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 → Sulfur Oxidation|
|Generation of Precursor Metabolites and Energy → Chemoautotrophic Energy Metabolism|
Some taxa known to possess this pathway include : Acidithiobacillus ferrooxidans
Expected Taxonomic Range: Bacteria
Acidithiobacilli are prominent bacteria that catalyse the oxidation of inorganic sulfur compounds under acidic conditions (pH 1 to 3) and ambient temperatures. Acidithiobacillus ferrooxidans is an important member of this family, which obtains energy via the oxidation of iron as well as sulfur, and is often found in pyrite deposits. Acidithiobacillus ferrooxidans is an important industrial microorganism, involved in bioleaching of copper ores and the biooxidation of gold-containing ores [Rawlings01]. The organism can oxidize sulfur compounds under both aerobic and anaerobic conditions, using either molecular oxygen or Fe3+ ions as electron acceptors, respectively [Corbett87].
Extracellular elemental sulfur most likely enters the periplasmic space by binding to thiol groups of special outer-membrane proteins and being transported as persulfide sulfur [Rohwerder03]. The thiol-bearing membrane proteins have not been identified conclusively, but candidates include a sulfide-binding protein isolated from Acidithiobacillus ferrooxidans [Sugio91] as well as several outer-membrane proteins which have been associated with sulfur oxidation in this organism [Buonfiglio93, Buonfiglio99, Ohmura96].
In all cases of elemental sulfur oxidation, an activation reaction is assumed to take place prior to oxidation, because elemental sulfur, which consists of stable polysulfane rings (such as S6, S7 or S8) forms orthorhombic crystals that are not soluble in water [Steudel00]. Many studies of sulfur oxidation by acidophilic bacteria showed that the first step of the pathway depends on low-molecular-mass thiols, such as glutathione (GSH), for activity [Suzuki66, Silver68]. Similar GSH-dependent activity has also been detected in Sulfobacillus thermosulfidooxidans [Krasilnikova04].
In an earlier work Sugio et al reported the purification of a sulfur:ferric ion oxidoreductase from Acidithiobacillus ferrooxidans. The presence of glutathione in the reaction buffer was absolutely essential [Sugio87]. The authors later reported that elemental sulfur was reduced by glutathione to hydrogen sulfide, and that hydrogen sulfide was the actual substrate for the enzyme. Based on these finding, the authors suggested that glutathione is one of the substrates of the enzyme [Sugio89]. Other works also reported that sulfur was enzymatically converted to hydrogen sulfide prior to further oxidation [Bacon89]. However, in a later study by Rohwerder and Sand [Rohwerder03] it was shown that cell-free systems from several members of the Acidithiobacillus and Acidiphilium spp., including Acidithiobacillus ferrooxidans, oxidized elemental sulfur only via S-sulfanylglutathione (GSSH), a non-enzymic reaction product from glutathione (GSH) and elemental sulfur. Thus, while GSH (and possibly other cellular thiols) plays a crucial role in elemental sulfur activation, it is not consumed during the enzymic sulfane sulfur oxidation. The enzyme that oxidizes the sulfur, the periplasmic glutathione-dependent sulfur dioxygenase, cannot accept sulfur or sulfide, but is specific for the sulfane sulfur of monoorganylpolysulfanes (RSnH, where n>1), and predominantly persulfides (where n=2), which is oxidized to sulfite in the presence of molecular oxygen and water. Organic persulfides such as S-sulfanylglutathione could not be replaced by other sulfane-sulfur-containing compounds (thiosulfate, polythionates, bisorganylpolysulfanes or monoarylthiosulfonates) as sulfur donor.
The product of the sulfur dioxygenase activity was shown to be sulfite, which is subsequently oxidized to sulfate by a sulfite:ferric ion oxidoreductase. Most of the sulfite:acceptor oxidoreductases that were isolated from other organisms were shown to use c-type cytochromes as electron acceptors [Vestal71, deJong00], and such an enzyme was also isolated from Acidithiobacillus ferrooxidans (see sulfite dehydrogenase) [Nakamura95a]. However, Sugio et al isolated from Acidithiobacillus ferrooxidans an enzyme that utilizes only Fe3+ as an electron accpetor [Sugio88]. It is possible that this organism has two systems for the oxidation of sulfur: one that utilizes a cytochrome c, and one that utilizes Fe3+ ions.
For a discussion of sulfide oxidation in this strain, see sulfide oxidation III (sulfur dioxygenase).
Buonfiglio93: Buonfiglio, V., Polidoro, M., Flora, L., Citro, G., Valenti, P., Orsi, N. (1993). "Identification of two outer membrane proteins involved in the oxidation of sulphur compounds in Thiobacillus ferrooxidans. ." FEMS Microbiol Rev 11: 43-50.
Buonfiglio99: Buonfiglio V, Polidoro M, Soyer F, Valenti P, Shively J (1999). "A novel gene encoding a sulfur-regulated outer membrane protein in Thiobacillus ferrooxidans." J Biotechnol 72(1-2);85-93. PMID: 10406099
deJong00: de Jong, G.A.H., Tang, J. A., Bos, P., de Vries, S., Kuenen, J. G. (2000). "Purification and characterization of a sulfite : cytochrome c oxidoreductase from Thiobacillus acidophilus." J Mol Catal B 8, 61-67.
Krasilnikova04: Krasil'nikova EN, Bogdanova TI, Zakharchuk LM, Tsaplina IA (2004). "[Sulfur metabolism enzymes in thermoacidophilus bacteria Sulfobacillus sibiricus]." Prikl Biokhim Mikrobiol 40(1);62-5. PMID: 15029700
Nakamura95a: Nakamura, K., Yoshikawa, H., Okubo, S., Kurosawa, H., Amano, Y. (1995). "Purification and properties of membrane-bound sulfite dehydrogenase from Thiobacillus thiooxidans." JCM 7814. Biosci. Biotechnol. Biochem. 59: 11-15.
Rawlings01: Rawlings, D. E. (2001). "The molecular genetics of Thiobacillus ferrooxidans and other mesophilic, acidophilic, chemolithotrophic, iron- or sulfur-oxidizing bacteria." Hydrometallurgy 59 :187-201.
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
Sugio87: Sugio T, Mizunashi W, Inagaki K, Tano T (1987). "Purification and some properties of sulfur:ferric ion oxidoreductase from Thiobacillus ferrooxidans." J Bacteriol 1987;169(11);4916-22. PMID: 3667519
Sugio88: Sugio T, Katagiri T, Moriyama M, Zhen YL, Inagaki K, Tano T (1988). "Existence of a new type of sulfite oxidase which utilizes ferric ions as an electron acceptor in Thiobacillus ferrooxidans." Appl Environ Microbiol 1988;54(1);153-7. PMID: 3345075
Sugio89: Sugio T, Katagiri T, Inagaki K, Tano T (1989). "Actual substrate for elemental sulfur oxidation by sulfur:ferric ion oxidoreductase purified from Thiobacillus ferrooxidans." Biochimica et Biophysica Acta 973:250-256.
Sugio91: Sugio, T., Suzuki, H., Oto, A., Inagaki, K., Tanaka, H., Tano, T. (1991). "Purification and some properties of a hydrogen sulfide binding protein that is involved in sulfur oxidation of Thiobacillus ferrooxidans." Agric Biol Chem 55: 2091-2094.
Franz07: Franz B, Lichtenberg H, Hormes J, Modrow H, Dahl C, Prange A (2007). "Utilization of solid "elemental" sulfur by the phototrophic purple sulfur bacterium Allochromatium vinosum: a sulfur K-edge X-ray absorption spectroscopy study." Microbiology 153(Pt 4);1268-74. PMID: 17379736
Sugio92: Sugio T, Hirose T, Ye LZ, Tano T (1992). "Purification and some properties of sulfite:ferric ion oxidoreductase from Thiobacillus ferrooxidans." J Bacteriol 1992;174(12);4189-92. PMID: 1597434
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