regulatory functions". Curr. Opin. Chem. Biol. 3: 152–157. PMID 10226040.
• Johnson, M.K. (1998). "Iron-sulfur proteins: new roles for old clusters". Curr. Opin. Chem. Biol. 2: 173–181. PMID 9667933.
• Nomenclature Committee of the International Union of Biochemistry (NC-IUB) (1979). "Nomenclature of iron-sulfur proteins. Recommendations 1978". Eur. J. Biochem. 93: 427–430. PMID 421685.
• Noodleman, L., Lovell, T., Liu, T., Himo, F. and Torres, R.A. (2002). "Insights into properties and energetics of iron-sulfur proteins from simple clusters to nitrogenase". Curr. Opin. Chem. Biol. 6: 259–273. PMID 12039013.
• Spiro, T.G., Ed. (1982). Iron-sulfur proteins. New York: Wiley. ISBN 0-471-07738-0. 

External links

• MeSH Iron-Sulfur+Proteins

v • d • e Carrier proteins, metalloproteins: iron-binding proteins
heme	Ferritin (Bacterioferritin) - Lactoferrin - Transferrin
nonheme	Hemerythrin - Inositol oxygenase - Iron-sulfur protein - Lipoxygenase - Tyrosine hydroxylase