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&ndash, 15. PMID 10766431.
  • Beinert, H. and Kiley, P.J. (1999). "Fe-S proteins in sensing and regulatory functions". Curr. Opin. Chem. Biol. 3: 152&ndash, 157. PMID 10226040.
  • Johnson, M.K. (1998). "Iron-sulfur proteins: new roles for old clusters". Curr. Opin. Chem. Biol. 2: 173&ndash, 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&ndash, 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&ndash, 273. PMID 12039013.
  • Spiro, T.G., Ed. (1982). Iron-sulfur proteins. New York: Wiley. ISBN 0-471-07738-0.

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