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Last modified: 23 September 1998

`Prismane' proteins
Iron-sulphur cluster Formal oxidation/spin states
I
Fe4S4 image
[Fe4S4](SgammaCys)4
 [Fe4S4]+ (quantum admixed S=3/2, S=1/2)
[Fe4S4]2+ (S=0)
II
Fe4S3O2X image
[Fe4S3O2X](SgammaCys)4(OepsilonGlu)2NepsilonHis
 [Fe4S2O2X]3+ (S=1/2);
[Fe4S2O2X]4+ (S=0, S=4);
[Fe4S2O2X]5+ (S=9/2, S=1/2);
[Fe4S2O2X]6+ (S=0)

Iron-sulphur proteins are characterised by the presence of polymetallic systems containing sulphide ions, in which the iron ions have variable oxidation states. Unusual polymetallic centres have been discovered in proteins from Desulfovibrio spp. The [Fe6S6] prismane core, proposed on the basis of the fact that the EPR spectra of these proteins are similar to those observed for the synthetic compounds containing such core, is that of two superimposed six­membered crowns with three alternating iron and sulphur ions each [1].

Four stable oxidation states (3+ to 6+) are observed in `prismane' proteins within a relatively narrow redox potential window of about 500 mV. The `prismane' protein is aerobically isolated predominantly in the one­electron­reduced 5+ state [2, 3]. It has been proposed that two iron ligands are non­cysteine type [1].

The 3­D structure of the `prismane' protein from D. vulgaris [4] has revealed that it does not contain a [Fe6S6] cluster. Instead, it contains two different clusters: a [Fe4S4] cubane (cluster I) and a novel `hybrid' cluster with an unusual mixture of bridging ligands and coordinations (cluster II). The protein comprises of three domains. Domain 1 (residues 1-221) includes two bundles, each consisting of three antiparallel alpha­helices; domains 2 and 3 (residues 223-375 and 376-553, respectively) both feature a central parallel ß­sheet with alpha­helices on either side. Clusters I and II are situated close to the interfaces of the three domains and some 12 Å apart.

Cluster I is a [Fe4S4] cubane bound near the N­terminus; the arrangement of the four ligand cysteines (Cys­3, Cys­5, Cys­15 and Cys­21) represent a novel [Fe4S4]­binding motif. As isolated, `prismane' protein contains a diamagnetic (S=0) cluster I. Mössbauer parameters suggest a delocalised mixed­valence [Fe2.5+4S4]2+ structure [5]. The dithionite­reduced [Fe4S4]+ cluster exhibits unusual magnetic properties which have been interpreted as quantum­mechanically admixed S=3/2, S=1/2 state [4].

Cluster II comprises both O and S bridges between iron atoms; X represents an unidentified ligand. Resonance Raman spectroscopy suggests that ligand X is a solvent­exchangeable oxygen. The coordination geometry of the iron in cluster II can be described as tetrahedral for Fe5 and Fe6 and trigonal bipyramidal for Fe7 and Fe8.

Fe5 and Fe6 are coordinated to one protein ligand each (Cys­434 and Cys­312, respectively) and are bridged by two S atoms, forming an [Fe2S2] unit or one face of a cubane cluster. Fe6 is also coordinated to an O atom which bridges Fe6 and Fe8 yielding another face of a cubane cluster. Fe7 is coordinated to three protein ligands (His­244, Glu­268 and Cys­459). It is also bound to an O atom which bridges Fe7 and Fe8 and to ligand X bridging Fe7 and Fe5. Fe8 is coordinated to Glu­494, two bridging oxygens and two sulphur atoms, one of which belongs to a persulphide group bound to Cys­406. The hybrid cluster behaves magnetically as a single entity resulting from the exchange coupling of four high­spin Fe ions [4].

The oxidation state nomenclature (3+, 4+, 5+, 6+), previously used for a putative [Fe6S6] cluster, has been retained and assigned to cluster II [4, Table 3]. (Note that the net charge on the cluster II at any oxidation level remains to be determined). Mössbauer analysis of the `prismane' protein in three oxidation states (4+, 5+, 6+) suggests that the redox behaviour of the protein is exclusively related with the µ­oxo bridged subcluster (Fe7-O-Fe8) of the hybrid cluster, while the oxidation state of the cluster I is not changed [5].

`Prismane' proteins in SWISS­PROT/TREMBL

Q01770 PRIS_DESDE Prismane protein; Desulfovibrio desulfuricans
P31101 PRIS_DESVH Prismane protein; Desulfovibrio vulgaris (strain Hildenborough)
P75825 PRIS_ECOLI Prismane protein homolog; Escherichia coli
Q58175 PRIS_METJA Prismane protein homolog; Methanococcus jannaschii
O27502 PRIS_METTH Prismane protein homolog; Methanobacterium thermoautotrophicum
P96095 PRIS_THIFE Prismane protein homolog; Thiobacillus ferrooxidans

`Prismane' proteins in alignment databases

Protein Family Pfam LPFC 3­D
alignment
09997; prismane
-
-

References

  1. Bertini, I., Ciurli, S. and Luchinat, C. (1995) The electronic structure of FeS centers in proteins and models. A contribution to the understanding of their electron transfer properties. Structure and Bonding 83, 1-53.
  2. Pierik, A.J., Hagen, W.R., Dunham, W.R. and Sands, R.H. (1992) Multi­frequency EPR and high­resolution Mössbauer spectroscopy of a putative [6Fe-6S] prismane­cluster­containing protein from Desulfovibrio vulgaris (Hildenborough). Characterization of a supercluster and superspin model protein. Eur. J. Biochem. 206, 705-719.
  3. Marritt, S.J., Farrar, J.A., Breton, J.L.J., Hagen, W.R. and Thomson, A.J. (1995) Characterization of the prismane protein from Desulfovibrio vulgaris (Hildenborough) by low­temperature magnetic circular dichroic spectroscopy. Eur. J. Biochem. 232, 501-505.
  4. Arendsen, A., Hadden, J., Card, G., McAlpine, A.S., Bailey, S., Zaitsev, V., Duke, E.H.M., Lindley, P., Kröckel, M., Trautwein, A.X., Feiters, M.C., Charnock, J.M., Garner, C.D., Marritt, S.J., Thomson, A.J., Kooter, I.M., Johnson, M.K., van den Berg, W.A.M., van Dongen, W.M.A.M. and Hagen, W.R. (1998) The "prismane" protein resolved: X­ray structure at 1.7 Å and multiple spectroscopy of two novel 4Fe clusters. J. Biol. Inorg. Chem. 3, 81-95.
  5. Kröckel, M., Trautwein, A.X., Arendsen, A.F. and Hagen, W.R. (1998) The prismane protein resolved: Mössbauer investigation of a 4Fe cluster with an unusual mixture of bridging ligands and metal coordinations. Eur. J. Biochem. 251, 454-461.
Bibliography on structural studies of `prismane' proteins