If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Locations of Mapped Genes:
|Superclasses:||Degradation/Utilization/Assimilation → Inorganic Nutrients Metabolism → Sulfur Compounds Metabolism → Sulfur Disproportionation|
|Generation of Precursor Metabolites and Energy → Other|
Pathway Summary from MetaCyc:
Disproportionation is a process in which a reactant is both oxidized and reduced in the same chemical reaction, forming two separate compounds. The disproportionation of thiosulfate, an important step in sulfur transformation in aquatic systems, was discovered in 1987 [Bak87]. Thiosulfate-disproportionating bacteria are numerically significant, and have been detected in high numbers in several environments, including freshwater, mud, and marine sediments (see thiosulfate disproportionation I (thiol-dependent)).
Several microorganisms have also been shown to grow by the disproportionation of elemental sulfur [Thamdrup93]. This observation was initially surprising, because under standard conditions the reaction is endergonic with G'° = 40.9 kJ. However, the removal of sulfide, which occurs in the presence of sulfide scavengers such as oxidized metals (for example iron and manganese oxides) makes the process energetically favorable [Frederiksen04].
About This Pathway
The first organisms that were shown to disproportionate sulfur were anaerobic [Thamdrup93, Finster98]. However, the thermoacidophilic chemolithotrophic sulfur-metabolizing archaeons of the genus Acidianus can obtain energy through the aerobic disproportionation of elemental sulfur to sulfate and hydrogen sulfide.
The initial enzyme in the aerobic sulfur oxidation pathway of Acidianus is a soluble, cytoplasmic sulfur oxygenase reductase (SOR) [Kletzin89]. The SOR catalyzes the oxygen- dependent disproportionation of elemental sulfur leading to sulfite, thiosulfate and hydrogen sulfide as reaction products. Sulfur acts simultaneously as electron donor and electron acceptor, as no external electron donor or cofactor, not even glutathione, is required.
SORs have been purified from Acidianus ambivalens and from Acidianus tengchongensis [Kletzin89, Sun03], and found to be very large complexes (an icosatetramer in the case of Acidianus ambivalens) composed of a single type of subunit [Moser92]. The monomers assemble to create a large hollow sphere enclosing a positively charged nanocompartment. Apolar channels provide access for linear sulfur compounds. The active sites, accessible from inside the sphere, contain a cysteine persulfide and a low-potential mononuclear non-heme iron ligated by a 2-His-1-carboxylate facial triad. the iron is likely the site of both sulfur reduction and sulfur oxidation [Urich06].
Pathway Evidence Glyph:
Finster98: Finster K, Liesack W, Thamdrup B (1998). "Elemental sulfur and thiosulfate disproportionation by Desulfocapsa sulfoexigens sp. nov., a new anaerobic bacterium isolated from marine surface sediment." Appl Environ Microbiol 1998;64(1);119-25. PMID: 9435068
Frederiksen04: Frederiksen TM, Finster K (2004). "The transformation of inorganic sulfur compounds and the assimilation of organic and inorganic carbon by the sulfur disproportionating bacterium Desulfocapsa sulfoexigens." Antonie Van Leeuwenhoek 85(2);141-9. PMID: 15028874
Kletzin89: Kletzin A (1989). "Coupled enzymatic production of sulfite, thiosulfate, and hydrogen sulfide from sulfur: purification and properties of a sulfur oxygenase reductase from the facultatively anaerobic archaebacterium Desulfurolobus ambivalens." J Bacteriol 171(3);1638-43. PMID: 2493451
Sun03: Sun CW, Chen ZW, He ZG, Zhou PJ, Liu SJ (2003). "Purification and properties of the sulfur oxygenase/reductase from the acidothermophilic archaeon, Acidianus strain S5." Extremophiles 7(2);131-4. PMID: 12664265
Thamdrup93: Thamdrup B, Finster K, Hansen JW, Bak F (1993). "Bacterial Disproportionation of Elemental Sulfur Coupled to Chemical Reduction of Iron or Manganese." Appl Environ Microbiol 59(1);101-108. PMID: 16348835
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
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