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J. Biol. Chem., Vol. 279, Issue 8, 6252-6260, February 20, 2004

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Cytochrome c₅₅₁ from Starkeya novella

CHARACTERIZATION, SPECTROSCOPIC PROPERTIES, AND PHYLOGENY OF A DIHEME PROTEIN OF THE SoxAX FAMILY^*

Ulrike Kappler, Kondo-Francois Aguey-Zinsou¶, Graeme R. Hanson||, Paul V. Bernhardt¶, and Alastair G. McEwan

From the Department of Microbiology and Parasitology and ^¶Department of Chemistry, School of Molecular and Microbial Sciences, and the ^||Centre for Magnetic Resonance, University of Queensland, Brisbane Qld 4072, Australia

Cytochromes from the SoxAX family have a major role in thiosulfate oxidation via the thiosulfate-oxidizing multi-enzyme system (TOMES). Previously characterized SoxAX proteins from Rhodovulum sulfidophilum and Paracoccus pantotrophus contain three heme c groups, two of which are located on the SoxA subunit. In contrast, the SoxAX protein purified from Starkeya novella was found to contain only two heme groups. Mass spectrometry showed that a disulfide bond replaced the second heme group found in the diheme SoxA subunits. Apparent molecular masses of 27,229 ± 10.3 Da and 20,258.6 ± 1 Da were determined for SoxA and SoxX with an overall mass of 49.7 kDa, indicating a heterodimeric structure. Optical redox potentiometry found that the two heme cofactors are reduced at similar potentials (versus NHE) that are as follows: +133 mV (pH 6.0); +104 mV (pH 7.0); +49 (pH 7.9) and +10 mV (pH 8.7). EPR spectroscopy revealed that both ferric heme groups are in the low spin state, and the spectra were consistent with one heme having a His/Cys axial ligation and the other having a His/Met axial ligation. The His/Cys ligated heme is present in different conformational states and gives rise to three distinct signals. Amino acid sequencing was used to unambiguously assign the protein to the encoding genes, soxAX, which are part of a complete sox gene cluster found in S. novella. Phylogenetic analysis of soxA- and soxX-related gene sequences indicates a parallel development of SoxA and SoxX, with the diheme and monoheme SoxA sequences located on clearly separated branches of a phylogenetic tree.

Received for publication, September 26, 2003 , and in revised form, November 7, 2003.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) AF139113.

^* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains redox potentiometry data as supplemental material.

Supported by the University of Queensland. To whom correspondence should be addressed. Tel.: 61-7-3365-1892; Fax: 61-7-3365-4620; E-mail: u.kappler{at}uq.edu.au.

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