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 (gmax = 3.45) and two "standard" cytochrome-like low-spin heme species with closely spaced g-tensor values were identified, LS1 (gz = 2.54, gy = 2.30, and gx = 1.87) and LS2 (gz = 2.43, gy = 2.26, and gx = 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: Em3 = +189 +/- 15 mV and Em2 = -432 +/- 15 mV (vs NHE, pH 7.0). Heme 1 was not reducible even with 20 mM titanium(III) citrate. The Em2 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.
Reijerse
Edward J
EJ
Max-Planck-Institut für Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mülheim/Ruhr, Germany. reijerse@mpi-muelheim.mpg.de
Sommerhalter
Monika
M
Hellwig
Petra
P
Quentmeier
Armin
A
Rother
Dagmar
D
Laurich
Christoph
C
Bothe
Eberhard
E
Lubitz
Wolfgang
W
Friedrich
Cornelius G
CG
D001426:Bacterial Proteins / Q000737:chemistry / Q000502:physiology*; D003574:Cytochrome c Group / Q000737:chemistry / Q000502:physiology*; D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy; D006418:Heme / Q000737:chemistry*; D008956:Models, Chemical; D042084:Paracoccus pantotrophus / Q000201:enzymology*; D013056:Spectrophotometry, Ultraviolet; D013885:Thiosulfates / Q000378:metabolism
D001426:Bacterial Proteins; D003574:Cytochrome c Group; C413327:SoxX protein, Paracoccus denitrificans; D013885:Thiosulfates; D006418:Heme