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The bacterial SoxAX cytochromes

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Abstract

SoxAX cytochromes are heme-thiolate proteins that play a key role in bacterial thiosulfate oxidation, where they initiate the reaction cycle of a multi-enzyme complex by catalyzing the attachment of sulfur substrates such as thiosulfate to a conserved cysteine present in a carrier protein. SoxAX proteins have a wide phylogenetic distribution and form a family with at least three distinct types of SoxAX protein. The types of SoxAX cytochromes differ in terms of the number of heme groups present in the proteins (there are diheme and triheme versions) as well as in their subunit structure. While two of the SoxAX protein types are heterodimers, the third group contains an additional subunit, SoxK, that stabilizes the complex of the SoxA and SoxX proteins. Crystal structures are available for representatives of the two heterodimeric SoxAX protein types and both of these have shown that the cysteine ligand to the SoxA active site heme carries a modification to a cysteine persulfide that implicates this ligand in catalysis. EPR studies of SoxAX proteins have also revealed a high complexity of heme dependent signals associated with this active site heme; however, the exact mechanism of catalysis is still unclear at present, as is the exact number and types of redox centres involved in the reaction.

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Acknowledgments

This work was supported by a fellowship (Australian Research Fellowship, DP0878525) from the Australian Research Council to UK. MJM is supported by a La Trobe Institute for Molecular Science Senior Research Fellowship.

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Authors and Affiliations

  1. School of Chemistry and Molecular Biosciences, The University of Queensland, St. Lucia, QLD, 4072, Australia

    Ulrike Kappler

  2. School of Molecular Sciences, La Trobe University, Melbourne, VIC, 3086, Australia

    Megan J. Maher

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Correspondence to Ulrike Kappler.

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18_2012_1098_MOESM1_ESM.pdf

Figure S1 Phylogenetic relationship of SoxA proteins. Two hundred sixteen SoxA amino acid sequences were analyzed using Mega 5.0. The tree shown was generated using the Neighbor-joining algorithm (Poisson model; uniform rate of evolution for all sites; gap treatment: pairwise-deletion; robustness testing: bootstrap method with 500 resampling cycles). This figure shows a tree that is identical to that shown in Fig. 3 but all branches are fully expanded. (PDF 58 kb)

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Kappler, U., Maher, M.J. The bacterial SoxAX cytochromes. Cell. Mol. Life Sci. 70, 977–992 (2013). https://doi.org/10.1007/s00018-012-1098-y

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