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.
Synonyms: sulfur respiration, sulphur respiration
|Superclasses:||Degradation/Utilization/Assimilation → Inorganic Nutrients Metabolism → Sulfur Compounds Metabolism → Sulfur Reduction|
Some taxa known to possess this pathway include : Acidianus ambivalens, Acidianus infernus, Acidithiobacillus ferrooxidans, Ammonifex degensii, Aquifex pyrophilus, Desulfomicrobium baculatum, Desulfurobacterium thermolithotrophum, Desulfuromonas acetexigens, Desulfuromonas acetoxidans, Pelobacter carbinolicus, Pyrobaculum islandicum, Pyrodictium abyssi, Pyrodictium brockii, Stetteria hydrogenophila, Stygiolobus azoricus, Sulfurospirillum arcachonense, Sulfurospirillum deleyianum, Thermoproteus tenax
The ability to reduce sulfur using H2 or organic substrates as electron donors is widespread among thermophiles. It is mostly found in hyperthermophilic archea living in volcanic habitats, and in mesophilic and thermophilic bacteria living in anoxic marine or brackish sediments, fresh water sediments, bovine rumen, hot water pools from solfataric fields, and volcanic hot springs [Hedderich99]. Some methanogens, especially the thermophilic and hyperthermophilic members of the genera Methanopyrus, Methanobacterium, Methanothermus, and Methanococcus are also able to reduce elemental sulfur to hydrogen sulfide [Stetter83].
Some of the organisms (including members of the archaebacterial genera Pyrococcus, Thermoproteus, Pyrodictum, Acidianus, Acidianus, Stygiolobus and Pyrobaculum) are able to use H2 and S0 as the sole energy source. These organisms must couple sulfur reduction to ADP phosphorylation. The majority of sulfur reducers, however, grow organotrophically, and can utilize complex substrates such as proteins. Since some of these organisms excrete organic products such as acetate or isovalerate, the metabolic significance of sulfur reduction in them is less clear [Schauder93].
Elemental sulfur is not a good substrate for enzymatic reactions because its solubility in water is very low (5 μg/L at 25 °) [Boulegue78]. One mechanism that solves the problem is the conversion of the sulfur ring to a more soluble sulfur intemediate (sometimes referred to as activated sulfur) prior to reduction [Belkin85, Zoephel88, Schauder93]. This mechanism has been documented in Wolinella succinogenes, in which the soluble compound has been shown to be a polysulfide [Klimmek91] (see sulfur reduction II (via polysulfide)). A second possibility is the physical attachment of the microbes to the elemental sulfur, resulting in a direct conversion of sulfur to sulfide. Such physical attachment has been shown to be important during aerobic sulfur oxidation by Acidithiobacillus ferrooxidans [Devasia93, Takeuchi97]. In addition, a recently purified sulfur reductase from the acidophilic hyperthermophile Acidianus ambivalens was shown to oxidize elemental sulfur in vitro without the need for hydrogen sulfide [Laska03]. Since polysulfides are unstable at low pH, it is possible that S0 is the actual substrate for acidophilic organisms. With the exception of a few organisms, it is still not clear which of these two possible mechanisms is used by the different sulfur reducers.
About This Pathway
Hyperthermophilic chemolithoautotrophic archaea gain energy by reducing S0 to hydrogen sulfide using electrons from H2. Sulfur reduction in these lithotrophic organisms must be coupled to ADP phosphorylation. In Pyrodictium brockii the respiratory chain was found to contain a hydrogenase, a quinone and a c-type cytochrome [Pihl90, Pihl91, Pihl92]. The electron transport chain of Pyrodictium abyssi differs: the nine subunit complex which makes up the H2:sulfur oxidoreductase represents the entire respiratory chain of the organism, containing a hydrogenase-sulfur reductase complex hydrogenase subunit, a hydrogenase-sulfur reductase complex sulfur reductase subunit, and electron transport components, all in one stable complex. The hydrogenase-sulfur reductase complex hydrogenase subunit, which is of the Ni/Fe type, is not sensitive to oxygen.The hydrogenase-sulfur reductase complex sulfur reductase subunit, on the other hand, is irreversibly damaged by it. The complex contains 50-55 mol sulfur, 50-55 mol iron and 1.6 mol nickel per mol native complex. The enzyme has a temperature optimum of about 100 ° C [Dirmeier98, Hedderich99].
While quite different from the hyperthermophiles, the acidophile chemolithtroph Acidithiobacillus ferrooxidans is also capable of reducing elemental sulfur to hydrogen sulfide. When five strains of Acidithiobacillus ferrooxidans were tested, all of them produced hydrogen sulfide from elemental sulfur at pH 1.5. Under argon gas hydrogen sulfide production stopped after 1 day of incubation. In contrast, under H2 hydrogen sulfide production continued for 6 days, producing much more sulfide, and suggesting that electrons from H2 were used to reduce elemental sulfur to give hydrogen sulfide [Ng00].
Variants: sulfur reduction II (via polysulfide)
Devasia93: Devasia P, Natarajan KA, Sathyanarayana DN, Rao GR (1993). "Surface Chemistry of Thiobacillus ferrooxidans Relevant to Adhesion on Mineral Surfaces." Appl Environ Microbiol 59(12);4051-4055. PMID: 16349107
Dirmeier98: Dirmeier R, Keller M, Frey G, Huber H, Stetter KO (1998). "Purification and properties of an extremely thermostable membrane-bound sulfur-reducing complex from the hyperthermophilic Pyrodictium abyssi." Eur J Biochem 1998;252(3);486-91. PMID: 9546664
Klimmek91: Klimmek, O., Kroeger, A., Steudel, R., Holdt, G (1991). "Growth of Wolinella succinogenes with polysulphide as terminal acceptor of phosphorylative electron transport." Arch. Microbiol. 155: 177–182.
Laska03: Laska S, Lottspeich F, Kletzin A (2003). "Membrane-bound hydrogenase and sulfur reductase of the hyperthermophilic and acidophilic archaeon Acidianus ambivalens." Microbiology 149(Pt 9);2357-71. PMID: 12949162
Ng00: Ng KY, Kamimura K, Sugio T (2000). "Production of hydrogen sulfide from tetrathionate by the iron-oxidizing bacterium Thiobacillus ferrooxidans NASF-1." J Biosci Bioeng 90(2);193-8. PMID: 16232841
Pihl90: Pihl TD, Schicho RN, Black LK, Schulman BA, Maier RJ, Kelly RM (1990). "Hydrogen-sulfur autotrophy in the hyperthermophilic archaebacterium, Pyrodictium brockii." Biotechnol Genet Eng Rev 8;345-77. PMID: 2128798
Pihl91: Pihl TD, Maier RJ (1991). "Purification and characterization of the hydrogen uptake hydrogenase from the hyperthermophilic archaebacterium Pyrodictium brockii." J Bacteriol 173(6);1839-44. PMID: 1900502
Pihl92: Pihl TD, Black LK, Schulman BA, Maier RJ (1992). "Hydrogen-oxidizing electron transport components in the hyperthermophilic archaebacterium Pyrodictium brockii." J Bacteriol 174(1);137-43. PMID: 1309514
Zoephel88: Zoephel, A., Kennedy, M. C., Beinert, H., Kroneck, P. M. H. (1988). "Investigations on microbial sulfur respiration 1. Activation and reduction of elemental sulfur in several strains of eubacteria." Arch Microbiol 150: 72- 77.
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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