The Small Subunit AroB of Arsenite Oxidase: LESSONS ON THE [2Fe-2S] RIESKE PROTEIN SUPERFAMILY |
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Authors: | Simon Duval Joanne M. Santini Wolfgang Nitschke Russ Hille Barbara Schoepp-Cothenet |
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Affiliation: | From the ‡Laboratoire de Bioénergétique et Ingénierie des Protéines UPR 9036, Institut de Biologie Structurale et Microbiologie, CNRS, F-13402 Marseille Cedex 20, France.;the §Institute of Structural and Molecular Biology, University College London, London WCIE 6BT, United Kingdom, and ;the ¶Department of Biochemistry, University of California, Riverside, California 92521 |
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Abstract: | Here, we describe the characterization of the [2Fe-2S] clusters of arsenite oxidases from Rhizobium sp. NT-26 and Ralstonia sp. 22. Both reduced Rieske proteins feature EPR signals similar to their homologs from Rieske-cyt b complexes, with g values at 2.027, 1.88, and 1.77. Redox titrations in a range of pH values showed that both [2Fe-2S] centers have constant Em values up to pH 8 at ∼+210 mV. Above this pH value, the Em values of both centers are pH-dependent, similar to what is observed for the Rieske-cyt b complexes. The redox properties of these two proteins, together with the low Em value (+160 mV) of the Alcaligenes faecalis arsenite oxidase Rieske (confirmed herein), are in line with the structural determinants observed in the primary sequences, which have previously been deduced from the study of Rieske-cyt b complexes. Since the published Em value of the Chloroflexus aurantiacus Rieske (+100 mV) is in conflict with this sequence analysis, we re-analyzed membrane samples of this organism and obtain a new value (+200 mV). Arsenite oxidase activity was affected by quinols and quinol analogs, which is similar to what is found with the Rieske-cyt b complexes. Together, these results show that the Rieske protein of arsenite oxidase shares numerous properties with its counterpart in the Rieske-cyt b complex. However, two cysteine residues, strictly conserved in the Rieske-cyt b-Rieske and considered to be crucial for its function, are not conserved in the arsenite oxidase counterpart. We discuss the role of these residues. |
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Keywords: | Bioenergetics Electron Transfer Enzyme Structure Evolution Iron-Sulfur Protein EPR Rieske Protein Arsenite Oxidase Bc1 Complex Redox Potentials |
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