This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Degradation/Utilization/Assimilation → Inorganic Nutrients Metabolism → Sulfur Compounds Metabolism → Thiosulfate Oxidation|
|Generation of Precursor Metabolites and Energy → Respiration → Aerobic Respiration|
Like fermentation, respiration is a process by which electrons are passed from an electron donor to a terminal electron acceptor. However, in respiration the electrons do not pass directly from the donor to the acceptor. Instead, they pass a number of membrane-bound electron carriers that function as a transport chain, passing the electrons from one to another in steps that follow the electrochemical gradients between the electron donor and the acceptor.
Each oxidized member of the electron transfer chain (which can be a flavoprotein, an electron-transfer quinone, a cytochrome, or other type of electron carrier) can be reduced by the reduced form of the preceding member, and the electrons flow through the chain all the way to the terminal acceptor, which could be oxygen in the case of aerobic respiration, or another type of molecule in anaerobic respiration.
Known terminal acceptors include organic compounds (fumarate, dimethyl sulfoxide, or trimethylamine N-oxide), or inorganic compounds (nitrate, nitrite, nitrous oxide, chlorate, perchlorate, oxidized manganese ions, ferric iron, gold, selenate, arsenate, sulfate and elemental sulfur).
During the process of electron transfer, a proton gradient is formed across the membrane due to three potential processes:
1. The use of some of the energy associated with the electron transfer for active pumping of protons out of the cell.
2. Exporting protons out of the cell during electron-to-hydrogen transfers.
3. Scalar reactions that consume protons inside the cell, or produce them outside the cell, without actually moving a proton from one compartment to another.
Upon passage of protons back into the cytoplasm, they drive multisubunit ATP synthase enzymes that generate ATP.
About This Pathway
Thiosulfate is one of the products of the initial step of the elemental sulfur oxidation pathway in the thermoacidophilic archaeon Acidianus ambivalens (see superpathway of sulfur oxidation (Acidianus ambivalens)). A novel thiosulphate:quinone oxidoreductase (TQO) activity was found in the membrane extracts of aerobically grown cells of this organism.
TQO oxidized thiosulfate with tetrathionate as product and ferric cyanide as electron acceptor with a maximum specific activity of 73.4 U per mg protein at 92 degrees C and pH 6. The enzyme could also use decyl ubiquinone (DQ) as electron acceptor, although activity was much lower, at 397 mU per mg protein at 80 degrees C.
A mixture of caldariella quinones, sulfolobus quinones and menaquinones was non-covalently bound to the protein [Muller04].
The larger subunit, which was glycosylated, was identical to DoxA, and the smaller was identical to DoxD, both of which described as subunits of the terminal quinol:oxygen oxidoreductase complex (cytochrome aa3) [Purschke97, Muller04].
The pathway has also been documented in Acidithiobacillus ferrooxidans [Brasseur04]. Thiosulfate is generated in that organism by chemical or enzymatic reactions among pathway intermediates during sulfur or sulfide oxidation [Rohwerder03].
Superpathways: superpathway of sulfur oxidation (Acidianus ambivalens)
Brasseur04: Brasseur G, Levican G, Bonnefoy V, Holmes D, Jedlicki E, Lemesle-Meunier D (2004). "Apparent redundancy of electron transfer pathways via bc(1) complexes and terminal oxidases in the extremophilic chemolithoautotrophic Acidithiobacillus ferrooxidans." Biochim Biophys Acta 1656(2-3);114-26. PMID: 15178473
Muller04: Muller FH, Bandeiras TM, Urich T, Teixeira M, Gomes CM, Kletzin A (2004). "Coupling of the pathway of sulphur oxidation to dioxygen reduction: characterization of a novel membrane-bound thiosulphate:quinone oxidoreductase." Mol Microbiol 53(4);1147-60. PMID: 15306018
Purschke97: Purschke WG, Schmidt CL, Petersen A, Schafer G (1997). "The terminal quinol oxidase of the hyperthermophilic archaeon Acidianus ambivalens exhibits a novel subunit structure and gene organization." J Bacteriol 179(4);1344-53. PMID: 9023221
Rohwerder03: Rohwerder T, Sand W (2003). "The sulfane sulfur of persulfides is the actual substrate of the sulfur-oxidizing enzymes from Acidithiobacillus and Acidiphilium spp." Microbiology 149(Pt 7);1699-710. PMID: 12855721
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