Catalyzes the oxidation of 2 molecules of thiosulfate to tetrathionate. 2 Fe(III)-[cytochrome c] + 2 thiosulfate = 2 Fe(II)-[cytochrome c] + 2 H(+) + tetrathionate Optimum pH is 4.2. Monomer. Binds 2 heme c groups covalently per subunit. Cells are unable to produce tetrathionate from thiosulfate.