Sulfur metabolism

Set of chemical reactions involving sulfur in living organisms

Sulfur is metabolized by all organisms, from bacteria and archaea to plants and animals. Sulfur can have an oxidation state from −2 to +6 and is reduced or oxidized by a diverse range of organisms.[1] The element is present in proteins, sulfate esters of polysaccharides, steroids, phenols, and sulfur-containing coenzymes.[2]

## Oxidation

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Reduced sulfur compounds are oxidized by most organisms, including higher animals and higher plants.[2] Some organisms can conserve energy (i.e., produce ATP) from the oxidation of sulfur and it can serve as the sole energy source for some lithotrophic bacteria and archaea.[3] Sulfur oxidizers use enzymes such as Sulfide:quinone reductase, sulfur dioxygenase and sulfite oxidase to oxidize sulfur compounds to sulfate.

### Sulfur-oxidizing microorganisms

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Reduced sulfur compounds, such as hydrogen sulfide, elemental sulfur, sulfite, thiosulfate, and various polythionates (e.g., tetrathionate), are oxidized by chemotrophic, phototrophic, and mixotrophic bacteria for energy.[1] Some chemosynthetic archaea use hydrogen sulfide as an energy source for carbon fixation, producing sugars.

#### Chemotrophic sulfur-oxidizing bacteria

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In order to have sufficient redox potential, microorganisms that use sulfur as an electron donor often use oxygen or nitrate as terminal electron acceptors.[4] Members of the chemotrophic Acidithiobacillus genus are able to oxidize a vast range of reduced sulfur compounds, but are restricted to acidic environments.[5] Chemotrophs that can produce sugars through chemosynthesis make up the base of some food chains. Food chains have formed in the absence of sunlight around hydrothermal vents, which emit hydrogen sulfide and carbon dioxide.

#### Phototrophic sulfur-oxidizing bacteria

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Microbial sulfur cycle

Some bacteria use light energy to couple sulfur oxidation to carbon dioxide (CO2) fixation for growth. These fall into two general groups: green sulfur bacteria (GSB) and purple sulfur bacteria (PSB).[6] However, some Cyanobacteria are also able to use hydrogen sulfide as an electron donor during anoxygenic photosynthesis.[7] All PSB are part of the class Gammaproteobacteria and are found in two families: Chromatiaceae and Ectothiorhodospiraceae. Typically, sulfur globules accumulate intracellularly in Chromatiaceae and extracellularly in Ectothiorhodospiraceae, which is one distinguishing feature between these two groups of PSB.[8] GSB are found within the family Chlorobiaceae generally oxidize sulfide or elemental sulfur, but some members are able to utilize thiosulfate.[9]

## Reduction

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Sulfur reduction occurs in plants, fungi, and many bacteria.[10] Sulfate can serve as an electron acceptor in anaerobic respiration and can also be reduced for the formation of organic compounds. Sulfate-reducing bacteria reduce sulfate and other oxidized sulfur compounds, such as sulfite, thiosulfate, and elemental sulfur, to sulfide.

### Dissimilatory sulfur reduction

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Some microorganisms are capable of reducing sulfate and elemental sulfur for energy by coupling sulfur reduction with the oxidation of molecular hydrogen or organic compounds such as acetate in anaerobic respiration.[11] These processes typically produce hydrogen sulfide as a byproduct, which can go on to serve as an electron donor in sulfur oxidation.[11] Sulfate reduction by sulfate-reducing bacteria is dissimilatory; the purpose of reducing the sulfate is to produce energy, and the sulfide is excreted. Dissimilatory sulfate reduction use the enzymes ATP sulfurylase, APS reductase, and sulfite reductase.[12]

### Assimilatory sulfur reduction

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In assimilatory sulfate reduction the sulfate is assimilated, or incorporated into organic compounds such as cysteine, methionine, or iron-sulfur clusters and enzyme cofactors.[13] In bacteria, sulfate and thiosulfate are transported into the cell by sulfate permeases where it can then be reduced and incorporated into biomolecules.[14] In some organisms (e.g., gut flora, cyanobacteria, and yeast),[15] assimilatory sulfate reduction is a more complex process that makes use of the enzymes ATP sulfurylase, APS kinase, PAPS reductase, and sulfite reductase.[10]

## Disproportionation

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Sulfur can also serve as both an electron donor and electron acceptor by microorganisms is disproportionation reactions. For example, Acidianus ambivalens uses sulfur oxygenase reductase (SOR) to convert elemental sulfur to sulfate, thiosulfate, and hydrogen sulfide through disproportionation.[16] Elemental sulfur disproportionation is restricted to environments where the concentration of the sulfide products are kept low, which typically happens in the presence of scavenging minerals that contain iron or manganese.[17] Disproportionation of thiosulfate often occurs in anoxic layers of marine and freshwater sediments.[18][19]

## Use by plants and animals

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Plants take up sulfate in their roots and reduce it to sulfide (see Sulfur assimilation). However, some Brassica species are able to assimilate atmospheric sources of sulfur in the absence of other sources.[20] Plants reduce APS directly to sulfite (using APS reductase) without phosphorylating APS to PAPS. From the sulfide they form the amino acids cysteine and methionine, sulfolipids, and other sulfur compounds. Animals obtain sulfur from cysteine and methionine in the protein that they consume.

Sulfur is the third most abundant mineral element in the body.[21] The amino acids cysteine and methionine are used by the body to make glutathione. Excess cysteine and methionine are oxidized to sulfate by sulfite oxidase, eliminated in the urine, or stored as glutathione (which can serve as a store for sulfur).[21] The lack of sulfite oxidase, known as sulfite oxidase deficiency, causes physical deformities, mental retardation, and death.

## See also

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* Microbial metabolism
* Sulfur cycle

## References

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1. ^ a b Loka Bharathi, P. A. (2008-01-01), "Sulfur Cycle", in Fath, Brian (ed.), Encyclopedia of Ecology (Second Edition), Oxford: Elsevier, pp. 192–199, doi:10.1016/b978-0-444-63768-0.00761-7, ISBN 978-0-444-64130-4, retrieved 2023-02-12
2. ^ a b Schiff JA (1979). "Pathways of assimilatory sulphate reduction in plants and microorganisms". In CIBA Foundation Symposium (ed.). Sulphur in Biology. John Wiley & Sons. pp. 49–50\. ISBN 9780470718230.
3. ^ Friedrich, Cornelius G. (1997-01-01), Poole, R. K. (ed.), "Physiology and Genetics of Sulfur-oxidizing Bacteria", Advances in Microbial Physiology, vol. 39, Academic Press, pp. 235–289, retrieved 2023-02-13
4. ^ Wasmund, Kenneth; Mußmann, Marc; Loy, Alexander (August 2017). "The life sulfuric: microbial ecology of sulfur cycling in marine sediments: Microbial sulfur cycling in marine sediments". Environmental Microbiology Reports. 9 (4): 323–344\. doi:10.1111/1758-2229.12538. PMC 5573963. PMID 28419734.
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6. ^ Dahl, Christiane; Friedrich, Cornelius G. (2008). Microbial sulfur metabolism. Berlin: Springer. ISBN 978-3-540-72682-1. OCLC 213092503.
7. ^ Cohen, Yehuda; Jørgensen, Bo Barker; Revsbech, Niels Peter; Poplawski, Ricardo (1986). "Adaptation to Hydrogen Sulfide of Oxygenic and Anoxygenic Photosynthesis among Cyanobacteria". Applied and Environmental Microbiology. 51 (2): 398–407\. doi:10.1128/aem.51.2.398-407.1986. ISSN 0099-2240. PMC 238881. PMID 16346996.
8. ^ Frigaard, Niels-Ulrik; Dahl, Christiane (2008-01-01), Poole, Robert K. (ed.), Sulfur Metabolism in Phototrophic Sulfur Bacteria, Advances in Microbial Physiology, vol. 54, Academic Press, pp. 103–200, doi:10.1016/s0065-2911(08)00002-7, ISBN 9780123743237, PMID 18929068, retrieved 2023-02-12
9. ^ Sakurai, Hidehiro; Ogawa, Takuro; Shiga, Michiko; Inoue, Kazuhito (2010-06-01). "Inorganic sulfur oxidizing system in green sulfur bacteria". Photosynthesis Research. 104 (2): 163–176\. doi:10.1007/s11120-010-9531-2. ISSN 1573-5079. PMID 20143161. S2CID 1091791.
10. ^ a b "Pathway: sulfate reduction I (assimilatory)." MetaCyc.
11. ^ a b Rabus, Ralf; Hansen, Theo A.; Widdel, Friedrich (2006), Dworkin, Martin; Falkow, Stanley; Rosenberg, Eugene; Schleifer, Karl-Heinz (eds.), "Dissimilatory Sulfate- and Sulfur-Reducing Prokaryotes", The Prokaryotes, New York, NY: Springer New York, pp. 659–768, doi:10.1007/0-387-30742-7_22, ISBN 978-0-387-25492-0, retrieved 2023-02-13
12. ^ "Pathway: sulfate reduction IV (dissimilatory)." MetaCyc.
13. ^ Moran, Mary Ann; Durham, Bryndan P. (2019). "Sulfur metabolites in the pelagic ocean". Nature Reviews Microbiology. 17 (11): 665–678\. doi:10.1038/s41579-019-0250-1. ISSN 1740-1534. PMID 31485034. S2CID 201834195.
14. ^ Aguilar-Barajas, Esther; Díaz-Pérez, César; Ramírez-Díaz, Martha I.; Riveros-Rosas, Héctor; Cervantes, Carlos (2011-08-01). "Bacterial transport of sulfate, molybdate, and related oxyanions". BioMetals. 24 (4): 687–707\. doi:10.1007/s10534-011-9421-x. ISSN 1572-8773. PMID 21301930. S2CID 1935648.
15. ^ "Pathway: sulfate reduction II (assimilatory)." MetaCyc.
16. ^ Janosch, Claudia; Remonsellez, Francisco; Sand, Wolfgang; Vera, Mario (2015-10-21). "Sulfur Oxygenase Reductase (Sor) in the Moderately Thermoacidophilic Leaching Bacteria: Studies in Sulfobacillus thermosulfidooxidans and Acidithiobacillus caldus". Microorganisms. 3 (4): 707–724\. doi:10.3390/microorganisms3040707. ISSN 2076-2607. PMC 5023260. PMID 27682113.
17. ^ Fike, David A.; Bradley, Alexander S.; Rose, Catherine V. (2015-05-30). "Rethinking the Ancient Sulfur Cycle". Annual Review of Earth and Planetary Sciences. 43 (1): 593–622\. doi:10.1146/annurev-earth-060313-054802. ISSN 0084-6597.
18. ^ Jørgensen, Bo Barker (1990). "The sulfur cycle of freshwater sediments: Role of thiosulfate". Limnology and Oceanography. 35 (6): 1329–1342\. doi:10.4319/lo.1990.35.6.1329.
19. ^ Jørgensen, Bo Barker (1990-07-13). "A Thiosulfate Shunt in the Sulfur Cycle of Marine Sediments". Science. 249 (4965): 152–154\. doi:10.1126/science.249.4965.152. ISSN 0036-8075. PMID 17836966. S2CID 220093825.
20. ^ Aghajanzadeh, Tahereh; Hawkesford, Malcolm J.; De Kok, Luit J. (2016-04-01). "Atmospheric H
2S and SO
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21. ^ a b Nimni ME, Han B, Cordoba F (2007). "Are we getting enough sulfur in our diet?". Nutrition & Metabolism (Lond). 4 (1): 24. doi:10.1186/1743-7075-4-24. ISSN 1743-7075. PMC 2198910. PMID 17986345.

## External links

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* Media related to Sulfur metabolism at Wikimedia Commons

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