Editing Sulfur (section)

===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>