-!- The enzyme catalyzes the reversible formation of a sulfur-sulfur bond between the sulfane atoms of two thiosulfate molecules, yielding tetrathionate and releasing two electrons. -!- In many bacterial species the enzyme is a diheme c-type cytochrome. -!- In a number of organisms, including Thiomonas intermedia and Sideroxydans lithotrophicus, a second diheme cytochrome (TsdB) acts as the electron acceptor. -!- However, some organisms, such as Allochromatium vinosum, lack TsdB. -!- The electron acceptor in these organisms may be the high-potential iron-sulfur protein (HiPIP).