The bacterial SoxAX cytochromes
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Cellular and Molecular Life Sciences
REVIEW
The bacterial SoxAX cytochromes Ulrike Kappler • Megan J. Maher
Received: 25 March 2012 / Revised: 9 July 2012 / Accepted: 17 July 2012 Ó Springer Basel AG 2012
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.
Electronic supplementary material The online version of this article (doi:10.1007/s00018-012-1098-y) contains supplementary material, which is available to authorized users. U. Kappler (&) School of Chemistry and Molecular Biosciences, The University of Queensland, St. Lucia, QLD 4072, Australia e-mail: [email protected] M. J. Maher School of Molecular Sciences, La Trobe University, Melbourne, VIC 3086, Australia
Keywords SoxAX cytochromes Cytochromes Heme thiolate proteins Sulfur oxidation Crystal structure Redox centres
Introduction The so-called SoxAX cytochromes are a group of c-type cytochromes that catalyze the formation of heterodisulfide bonds between inorganic sulfur compounds and a conserved cysteine on a sulfur carrier protein [1, 2]. The reaction involves a heme group located in the SoxAX active site, which has a His/Cys axial ligation. Unlike the well-known His/Met ligated hemes present in many proand eukaryotic cytochromes, proteins containing His/Cys ligated heme groups are relatively rare in nature, and fulfill a range of special and diverse functions [3]. A well-known example of heme thiolate proteins are the cytochrome P450s that play a role in xenobiotic metabolism; however, in these proteins, the heme group is essentially five coordinate, which increases its catalytic reactivity [3, 4]. There are several examples of proteins that contain six coordinate heme thiolate groups, including human cystathionine beta synthase, an enzyme involved in the formation of cystathionine from homoc
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