Note: a dashed line (without arrowheads) between two compound names is meant to imply that the two names are just different instantiations of the same compound -- i.e. one may be a specific name and the other a general name, or they may both represent the same compound in different stages of a polymerization-type pathway. 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: polysulfide respiration, polysulphide respiration
|Superclasses:||Degradation/Utilization/Assimilation → Inorganic Nutrients Metabolism → Sulfur Compounds Metabolism → Sulfur Reduction|
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
Elemental sulfur, while having little solubility, is readily converted to polysulfide in aqueous solutions containing sulfide. The sulfur ring is cleaved by a nucleophilic attack of the sulfide, forming a linear polysulfide, as shown in the equation:
Evidence gathered from experiments with several different bacteria and archaea suggested that a sulfur solubilization step is involved in the reduction of elemental sulfur. For example, sulfur reduction by membrane fractions of Sulfurospirillum deleyianum was facilitated by the addition of sulfide to the medium [Zoephel88], and polysulfides were involved in sulfur reduction by Desulfovibrio desulfuricans [Cammack84].
A study about the role of polysulfides in sulfur reduction by the hyperthermophilic archaeon Pyrococcus furiosus found that the organism does not need to be in direct contact with the sulfur to metabolize it, and that polysulfides serve as substrates for sulfur reduction [Blumentals90]. A cytoplasmic sulfhydrogenase II was isolated from this organism that catalyzed the reduction of polysulfide to hydrogen sulfide with NADPH as the electron accpetor [Ma94].
In some strains of sulfate reducing bacteria, the sulfur reductase appears to be cytochrome c3. Purified cytochromes from Desulfovibrio desulfuricans strain Norway 4 and another sulfate-reducing strain were able to reduce colloidal sulfur to hydrogen sulfide in vitro [Fauque79].
Perhaps the best characterized system is found in Wolinella succinogenes, where it has been shown that the main substrate for sulfur reduction is polysulfides [Klimmek91]. A reductase that was originally purified from this bacterium as sulfur reductase [Schroeder88] was eventually renamed to polysulfide reductase [Krafft92], and the genes encoding it were cloned and sequenced. This enzyme is a membrane-bound molybdoenzyme, which oxidizes S0 with electrons obtained from either a hydrogenase or a formate dehydrogenase, through a methyl-menaquinone-6 electron carrier [Dietrich02].
It should be noted that in addition to this polysulfide reductase, Wolinella succinogenes appears to have another sulfur/polysulfide reductase, as mutants lacking this enzyme were still able to grow by the reduction of either elemental sulfur or polysulfides, although the growth yield was about 25% of that obtained by the wild type [Ringel96].
Variants: sulfur reduction I
Blumentals90: Blumentals II, Itoh M, Olson GJ, Kelly RM (1990). "Role of Polysulfides in Reduction of Elemental Sulfur by the Hyperthermophilic Archaebacterium Pyrococcus furiosus." Appl Environ Microbiol 56(5);1255-1262. PMID: 16348181
Cammack84: Cammack, R., Fauque, G., Moura, J.J.G., Le Gall, J. (1984). "ESR studies of cytochrome c3 from Desulfovibrio desulfuricans strain Norway 4: midpoint potentials of the four haems, and interactions with ferredoxin and colloidal sulphur." Biochim. Biophys. Acta 784: 68-74.
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
Dietrich02: Dietrich W, Klimmek O (2002). "The function of methyl-menaquinone-6 and polysulfide reductase membrane anchor (PsrC) in polysulfide respiration of Wolinella succinogenes." Eur J Biochem 269(4);1086-95. PMID: 11856339
Fauque79: Fauque G, Herve D, Le Gall J (1979). "Structure-function relationship in hemoproteins: the role of cytochrome c3 in the reduction of colloidal sulfur by sulfate-reducing bacteria." Arch Microbiol 121(3);261-4. PMID: 229785
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.
Krafft92: Krafft T, Bokranz M, Klimmek O, Schroder I, Fahrenholz F, Kojro E, Kroger A (1992). "Cloning and nucleotide sequence of the psrA gene of Wolinella succinogenes polysulphide reductase." Eur J Biochem 206(2);503-10. PMID: 1597189
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
Ma94: Ma K, Adams MW (1994). "Sulfide dehydrogenase from the hyperthermophilic archaeon Pyrococcus furiosus: a new multifunctional enzyme involved in the reduction of elemental sulfur." J Bacteriol 176(21);6509-17. PMID: 7961401
Ringel96: Ringel, M., Gross, R., Krafft, T., Kroeger, A., Schauder, R. (1996). "Growth of Wolinella succinogenes with elemental sulfur in the absence of polysulfide." Archives of Microbiology 165(1): 62-64.
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
Huang91: Huang CJ, Barrett EL (1991). "Sequence analysis and expression of the Salmonella typhimurium asr operon encoding production of hydrogen sulfide from sulfite." J Bacteriol 173(4);1544-53. PMID: 1704886
Ma00: Ma K, Weiss R, Adams MW (2000). "Characterization of hydrogenase II from the hyperthermophilic archaeon Pyrococcus furiosus and assessment of its role in sulfur reduction." J Bacteriol 182(7);1864-71. PMID: 10714990
Ma93a: Ma K, Schicho RN, Kelly RM, Adams MW (1993). "Hydrogenase of the hyperthermophile Pyrococcus furiosus is an elemental sulfur reductase or sulfhydrogenase: evidence for a sulfur-reducing hydrogenase ancestor." Proc Natl Acad Sci U S A 90(11);5341-4. PMID: 8389482
Pedroni95: Pedroni P, Della Volpe A, Galli G, Mura GM, Pratesi C, Grandi G (1995). "Characterization of the locus encoding the [Ni-Fe] sulfhydrogenase from the archaeon Pyrococcus furiosus: evidence for a relationship to bacterial sulfite reductases." Microbiology 141 ( Pt 2);449-58. PMID: 7704275
vanHaaster08: van Haaster DJ, Silva PJ, Hagedoorn PL, Jongejan JA, Hagen WR (2008). "Reinvestigation of the steady-state kinetics and physiological function of the soluble NiFe-hydrogenase I of Pyrococcus furiosus." J Bacteriol 190(5);1584-7. PMID: 18156274
Zophel91: Zophel A, Kennedy MC, Beinert H, Kroneck PM (1991). "Investigations on microbial sulfur respiration. Isolation, purification, and characterization of cellular components from Spirillum 5175." Eur J Biochem 195(3);849-56. PMID: 1847872
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