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 heterodimeric c-type cytochrome that plays an essential role in photosynthetic thiosulfate and sulfide oxidation. The three heme sites of SoxAX have been analyzed using electronic absorption, electron paramagnetic resonance, and magnetic circular dichroism spectroscopies. Heme-3 in the ferric state is characterized 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. Hemes-1 and -2 both have thiolate plus nitrogenous ligand sets in the ferric state and give rise to rhombic EPR spectra. Heme-1, whose ligands derive from cysteinate and histidine residues, remains ferric in the presence of dithionite ion. Ferric heme-2 exists with a preparation-dependent mixture of two different ligand sets, one being cysteinate/histidine, the other an unidentified pair with a weaker crystal-field strength. Upon reduction of the SoxAX complex with dithionite, a change occurs in the ligands of heme-2 in which the thiolate is either protonated or replaced by an unidentified ligand. Sequence analysis places the histidine/methionine-coordinated heme in SoxX and the thiolate-liganded hemes in SoxA. SoxAX is the first naturally occurring c-type cytochrome in which a thiolate-coordinated heme has been identified.