binding of thiosulfate to an unusual and reactive cysteine S-sulphane adduct, forming a S-thiosulfonate adduct. Step 2 follows with an additional thiosulfate to subsequently form tetrathionate while reducing both hemes and leaving a typical cysteine residue. In the active site, the cysteine residue bound to the catalytic iron heme is essential for enzymatic activity, as the abolishment of this residue completely eliminated the enzyme's ability to both oxidize thiosulfate and reduce tetrathionate. In A. vinosum, although the process is reversible, the reaction to form two thiosulfate ions is much slower than the formation of tetrathionate despite the reduction of tetrathionate to thiosulfate having a potential of +198 mV. Reduction of the enzyme results in a ligand switch from Lys208 to Met209 in the second heme. Mutant proteins that replace Met209 with asparagine or glycine have similar substrate affinities to the wildtype variant but have much lower specific activities, suggesting that heme 2 is the electron exit point in the last steps of the mechanism. Upon the reduction of heme 2 and the ligand switch, the redox potential is increased and hinders the back reaction to form thiosulfate. Here, it is suggested that a high potential iron-sulfur protein (HiPIP) serves as the electron acceptor in the oxidation of both hemes to their initial state. Function The oxidation of thiosulfate to tetrathionate is observed in several thiobacilli, phototrophs, and heterotrophs, as thiosulfate and tetrathionate play the role of electron donor and electron acceptor, respectively, in many bacterial species. Both compounds are intermediates in and play an important role in the biogeochemical sulfur cycle, the process of conversion between sulfide and sulfate. Thus, thiosulfate dehydrogenase is essential for the conversion between the intermediates in the sulfur cycle. The sulfur cycle enables a variety of bacteria to utilize generated thiosulfate as an electron donor for aerobic growth and anaerobic carbon dioxide fixation for photosynthesis. Pseudomonas and Halomonas are examples of the many thiobacteria that utilize thiosulfate dehydrogenase to derive energy from thiosulfate as a supplemental energy source. Tetrathionate can serve as a respiratory electron acceptor during anaerobic respiration by tetrathionate reduction. Industrial applications Thiobacteria such as Acidithiobacillus ferrooxidans have become essential to industrial bioleaching applications, as the microorganisms are able to oxidize iron and sulfur from iron-sulfur minerals as energy sources, supporting their own autotrophic growth while producing ferric iron and sulfuric acid. Thus, bacteria have been isolated from mineral deposits and used in