Novel heme ligation in a c-type cytochrome involved in thiosulfate oxidation: EPR and MCD of SoxAX from Rhodovulum sulfidophilum

The SoxAX complex of the bacterium Rhodovulum sulfidophilum is a heterodime ric c-type cytochrome that plays an essential role in photosynthetic thiosu lfate and sulfide oxidation. The three heme sites of SoxAX have been analyz ed using electronic absorption, electron paramagnetic resonance, and magnet ic circular dichroism spectroscopies. Heme-3 in the ferric state is charact erized by a Large g(max) EPR signal and has histidine and methionine axial heme iron ligands which are retained on reduction to the ferrous state. Hem es-1 and -2 both have thiolate plus nitrogenous ligand sets in the ferric s tate and give rise to rhombic EPR spectra. Heme-1, whose ligands derive fro m cysteinate and histidine residues, remains ferric in the presence of dith ionite ion. Ferric heme-2 exists with a preparation-dependent mixture of tw o different ligand sets, one being cysteinate/histidine, the other an unide ntified pair with a weaker crystal-field strength. Upon reduction of the So xAX complex with dithionite, a change occurs in the ligands of heme-2 in wh ich the thiolate is either protonated or replaced by an unidentified ligand . Sequence analysis places the histidine/methionine-coordinated heme in Sox X and the thiolate-liganded hemes in SoxA. SoxAX is the first naturally occ urring c-type cytochrome in which a thiolate-coordinated heme has been iden tified.