Editing Sulfur (section)

==Biological role==
Sulfur is an essential component of all living [[cell (biology)|cells]]. It is the eighth most abundant element in the human body by weight,<ref name="sulphurinstitute-2021">{{cite news |title=Sulphur and the Human Body |url=https://www.sulphurinstitute.org/pub/?id=8c64bf34-bc30-5bd9-0719-f6de83f7e841 |access-date=3 April 2021 |publisher=The Sulfur Institute}}</ref> about equal in abundance to [[potassium]], and slightly greater than [[sodium]] and [[chlorine]].<ref>{{Cite web |title=What is the body made of? |url=https://www.newscientist.com/question/what-is-the-body-made-of/ |url-status=live |archive-url=https://web.archive.org/web/20211103173257/https://www.newscientist.com/question/what-is-the-body-made-of/ |archive-date=November 3, 2021 |access-date=November 9, 2021 |website=[[New Scientist]]}}</ref> A {{convert|70|kg|abbr=on}} human body contains about {{Convert|140|g}} of sulfur.<ref>{{Cite web |last=Helmenstine |first=Anne |date=February 3, 2019 |title=Elemental Composition of the Human Body by Mass |url=https://www.thoughtco.com/elemental-composition-human-body-by-mass-608192 |url-status=live |archive-url=https://web.archive.org/web/20210413013702/https://www.thoughtco.com/elemental-composition-human-body-by-mass-608192 |archive-date=April 13, 2021 |access-date=November 21, 2021 |website=[[ThoughtCo.]]}}</ref> The main dietary source of sulfur for humans is [[sulfur-containing amino acids]],<ref name="Parcell-2002">{{Cite journal |last=Parcell |first=Stephen |date=February 2002 |title=Sulfur in human nutrition and applications in medicine |url=https://pubmed.ncbi.nlm.nih.gov/11896744/ |journal=Alternative Medicine Review |volume=7 |issue=1 |pages=22–44 |issn=1089-5159 |pmid=11896744}}</ref> which can be found in plant and animal proteins.<ref name="Ingenbleek-2013">{{Cite journal |last1=Ingenbleek |first1=Yves |last2=Kimura |first2=Hideo |date=July 2013 |title=Nutritional essentiality of sulfur in health and disease |journal=Nutrition Reviews |volume=71 |issue=7 |pages=413–432 |doi=10.1111/nure.12050 |issn=1753-4887 |pmid=23815141|doi-access=free }}</ref>

=== Transferring sulfur between inorganic and biomolecules ===
{{See also|Sulfur cycle|Sulfur metabolism}}

In the 1880s, while studying ''[[Beggiatoa]]'' (a bacterium living in a sulfur rich environment), [[Sergei Winogradsky]] found that it oxidized [[hydrogen sulfide]] (H<sub>2</sub>S) as an energy source, forming intracellular sulfur droplets. Winogradsky referred to this form of metabolism as inorgoxidation (oxidation of inorganic compounds).<ref>{{Cite journal |last=Dworkin |first=Martin |date=March 2012 |title=Sergei Winogradsky: a founder of modern microbiology and the first microbial ecologist |journal=FEMS Microbiology Reviews |volume=36 |issue=2 |pages=364–379 |doi=10.1111/j.1574-6976.2011.00299.x |issn=1574-6976 |pmid=22092289|doi-access=free }}</ref> Another contributor, who continued to study it was [[Selman Waksman]].<ref>{{Cite journal |last1=Waksman |first1=S. A. |last2=Starkey |first2=R. L. |title=On the Growth and Respiration of Sulfur-Oxidizing Bacteria |date=1923-01-20 |journal=The Journal of General Physiology |volume=5 |issue=3 |pages=285–310 |doi=10.1085/jgp.5.3.285 |issn=0022-1295 |pmc=2140527 |pmid=19871997}}</ref> Primitive bacteria that live around deep ocean [[hydrothermal vent|volcanic vents]] oxidize hydrogen sulfide for their nutrition, as discovered by [[Robert Ballard]].<ref name="BBC_Elements-2014" />

Sulfur oxidizers can use as energy sources reduced sulfur compounds, including hydrogen sulfide, elemental sulfur, [[sulfite]], [[thiosulfate]], and various [[polythionates]] (e.g., [[tetrathionate]]).<ref>{{cite journal |author= Pronk JT |author2= Meulenberg R |author3= Hazeu W |author4= Bos P |author5= Kuenen JG |date= 1990 |title= Oxidation of reduced inorganic sulphur compounds by acidophilic thiobacilli |journal= FEMS Microbiology Letters |volume= 75 |issue= 2–3 |pages= 293–306 |doi= 10.1111/j.1574-6968.1990.tb04103.x |df= dmy-all|doi-access= free }}</ref> They depend on enzymes such as [[sulfur dioxygenase|sulfur oxygenase]] and [[sulfite oxidase]] to oxidize sulfur to sulfate. Some [[lithotroph]]s can even use the energy contained in sulfur compounds to produce sugars, a process known as [[chemosynthesis]]. Some [[bacteria]] and [[archaea]] use hydrogen sulfide in place of water as the [[electron donor]] in chemosynthesis, a process similar to [[photosynthesis]] that produces sugars and uses oxygen as the [[electron acceptor]]. Sulfur-based chemosynthesis may be simplifiedly compared with photosynthesis:

{{block indent|H<sub>2</sub>S + CO<sub>2</sub> → sugars + S}}
{{block indent|H<sub>2</sub>O + CO<sub>2</sub> → sugars + O<sub>2</sub>}}

There are bacteria combining these two ways of nutrition: [[green sulfur bacteria]] and [[purple sulfur bacteria]].<ref>{{Citation |last1=Frigaard |first1=Niels-Ulrik |title=Sulfur Metabolism in Phototrophic Sulfur Bacteria |date=2008-01-01 |url=https://www.sciencedirect.com/science/article/pii/S0065291108000027 |volume=54 |pages=103–200 |editor-last=Poole |editor-first=Robert K. |publisher=Academic Press |language=en |access-date=2022-05-17 |last2=Dahl |first2=Christiane|series=Advances in Microbial Physiology |doi=10.1016/S0065-2911(08)00002-7 |pmid=18929068 |isbn=9780123743237 }}</ref> Also sulfur-oxidizing bacteria can go into symbiosis with larger organisms, enabling the later to use hydrogen sulfide as food to be oxidized. Example: the [[giant tube worm]].<ref>{{Cite journal |last=Cavanaugh |first=Colleen M. |date=1994 |title=Microbial Symbiosis: Patterns of Diversity in the Marine Environment |journal=American Zoologist |volume=34 |pages=79–89 |doi=10.1093/icb/34.1.79 |doi-access=free }}</ref>

There are [[sulfate-reducing bacteria]], that, by contrast, "breathe sulfate" instead of oxygen. They use organic compounds or molecular hydrogen as the energy source. They use sulfur as the electron acceptor, and reduce various oxidized sulfur compounds back into sulfide, often into hydrogen sulfide. They can grow on other partially oxidized sulfur compounds (e.g. thiosulfates, thionates, polysulfides, sulfites).

There are studies pointing that many deposits of native sulfur in places that were the bottom of [[Tethys Ocean|the ancient oceans]] have biological origin.<ref>{{Cite journal |last1=Jones |first1=Galen E. |last2=Starkey |first2=Robert L. |last3=Feely |first3=Herbert W. |last4=Kulp |first4=J. Laurence |date=1956-06-22 |title=Biological Origin of Native Sulfur in Salt Domes of Texas and Louisiana |url=https://www.science.org/doi/10.1126/science.123.3208.1124 |journal=Science |language=en |volume=123 |issue=3208 |pages=1124–1125 |doi=10.1126/science.123.3208.1124 |pmid=17793426 |bibcode=1956Sci...123.1124J |issn=0036-8075}}</ref><ref>{{Cite journal |last1=Philip |first1=G. |last2=Wali |first2=A. M. A. |last3=Aref |first3=M. A. M. |date=1994-09-01 |title=On the origin of native sulfur deposits in Gebel El Zeit, Gulf of Suez, Egypt |url=https://doi.org/10.1007/BF03175232 |journal=Carbonates and Evaporites |language=en |volume=9 |issue=2 |pages=223–232 |doi=10.1007/BF03175232 |bibcode=1994CarEv...9..223P |s2cid=128827551 |issn=1878-5212}}</ref><ref>{{Cite web |title=Petrography and mineralogy of the crystalline limestone of Fatha Formation from Mishraq area, Iraq |url=https://www.researchgate.net/publication/330038794 |access-date=2022-04-15 |website=ResearchGate |language=en}}</ref> These studies indicate that this native sulfur have been obtained through biological activity, but what is responsible for that (sulfur-oxidizing bacteria or sulfate-reducing bacteria) is still unknown for sure.

Sulfur is absorbed by [[plant]]s [[root]]s from soil as [[sulfate]] and transported as a phosphate ester. Sulfate is reduced to sulfide via sulfite before it is incorporated into [[cysteine]] and other organosulfur compounds.<ref name="Heldt-1996">{{cite book |last=Heldt |first=Hans-Walter |title=Pflanzenbiochemie |publisher=Spektrum Akademischer Verlag |year=1996 |isbn=978-3-8274-0103-8 |place=Heidelberg |pages=321–333 |language=de}}</ref>

{{block indent|{{chem2|SO4(2-)}} → {{chem2|SO3(2-)}} → {{chem2|H2S}} → cysteine (thiol) → methionine (thioether)}}

While the plants' role in transferring sulfur to animals by [[food chain]]s is more or less understood, the role of sulfur bacteria is just getting investigated.<ref>{{Cite journal |last1=Kuenen |first1=J. G. |last2=Beudeker |first2=R. F. |date=1982-09-13 |title=Microbiology of thiobacilli and other sulphur-oxidizing autotrophs, mixotrophs and heterotrophs |url=https://pubmed.ncbi.nlm.nih.gov/6127737/ |journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |volume=298 |issue=1093 |pages=473–497 |doi=10.1098/rstb.1982.0093 |issn=0962-8436 |pmid=6127737|bibcode=1982RSPTB.298..473K }}</ref><ref>{{Cite journal |last1=Wasmund |first1=Kenneth |last2=Mußmann |first2=Marc |last3=Loy |first3=Alexander |date=August 2017 |title=The life sulfuric: microbial ecology of sulfur cycling in marine sediments: Microbial sulfur cycling in marine sediments |journal=Environmental Microbiology Reports |language=en |volume=9 |issue=4 |pages=323–344 |doi=10.1111/1758-2229.12538 |pmc=5573963 |pmid=28419734}}</ref>

===Protein and organic metabolites===
In all forms of life, most of the sulfur is contained in two [[proteinogenic amino acid]]s ([[cysteine]] and [[methionine]]), thus the element is present in all [[protein]]s that contain these amino acids.<ref>{{Cite journal |last1=Gutiérrez-Preciado |first1=A. |last2=Romero |first2=H. |last3=Peimbert |first3=M. |date=2010 |title=An Evolutionary Perspective on Amino Acids |url=https://www.nature.com/scitable/topicpage/an-evolutionary-perspective-on-amino-acids-14568445/ |journal=Nature Education |volume=3 |issue=9 |page=29}}</ref> Some of the sulfur is present in certain metabolites—many of which are [[Cofactor (biochemistry)|cofactors]]—and sulfated polysaccharides of [[connective tissue]] ([[chondroitin sulfate]]s, [[heparin]]).
[[File:Disulfide-bridges-made-in-Avogadro.png|alt=Disulfide bonds between two alpha-helix|left|thumb|Schematic representation of disulfide bridges (in yellow) between two protein helices]]
The functionality of a given protein is heavily dependent on its structure. Proteins reach this structure through the process of [[protein folding]], which is facilitated by a variety of intra- and inter-molecular bonds. While much of the folding is driven by the formation of [[hydrogen bond]]s, [[covalent bond|covalent bonding]] of cysteine residues into disulfide bridges imposes constraints that stabilize particular conformations while preventing others from forming. As the [[bond energy]] of a covalent disulfide bridge is higher than the energy of a [[Coordinate covalent bond|coordinate bond]] or hydrophobic interaction, greater numbers of disulfide bridges lead to higher energies required for protein [[Denaturation (biochemistry)|denaturation]]. Disulfide bonds often serve to stabilize protein structures in the more oxidizing conditions of the extracellular environment.<ref>{{Cite book |last1=Alberts |first1=Bruce |url=https://www.ncbi.nlm.nih.gov/books/NBK26830/#_A436_ |title=Molecular Biology of the Cell. 4th edition. |last2=Johnson |first2=Alexander |last3=Lewis |first3=Julian |last4=Raff |first4=Martin |last5=Roberts |first5=Keith |last6=Walter |first6=Peter |publisher=Garland Science |year=2002 |isbn=978-0-8153-3218-3 |location=New York |language=en}}</ref> Within the [[cytoplasm]], disulfide bonds may instead be reduced (i.e. in -SH form) to their constituent cysteine residues by [[thioredoxin]]s.<ref>{{Cite journal |last1=Arnér |first1=Elias S. J. |last2=Holmgren |first2=Arne |date=25 December 2001 |title=Physiological functions of thioredoxin and thioredoxin reductase: Thioredoxin and thioredoxin reductase |journal=European Journal of Biochemistry |language=en |volume=267 |issue=20 |pages=6102–6109 |doi=10.1046/j.1432-1327.2000.01701.x |pmid=11012661 |doi-access=free }}</ref>

Many important cellular enzymes use prosthetic groups ending with sulfhydryl (-SH) moieties to handle reactions involving acyl-containing biochemicals: two common examples from basic metabolism are [[coenzyme A]] and [[alpha-lipoic acid]].<ref name="Nelson-2000">{{cite book|isbn= 978-1-57259-153-0|last1= Nelson|first1= D. L.|last2= Cox|first2= M. M.|title= Lehninger, Principles of Biochemistry|edition= 3rd|publisher= Worth Publishing|place= New York|date= 2000|url-access= registration|url= https://archive.org/details/lehningerprincip01lehn}}</ref> Cysteine-related metabolites [[homocysteine]] and [[taurine]] are other sulfur-containing amino acids that are similar in structure, but not coded by [[DNA]], and are not part of the [[primary structure]] of proteins, take part in various locations of mammalian physiology.<ref>{{Cite journal |last=Selhub |first=J. |date=1999-07-01 |title=Homocysteine metabolism |url=https://www.annualreviews.org/doi/10.1146/annurev.nutr.19.1.217 |journal=Annual Review of Nutrition |volume=19 |issue=1 |pages=217–246 |doi=10.1146/annurev.nutr.19.1.217 |pmid=10448523 |issn=0199-9885}}</ref><ref>{{Cite journal |last=Huxtable |first=R. J. |date=1992-01-01 |title=Physiological actions of taurine |url=https://journals.physiology.org/doi/abs/10.1152/physrev.1992.72.1.101 |journal=Physiological Reviews |volume=72 |issue=1 |pages=101–163 |doi=10.1152/physrev.1992.72.1.101 |pmid=1731369 |issn=0031-9333}}</ref> Two of the 13 classical vitamins, [[biotin]] and [[thiamine]], contain sulfur, and serve as cofactors to several enzymes.<ref>{{Cite web |title=The Function of Biotin |url=https://www.chem.uwec.edu/webpapers2001/barkacs/pages/function.html |access-date=2022-06-10 |website=www.chem.uwec.edu}}</ref><ref>{{Cite web |last=Edwards |first=Katie A. |title=Thiamine Biochemistry |url=http://thiamine.dnr.cornell.edu/Thiamine_biochemistry.html |access-date=2022-06-10 |website=thiamine.dnr.cornell.edu}}</ref>
In intracellular chemistry, sulfur operates as a carrier of reducing hydrogen and its electrons for cellular repair of oxidation. Reduced [[glutathione]], a sulfur-containing tripeptide, is a reducing agent through its sulfhydryl (–SH) moiety derived from [[cysteine]].

[[Methanogenesis]], the route to most of the world's methane, is a multistep biochemical transformation of [[carbon dioxide]]. This conversion requires several organosulfur cofactors. These include [[coenzyme M]], {{chem2|CH3SCH2CH2SO3-}}, the immediate precursor to [[methane]].<ref>{{cite journal|last1= Thauer|first1= R. K.|title= Biochemistry of methanogenesis: a tribute to Marjory Stephenson:1998 Marjory Stephenson Prize Lecture|journal= Microbiology|volume= 144|issue= 9|pages= 2377–2406|date= 1998|pmid= 9782487|doi= 10.1099/00221287-144-9-2377|doi-access= free}}</ref>

===Metalloproteins and inorganic cofactors===
Metalloproteins—in which the active site is a transition metal ion (or metal-sulfide cluster) often coordinated by sulfur atoms of cysteine residues<ref>{{Cite journal |last1=Pace |first1=Nicholas J. |last2=Weerapana |first2=Eranthie |date=2014-04-17 |title=Zinc-binding cysteines: diverse functions and structural motifs |journal=Biomolecules |volume=4 |issue=2 |pages=419–434 |doi=10.3390/biom4020419 |issn=2218-273X |pmc=4101490 |pmid=24970223|doi-access=free }}</ref>—are essential components of enzymes involved in electron transfer processes. Examples include [[plastocyanin]] (Cu<sup>2+</sup>) and [[nitrous-oxide reductase|nitrous oxide reductase]] (Cu–S). The function of these enzymes is dependent on the fact that the transition metal ion can undergo [[redox reaction]]s. Other examples include many zinc proteins,<ref>{{Cite journal |last1=Giles |first1=Niroshini M |last2=Watts |first2=Aaron B |last3=Giles |first3=Gregory I |last4=Fry |first4=Fiona H |last5=Littlechild |first5=Jennifer A |last6=Jacob |first6=Claus |date=2003-08-01 |title=Metal and Redox Modulation of Cysteine Protein Function |journal=Chemistry & Biology |language=en |volume=10 |issue=8 |pages=677–693 |doi=10.1016/S1074-5521(03)00174-1 |pmid=12954327 |issn=1074-5521|doi-access=free }}</ref> as well as [[iron–sulfur cluster]]s. Most pervasive are the [[ferrodoxin]]s, which serve as electron shuttles in cells. In bacteria, the important [[nitrogenase]] enzymes contain an Fe–Mo–S cluster and is a [[catalyst]] that performs the important function of [[nitrogen fixation]], converting atmospheric nitrogen to ammonia that can be used by microorganisms and plants to make proteins, DNA, RNA, alkaloids, and the other organic nitrogen compounds necessary for life.<ref>{{cite book|isbn= 978-0-935702-73-6 |first1= S. J.|last1= Lippard|first2= J. M.|last2= Berg|title= Principles of Bioinorganic Chemistry|publisher= University Science Books|date= 1994}}</ref>

Sulfur is also present in [[molybdenum cofactor]].<ref>{{Cite journal |last1=Schwarz |first1=Günter |last2=Mendel |first2=Ralf R. |date=2006 |title=Molybdenum cofactor biosynthesis and molybdenum enzymes |journal=Annual Review of Plant Biology |language=en |volume=57 |issue=1 |pages=623–647 |doi=10.1146/annurev.arplant.57.032905.105437 |pmid=16669776 |bibcode=2006AnRPB..57..623S |issn=1543-5008}}</ref>
:[[File:FdRedox.png|center|upright=3|Easiness of electron flow in a cluster provides catalytic effect of a respective enzyme.|thumb]]

===Sulfate===
{{Main|Sulfation#Sulfation in biology}}

===Deficiency===
In humans [[methionine]] is an [[essential amino acid]]; [[cysteine]] is conditionally essential and may be synthesized from non-essential [[serine]] via sulfur salvaged from methionine. Sulfur deficiency is uncommon due to the ubiquity of cysteine and methionine in food.{{cn|date=February 2025}} 

Isolated sulfite oxidase deficiency is a rare, fatal genetic disease caused by mutations to [[sulfite oxidase]], which is needed to metabolize sulfites to sulfates.<ref>{{Cite journal |last1=Karakas |first1=E |last2=Kisker |first2=C |date=October 2005 |title=Structural analysis of missense mutations causing isolated sulfite oxidase deficiency |url=https://pubs.rsc.org/en/content/articlelanding/2005/dt/b505789m |journal=Dalton Transactions |issue=21 |pages=3459–63 |doi=10.1039/B505789M |pmid=16234925 |issn=1477-9234}}</ref>