Spectroscopic Characterization of a Novel Tetranuclear Fe Cluster in an Iron-Sulfur Protein Isolated from Desulfovibrio desulfuricans

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• 作者：Pedro Tavares ; Alice S. Pereira ; Carsten Krebs ; Natarajan Ravi ; José ; J. G. Moura ; Isabel Moura ; and Boi Hanh Huynh
• 刊名：Biochemistry
• 出版年：1998
• 出版时间：March 3, 1998
• 年：1998
• 卷：37
• 期：9
• 页码：2830 - 2842
• 全文大小：157K
• 年卷期：v.37,no.9(March 3, 1998)
• ISSN：1520-4995

文摘

Mössbauer and EPR spectroscopies were used to characterizethe Fe clusters in an Fe-Sprotein isolated from Desulfovibrio desulfuricans (ATCC27774). This protein was previously thoughtto contain hexanuclear Fe clusters, but a recent X-ray crystallographicmeasurement on a similar proteinisolated from Desulfovibrio vulgaris showed that the proteincontains two tetranuclear clusters, a cubane-type [4Fe-4S] cluster and a mixed-ligand cluster of novel structure[Lindley et al. (1997) Abstract,Chemistry of Metals in Biological Systems, European ResearchConference, Tomar, Portugal]. Threeprotein samples poised at different redox potentials (as-purified, 40and 320 mV) were investigated. Inall three samples, the [4Fe-4S] cluster was found to be present inthe diamagnetic 2+ oxidation stateand exhibited typical Mössbauer spectra. The novel-structurecluster was found to be redox active. Inthe 320-mV and as-purified samples, the cluster is at a redoxequilibrium between its fully oxidized andone-electron reduced states. In the 40-mV sample, the cluster isin a two-electron reduced state. Distinctspectral components associated with the four Fe sites of cluster 2 inthe three oxidation states were identified.The spectroscopic parameters obtained for the Fe sites reflectdifferent ligand environments, making itpossible to assign the spectral components to individual Fe sites.In the fully oxidized state, all four ironions are high-spin ferric and antiferromagnetically coupled to form adiamagnetic S = 0 state. In theone-electron and two-electron reduced states, the reducing electronswere found to localize, consecutively,onto two Fe sites that are rich in oxygen/nitrogen ligands. Basedon the X-ray structure and the Mössbauerparameters, attempts could be made to identify the reduced Fe sites.For the two-electron reduced cluster,EPR and Mössbauer data indicate that the cluster is paramagneticwith a nonzero interger spin. For theone-electron reduced cluster, the data suggest a half-integer spin of⁹/₂. Characteristic fine andhyperfineparameters for all four Fe sites were obtained. Structuralimplications and the nature of the spin-couplinginteractions are discussed.