The unusal redox centers of SoxXA, a novel c-type heme-enzyme essential for chemotrophic sulfur-oxidation of Paracoccus pantotrophus

The heterodimeric hemoprotein SoxXA, essential for lithotrophic sulfur oxidation of the aerobic bacterium Paracoccus pantotrophus, was examined by a combination of spectroelectrochemistry and EPR spectroscopy. The EPR spectra for SoxXA showed contributions from three paramagnetic heme iron centers. One highly anisotropic low-spin (HALS) species (g(max) = 3.45) and two standard cytochrome-like low-spin heme species with closely spaced g-tensor values were identified, LS1 (g(z) = 2.54, g(y) = 2.30, and g(x) = 1.87) and LS2 (g(z) = 2.43, g(y) = 2.26, and g(x) = 1.90). The crystal structure of SoxXA from P. pantotrophus confirmed the presence of three heme groups, one of which (heme 3) has a His/Met axial coordination and is located on the SoxX subunit [Dambe et al. (2005) J. Struct. Biol. 152, 229-234]. This heme was assigned to the HALS species in the EPR spectra of the isolated SoxX subunit. The LS1 and LS2 species were associated with heme 1 and heme 2 located on the SoxA subunit, both of which have EPR parameters characteristic for an axial His/thiolate coordination. Using thin-layer spectroelectrochemistry the midpoint potentials of heme 3 and heme 2 were determined: E-m3 = +189 +/- 15 mV and E-m2 = -432 +/- 15 mV (vs NHE, pH 7.0). Heme 1 was not reducible even with 20 mM titanium(III) citrate. The E-m2 midpoint potential turned out to be pH dependent. It is proposed that heme 2 participates in the catalysis and that the cysteine persulfide ligation leads to the unusually low redox potential (-436 mV). The pH dependence of its redox potential may be due to (de) protonation of the Arg247 residue located in the active site.