C-type diheme cytochrome, which is part of the SoxAX cytochrome complex involved in sulfur oxidation. The SoxAX complex catalyzes the formation of a heterodisulfide bond between the conserved cysteine residue on a sulfur carrier SoxYZ complex subunit SoxY and thiosulfate or other inorganic sulfur substrates. This leads to the liberation of two electrons, which may be transferred from the SoxAX complex to another cytochrome c and which then may be used for reductive CO(2) fixation. 2 Fe(III)-[cytochrome c] + L-cysteinyl-[SoxY protein] + thiosulfate = 2 Fe(II)-[cytochrome c] + 2 H(+) + S-sulfosulfanyl-L-cysteinyl-[SoxY protein] 2 Fe(III)-[cytochrome c] + S-sulfanyl-L-cysteinyl-[SoxY protein] + thiosulfate = 2 Fe(II)-[cytochrome c] + 2 H(+) + S-(2-sulfodisulfanyl)-L-cysteinyl-[SoxY protein] Binds 2 heme c groups covalently per subunit. E(0) is approximately -340 mV for His/Cys(-) ligated heme (heme 1) and -400 mV for His/CysS(-) ligated heme (heme 2) at pH 7.0. Heterodimer of SoxA and SoxX. By thiosulfate. Cysteine persulfide at Cys-248. The measured mass is that of mature SoxA with a diheme bound and with an undetermined modification. Unable to grow photoautotrophically on or oxidize either thiosulfate or sulfide. Thiosulfate:cytochrome c oxidoreductase activity is zero in periplasmic extracts prepared from cells grown on thiosulfate plus malate. Belongs to the SoxA family.