Cysteine: Difference between revisions

{{Short description|Proteinogenic amino acid}}

{{Distinguish|cytosine|cystine|cytisine|cytidine|Sistine}}

{{Redirect|Cys}}

{{Chembox

| Verifiedfields = changed

| Watchedfields = changed

| verifiedrevid = 443554430

| Name = {{sm|l}}-Cysteine

| ImageFile_Ref = {{chemboximage|correct|??}}

| ImageFile = L-Cystein - L-Cysteine.svg

| ImageCaption = [[Skeletal formula]] of <small>L</small>-cysteine

| ImageFileL1 = Cysteine-from-xtal-3D-bs-17.png

| ImageCaptionL1 = [[Ball-and-stick model]]

| ImageFileR1 = L-cysteine-from-xtal-Mercury-3D-sf.png

| ImageCaptionR1 = [[Space-filling model]]

| IUPACName = Cysteine

| OtherNames = {{Unbulleted list|2-Amino-3-sulfhydrylpropanoic acid}}

| SystematicName =

| Section1 = {{Chembox Identifiers

| IUPHAR_ligand = 4782

| Abbreviations = '''Cys''', '''C'''

| UNII_Ref = {{fdacite|correct|FDA}}

| UNII1_Ref = {{fdacite|correct|FDA}}

| UNII1 = A9U1687S1S

| UNII1_Comment = (hydrochloride)

| InChIKey = XUJNEKJLAYXESH-REOHCLBHBU

| CASNo_Ref = {{cascite|correct|CAS}}

| CASNo2_Ref = {{cascite|correct|CAS}}

| CASNo2 = 52-89-1

| CASNo2_Comment = (hydrochloride) <!-- also verified at CAS Common Chemistry -->

| CASNo3 = 921-01-7

| CASNo3_Comment = D-cysteine

| EC_number = 200-158-2

| PubChem = 5862

| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}

| ChemSpiderID1_Ref = {{chemspidercite|correct|chemspider}}

| ChemSpiderID1_Comment = (<small>L</small>-form)

| ChEBI_Ref = {{ebicite|correct|EBI}}

| ChEBI = 15356

| SMILES1 = C([C@@H](C(=O)[O-])[NH3+])S

| SMILES1_Comment = [[Zwitterion]]

| Section2 = {{Chembox Properties

| Properties_ref = <ref>{{RubberBible62nd|page=C-259}}.</ref>

| C=3 | H=7 | N=1 | O=2 | S=1

| Appearance = white crystals or powder

| Density =

| MeltingPtC = 240

| MeltingPt_notes = decomposes

| SpecRotation = +9.4° (H₂O, ''c'' = 1.3)

| Solubility = 277g/L (at 25 °C)<ref>{{cite web| url = https://pubchem.ncbi.nlm.nih.gov/compound/Cysteine#section=Solubility | title = PubChem data }}</ref>

| SolubleOther = 1.5g/100g ethanol 19 °C <ref>{{Cite book | url = https://books.google.com/books?id=xteiARU46SQC&pg=PA15 | title = Food Chemistry | isbn = 9783540699330 | last1 = Belitz | first1 = H.-D | last2 = Grosch | first2 = Werner | last3 = Schieberle | first3 = Peter | date = 2009-02-27 | publisher = Springer }}</ref>

| pKa = 1.71 ([[conjugate acid]]), 8.33 ([[thiol]]), 10.78<ref>Kirste, Burkhard (23&nbsp;Jan&nbsp;1998). "[http://www.chemie.fu-berlin.de/chemistry/bio/aminoacid/cystein_en.html Cysteine]". ''[https://web.archive.org/web/20161107053630/http://www.chemie.fu-berlin.de/chemistry/bio/amino-acids_en.html Overview of Amino Acids]''. [[Free University of Berlin]] Dep't. of Biology, Chemistry, and Pharmacy. {{Webarchive|url=https://web.archive.org/web/20161110090138/http://www.chemie.fu-berlin.de/chemistry/bio/aminoacid/cystein_en.html |date=2016-11-10 }}</ref>

| Section3 =

| Section4 =

| Section5 =

| Section6 =

'''Cysteine''' (symbol '''Cys''' or '''C''';<ref>{{IUPAC-IUB amino acids 1983}}</ref> {{IPAc-en|ˈ|s|ɪ|s|t|ɪ|iː|n}})<ref>{{cite web|url=https://en.oxforddictionaries.com/definition/cysteine|archive-url=https://web.archive.org/web/20160925134245/https://en.oxforddictionaries.com/definition/cysteine|url-status=dead|archive-date=September 25, 2016|title=cysteine - Definition of cysteine in English by Oxford Dictionaries|website=Oxford Dictionaries - English|access-date=15 April 2018}}</ref> is a semiessential<ref name=microbial>{{cite web|title=The primary structure of proteins is the amino acid sequence|url=http://www.microbiologytext.com/index.php?module=Book&func=displayarticle&art_id=661|work=The Microbial World|publisher=University of Wisconsin-Madison Bacteriology Department|access-date=16 September 2012|archive-date=25 May 2013|archive-url=https://web.archive.org/web/20130525095344/http://www.microbiologytext.com/index.php?module=Book&func=displayarticle&art_id=661|url-status=dead}}</ref> [[proteinogenic amino acid]] with the [[chemical formula|formula]] {{chem2|HOOC\sCH(\sNH2)\sCH2\sSH}}. The [[thiol]] side chain in cysteine often participates in [[enzymatic]] reactions as a [[nucleophile]]. Cysteine is chiral, but both <small>D</small> and <small>L</small>-cysteine are found in nature. <small>L</small>{{nbh}}Cysteine is a protein monomer in all biota, and <small>D</small>-cysteine acts as a signaling molecule in mammalian nervous systems.<ref>{{cite journal|doi=10.1073/pnas.2110610118|doi-access=free|title=D-cysteine is an endogenous regulator of neural progenitor cell dynamics in the mammalian brain|first1=Evan&nbsp;R.|last1=Semenza|first2=Maged&nbsp;M.|last2=Harraz|first3=Efrat|last3=Abramson|first4=Adarsha&nbsp;P.|last4=Malla|first5=Chirag|last5=Vasavda|first6=Moataz&nbsp;M.|last6=Gadalla|first7=Michael&nbsp;D.|last7=Kornberg|first8=Solomon&nbsp;H.|last8=Snyder|first9=Robin|last9=Roychaudhuri|orig-date=18 Aug 2021|date=23 Sep 2021|journal=[[PNAS]]|volume=118|issue=39|pages=e2110610118 |pmid=34556581 |pmc=8488618|bibcode=2021PNAS..11810610S }}</ref> Cysteine is named after its discovery in urine, which comes from the urinary bladder or cyst, from [[Greek language|Greek]] κύστη ''kýsti'', "bladder".<ref>{{Cite journal |last=Saffran |first=M. |date=April 1998 |title=Amino acid names and parlor games: from trivial names to a one-letter code, amino acid names have strained students' memories. Is a more rational nomenclature possible? |url=http://dx.doi.org/10.1016/s0307-4412(97)00167-2 |journal=Biochemical Education |volume=26 |issue=2 |pages=116–118 |doi=10.1016/s0307-4412(97)00167-2 |issn=0307-4412}}</ref>

The thiol is susceptible to oxidation to give the [[disulfide bond|disulfide]] derivative [[cystine]], which serves an important structural role in many [[protein]]s. In this case, the symbol '''Cyx''' is sometimes used.<ref name=":0">{{Cite web |title=Amber Workshop - Tutorial A1 - Section 1: Do some editing of the PDB file |url=https://ambermd.org/tutorials/advanced/tutorial1_orig/section1.htm |access-date=2022-06-02 |website=ambermd.org |archive-date=2022-05-22 |archive-url=https://web.archive.org/web/20220522002234/https://ambermd.org/tutorials/advanced/tutorial1_orig/section1.htm |url-status=dead }}</ref><ref name=":1">{{cite journal |last1=Lee |first1=Jumin |last2=Hitzenberger |first2=Manuel |last3=Rieger |first3=Manuel |last4=Kern |first4=Nathan R. |last5=Zacharias |first5=Martin |last6=Im |first6=Wonpil |title=CHARMM-GUI supports the Amber force fields |journal=The Journal of Chemical Physics |date=21 July 2020 |volume=153 |issue=3 |pages=035103 |doi=10.1063/5.0012280 |pmid=32716185 |s2cid=220796795 |doi-access=free }}</ref> The deprotonated form can generally be described by the symbol '''Cym''' as well.<ref name=":1" /><ref name=":02">{{Cite web |title=Amber Workshop - Tutorial A1 - Section 1: Do some editing of the PDB file |url=https://ambermd.org/tutorials/advanced/tutorial1_orig/section1.htm |access-date=2022-06-02 |website=ambermd.org |archive-date=2022-05-22 |archive-url=https://web.archive.org/web/20220522002234/https://ambermd.org/tutorials/advanced/tutorial1_orig/section1.htm |url-status=dead }}</ref>

When used as a food additive, cysteine has the [[E number]] E920.

Cysteine is [[Genetic code|encoded]] by the [[codon]]s UGU and UGC.

==Structure==

Like other amino acids (not as a residue of a protein), cysteine exists as a [[zwitterion]]. Cysteine has {{sm|l}} [[dextrorotation and levorotation|chirality]] in the older {{sm|d}}/{{sm|l}} notation based on homology to {{sm|d}}- and {{sm|l}}-glyceraldehyde. In the newer ''R''/''S'' system of designating chirality, based on the atomic numbers of atoms near the asymmetric carbon, cysteine (and selenocysteine) have ''R'' chirality, because of the presence of sulfur (or selenium) as a second neighbor to the asymmetric carbon atom. The remaining chiral amino acids, having lighter atoms in that position, have ''S'' chirality. Replacing sulfur with [[selenium]] gives [[selenocysteine]].

[[File:Betain-Cystein.png|thumb|left|250px|(''R'')-Cysteine (left) and (''S'')-Cysteine (right) in zwitterionic form at neutral pH]]

==Dietary sources==

Cysteinyl is a residue in high-[[protein]] foods. Some foods considered rich in cysteine include poultry, eggs, beef, and whole grains. In high-protein diets, cysteine may be partially responsible for reduced blood pressure and stroke risk.<ref>{{cite journal |last1=Larsson |first1=Susanna C. |last2=Håkansson |first2=Niclas |last3=Wolk |first3=Alicja |title=Dietary Cysteine and Other Amino Acids and Stroke Incidence in Women |journal=Stroke |date=April 2015 |volume=46 |issue=4 |pages=922–926 |doi=10.1161/STROKEAHA.114.008022 |pmid=25669310 |s2cid=14895681 |url=http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-397364 }}</ref> Although classified as a non[[essential amino acid]],<ref>{{Cite web |title=Cysteine - Health Encyclopedia - University of Rochester Medical Center |url=https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=19&contentid=Cysteine#:~:text=Cysteine%20is%20a%20non-essential,is%20important%20for%20making%20collagen. |access-date=2024-05-13 |website=www.urmc.rochester.edu}}</ref> in rare cases, cysteine may be essential for infants, the elderly, and individuals with certain metabolic diseases or who suffer from [[malabsorption]] [[syndrome]]s. Cysteine can usually be synthesized by the human body under normal physiological conditions if a sufficient quantity of [[methionine]] is available.

== Industrial sources<span class="anchor" id="Dietary restrictions"></span> ==

The majority of {{sm|l}}-cysteine is obtained industrially by [[hydrolysis]] of animal materials, such as poultry feathers or hog hair. Despite widespread rumor,<ref name=Kashrut/> human hair is rarely a source material.<ref>{{cite web |url=https://www.vrg.org/vrgnews/2007jul.htm#s2 |website=vrg.org |access-date=26 August 2024 |title=VRG-News July 2007 -- the Vegetarian Resource Group }}</ref> Indeed, food additive or cosmetic product manufactures may not legally source from human hair in the European Union.<ref>{{cite web |url=http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2012:083:0001:0295:EN:PDF |title=EU Chemical Requirements |access-date=May 24, 2020 |quote=...L-cysteine hydrochloride or hydrochloride monohydrate. Human hair may not be used as a source for this substance}}</ref><ref>{{cite web |url=https://eur-lex.europa.eu/legal-content/en/TXT/?uri=CELEX%3A32009R1223 |title=Regulation (EC) No 1223/2009 of the European Parliament and of the Council of 30 November 2009 on cosmetic products |access-date=July 28, 2021 |quote=...ANNEX II LIST OF SUBSTANCES PROHIBITED IN COSMETIC PRODUCTS...416 Cells, tissues or products of human origin}}</ref>

Some animal-originating sources of {{sm|l}}-cysteine as a food additive contravene kosher, halal, vegan, or vegetarian diets.<ref name=Kashrut>See, e.g., {{cite news|title=Like mountains hanging by a hair: Kosher issues in L-cysteine (Commentary on Chagiga I:8)|first=Zushe&nbsp;Yosef|last=Blech|work=MK News and Views|volume=IV|issue=6|date=May 2003|publisher=[[Montreal Kosher]]|via=kashrut.com|url=http://www.kashrut.com/articles/L_cysteine/}} Rabbi Blech does not address hog hair-derived cysteine, which is almost certainly ''[[treyf]]''.</ref> To avoid this problem, synthetic {{sm|l}}-cysteine, compliant with Jewish [[kosher]] and Muslim [[halal]] laws, is also available, albeit at a higher price.<ref>{{Cite web |url= http://www.vrg.org/nutshell/faqingredients.htm#cystine |title= Questions About Food Ingredients: What is L-cysteine/cysteine/cystine? |publisher= Vegetarian Resource Group }}</ref> The typical synthetic route involves fermentation with an artificial ''[[Escherichia coli|E.&nbsp;coli]]'' strain.<ref name=Ullmann/>

Alternatively, [[Evonik]] (formerly Degussa) introduced a route from substituted [[thiazoline]]s.<ref name="Martens">{{cite journal |last1=Martens |first1=Jürgen |author-link2=Heribert Offermanns |last2=Offermanns |first2=Heribert |last3=Scherberich |first3=Paul |year=1981 |title=Facile Synthesis of Racemic Cysteine |journal=Angewandte Chemie International Edition in English |volume=20 |issue=8 |pages=668 |doi=10.1002/anie.198106681}}</ref> ''Pseudomonas thiazolinophilum'' hydrolyzes racemic 2{{nbh}}amino-Δ²{{nbh}}thiazoline-4{{nbh}}carboxylic acid to {{sm|l}}{{nbh}}cysteine.<ref name=Ullmann>{{cite book |doi=10.1002/14356007.a02_057.pub2 |chapter=Amino Acids |title=Ullmann's Encyclopedia of Industrial Chemistry |year=2007 |last1=Drauz |first1=Karlheinz |last2=Grayson |first2=Ian |last3=Kleemann |first3=Axel |last4=Krimmer |first4=Hans-Peter |last5=Leuchtenberger |first5=Wolfgang |last6=Weckbecker |first6=Christoph |isbn=978-3-527-30673-2}}</ref>

== Biosynthesis ==

[[File:Cysteine biosynthesis.svg|thumb|150px|Cysteine synthesis: [[Cystathionine beta synthase]] catalyzes the upper reaction and [[cystathionine gamma-lyase]] catalyzes the lower reaction.]]

In animals, biosynthesis begins with the amino acid [[serine]]. The sulfur is derived from [[methionine]], which is converted to [[homocysteine]] through the intermediate [[S-adenosylmethionine|''S''-adenosylmethionine]]. [[Cystathionine beta-synthase]] then combines homocysteine and serine to form the asymmetrical thioether [[cystathionine]]. The enzyme [[cystathionine gamma-lyase]] converts the cystathionine into cysteine and [[alpha-ketobutyrate]]. In [[plant]]s and [[bacteria]], cysteine biosynthesis also starts from serine, which is converted to [[O-acetylserine|''O''-acetylserine]] by the enzyme [[serine transacetylase]]. The enzyme [[cysteine synthase]], using sulfide sources, converts this ester into cysteine, releasing acetate.<ref>{{cite journal |author=Hell R |title=Molecular physiology of plant sulfur metabolism |journal=Planta |volume=202 |issue=2 |pages=138–48 |year=1997 |pmid=9202491 |doi=10.1007/s004250050112|bibcode=1997Plant.202..138H |s2cid=2539629 }}</ref>

==Biological functions==

The cysteine sulfhydryl group is [[nucleophile|nucleophilic]] and easily oxidized. The reactivity is enhanced when the thiol is ionized, and cysteine [[amino acid residue|residue]]s in proteins have [[acid dissociation constant|pK_a]] values close to neutrality, so are often in their reactive [[thiolate]] form in the cell.<ref>{{cite journal |vauthors=Bulaj G, Kortemme T, Goldenberg DP |title=Ionization-reactivity relationships for cysteine thiols in polypeptides |journal=Biochemistry |volume=37 |issue=25 |pages=8965–72 |date=June 1998 |pmid=9636038 |doi=10.1021/bi973101r}}</ref> Because of its high reactivity, the sulfhydryl group of cysteine has numerous biological functions.

===Precursor to the antioxidant glutathione===

Due to the ability of thiols to undergo redox reactions, cysteine and cysteinyl residues have [[antioxidant]] properties. Its antioxidant properties are typically expressed in the tripeptide [[glutathione]], which occurs in humans and other organisms. The systemic availability of oral glutathione (GSH) is negligible; so it must be biosynthesized from its constituent amino acids, cysteine, [[glycine]], and [[glutamic acid]]. While glutamic acid is usually sufficient because amino acid nitrogen is recycled through glutamate as an intermediary, dietary cysteine and glycine supplementation can improve synthesis of glutathione.<ref>{{cite journal |last1=Sekhar |first1=Rajagopal V |last2=Patel |first2=Sanjeet G |title=Deficient synthesis of glutathione underlies oxidative stress in aging and can be corrected by dietary cysteine and glycine supplementation |journal=The American Journal of Clinical Nutrition |date=2011 |volume=94 |issue=3 |pages=847–853 |doi=10.3945/ajcn.110.003483 |pmid=21795440 |url= |pmc=3155927 }} {{Open Access}}</ref>

Cysteine is an important source of [[sulfide]] in human [[metabolism]]. The sulfide in [[iron-sulfur cluster]]s and in [[nitrogenase]] is extracted from cysteine, which is converted to [[alanine]] in the process.<ref>{{cite journal |vauthors=Lill R, Mühlenhoff U |title=Iron-sulfur protein biogenesis in eukaryotes: components and mechanisms |journal=Annu. Rev. Cell Dev. Biol. |volume=22 |pages=457–86 |year=2006 |pmid=16824008 |doi=10.1146/annurev.cellbio.22.010305.104538|url=http://nbn-resolving.de/urn:nbn:de:bvb:12-bsb00055966-1 }}</ref>

Beyond the iron-sulfur proteins, many other metal cofactors in enzymes are bound to the thiolate substituent of cysteinyl residues. Examples include zinc in [[zinc finger]]s and [[alcohol dehydrogenase]], copper in the [[plastocyanin|blue copper protein]]s, iron in [[cytochrome P450]], and nickel in the [NiFe]-[[hydrogenase]]s.<ref>{{cite book |first1=Stephen J. |last1=Lippard |first2=Jeremy M. |last2=Berg |title=Principles of Bioinorganic Chemistry |publisher=University Science Books |location=Mill Valley, CA |year=1994 |isbn=978-0-935702-73-6}}{{page needed|date=July 2013}}</ref> The sulfhydryl group also has a high [[Affinity (pharmacology)|affinity]] for [[Heavy metal (chemistry)|heavy metal]]s, so that proteins containing cysteine, such as [[metallothionein]], will [[ligand|bind]] metals such as mercury, lead, and cadmium tightly.<ref>{{cite journal |vauthors=Baker DH, Czarnecki-Maulden GL |title=Pharmacologic role of cysteine in ameliorating or exacerbating mineral toxicities |journal=J. Nutr. |volume=117 |issue=6 |pages=1003–10 |date=June 1987 |pmid=3298579 |doi=10.1093/jn/117.6.1003 |doi-access=free }}</ref>

===Roles in protein structure===

In the translation of messenger RNA molecules to produce polypeptides, cysteine is coded for by the UGU and UGC [[codon]]s.

Cysteine has traditionally been considered to be a [[hydrophilic]] amino acid, based largely on the chemical parallel between its [[thiol group|sulfhydryl group]] and the [[hydroxyl]] groups in the side chains of other polar amino acids. However, the cysteine side chain has been shown to stabilize hydrophobic interactions in micelles to a greater degree than the side chain in the nonpolar amino acid glycine and the polar amino acid serine.<ref>{{cite journal |author=Heitmann P |title=A model for sulfhydryl groups in proteins. Hydrophobic interactions of the cystein side chain in micelles |journal=Eur. J. Biochem. |volume=3 |issue=3 |pages=346–50 |date=January 1968 |pmid=5650851 |doi=10.1111/j.1432-1033.1968.tb19535.x|doi-access=free }}</ref> In a statistical analysis of the frequency with which amino acids appear in various proteins, cysteine residues were found to associate with hydrophobic regions of proteins. Their hydrophobic tendency was equivalent to that of known nonpolar amino acids such as [[methionine]] and [[tyrosine]] (tyrosine is polar aromatic but also hydrophobic<ref>{{cite web|url=http://wbiomed.curtin.edu.au/biochem/tutorials/AAs/AA.html|title=A Review of Amino Acids (tutorial)|publisher=Curtin University|access-date=2015-09-09|archive-url=https://web.archive.org/web/20150907052410/http://wbiomed.curtin.edu.au/biochem/tutorials/AAs/AA.html|archive-date=2015-09-07|url-status=dead}}</ref>), those of which were much greater than that of known polar amino acids such as serine and [[threonine]].<ref>{{cite journal |vauthors=Nagano N, Ota M, Nishikawa K |title=Strong hydrophobic nature of cysteine residues in proteins |journal=FEBS Lett. |volume=458 |issue=1 |pages=69–71 |date=September 1999 |pmid=10518936 |doi=10.1016/S0014-5793(99)01122-9|s2cid=34980474 |doi-access=free |bibcode=1999FEBSL.458...69N }}</ref> [[Hydrophobicity scales]], which rank amino acids from most hydrophobic to most hydrophilic, consistently place cysteine towards the hydrophobic end of the spectrum, even when they are based on methods that are not influenced by the tendency of cysteines to form disulfide bonds in proteins. Therefore, cysteine is now often grouped among the hydrophobic amino acids,<ref>{{cite web | url = http://www.russelllab.org/aas/hydrophobic.html | title = Hydrophobic amino acids | access-date = 2012-09-16 | last1 = Betts | first1 = M.J. |author2=R.B. Russell | year = 2003 | work = Amino Acid Properties and Consequences of Substitutions, In: Bioinformatics for Geneticists | publisher = Wiley}}</ref><ref>{{cite web |url=http://webhost.bridgew.edu/fgorga/proteins/nonpolar.htm |title=Introduction to Protein Structure--Non-Polar Amino Acids |access-date=2012-09-16 |last1=Gorga |first1=Frank R. |date=1998–2001 |url-status=dead |archive-url=https://web.archive.org/web/20120905162400/http://webhost.bridgew.edu/fgorga/proteins/nonpolar.htm |archive-date=2012-09-05 }}</ref> though it is sometimes also classified as slightly polar,<ref>{{cite web | url = http://www.elmhurst.edu/~chm/vchembook/561aminostructure.html | title = Virtual Chembook--Amino Acid Structure | access-date = 2012-09-16 | publisher = Elmhurst College | url-status = dead | archive-url = https://web.archive.org/web/20121002050150/http://www.elmhurst.edu/~chm/vchembook/561aminostructure.html | archive-date = 2012-10-02 }}</ref> or polar.<ref name=microbial/>

Most cysteine residues are covalently bonded to other cysteine residues to form [[disulfide bond]]s, which play an important role in the folding and stability of some proteins, usually proteins secreted to the extracellular medium.<ref name=Sevier>{{cite journal |vauthors=Sevier CS, Kaiser CA |title=Formation and transfer of disulphide bonds in living cells |journal=Nat. Rev. Mol. Cell Biol. |volume=3 |issue=11 |pages=836–47 |date=November 2002 |pmid=12415301 |doi=10.1038/nrm954|s2cid=2885059 |doi-access=free }}</ref> Since most cellular compartments are [[reducing environment]]s, disulfide bonds are generally unstable in the [[cytosol]] with some exceptions as noted below.

[[Image:Cystine-skeletal.png|thumb|right|150px|Figure 2: [[Cystine]] (shown here in its neutral form), two cysteines bound together by a disulfide bond]]

Disulfide bonds in proteins are formed by oxidation of the sulfhydryl group of cysteine residues. The other sulfur-containing amino acid, methionine, cannot form disulfide bonds. More aggressive oxidants convert cysteine to the corresponding [[sulfinic acid]] and [[sulfonic acid]]. Cysteine residues play a valuable role by crosslinking proteins, which increases the rigidity of proteins and also functions to confer proteolytic resistance (since protein export is a costly process, minimizing its necessity is advantageous). Inside the cell, disulfide bridges between cysteine residues within a polypeptide support the protein's tertiary structure. [[Insulin]] is an example of a protein with cystine crosslinking, wherein two separate peptide chains are connected by a pair of disulfide bonds.

[[Protein disulfide isomerase]]s catalyze the proper formation of [[disulfide bond]]s; the cell transfers [[dehydroascorbic acid]] to the [[endoplasmic reticulum]], which oxidizes the environment. In this environment, cysteines are, in general, oxidized to cystine and are no longer functional as a nucleophiles.

Aside from its oxidation to cystine, cysteine participates in numerous [[post-translational modification]]s. The [[nucleophilic]] sulfhydryl group allows cysteine to conjugate to other groups, e.g., in [[prenylation]]. [[Ubiquitin]] [[ligase]]s transfer ubiquitin to its pendant, proteins, and [[caspase]]s, which engage in proteolysis in the apoptotic cycle. [[Intein]]s often function with the help of a catalytic cysteine. These roles are typically limited to the intracellular milieu, where the environment is reducing, and cysteine is not oxidized to cystine.

== Evolutionary role of cysteine ==

Cysteine is considered a "newcomer" amino acid, being the 17th amino acid incorporated into the [[genetic code]].<ref>{{Cite journal |last1=Osawa |first1=S |last2=Jukes |first2=T H |last3=Watanabe |first3=K |last4=Muto |first4=A |date=March 1992 |title=Recent evidence for evolution of the genetic code |journal=Microbiological Reviews |language=en |volume=56 |issue=1 |pages=229–264 |doi=10.1128/mr.56.1.229-264.1992 |issn=0146-0749 |pmc=372862 |pmid=1579111}}</ref><ref>{{Cite journal |last=Trifonov |first=Edward N. |date=September 2009 |title=The origin of the genetic code and of the earliest oligopeptides |url=https://linkinghub.elsevier.com/retrieve/pii/S0923250809000576 |journal=Research in Microbiology |language=en |volume=160 |issue=7 |pages=481–486 |doi=10.1016/j.resmic.2009.05.004|pmid=19524038 }}</ref> Similar to other later-added amino acids such as [[methionine]], [[tyrosine]], and [[tryptophan]], cysteine exhibits strong nucleophilic and redox-active properties.<ref>{{Cite journal |last1=Paulsen |first1=Candice E. |last2=Carroll |first2=Kate S. |date=2013-07-10 |title=Cysteine-Mediated Redox Signaling: Chemistry, Biology, and Tools for Discovery |journal=Chemical Reviews |language=en |volume=113 |issue=7 |pages=4633–4679 |doi=10.1021/cr300163e |issn=0009-2665 |pmc=4303468 |pmid=23514336}}</ref><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=August 2003 |title=Metal and Redox Modulation of Cysteine Protein Function |url=https://doi.org/10.1016/S1074-5521(03)00174-1 |journal=Chemistry & Biology |volume=10 |issue=8 |pages=677–693 |doi=10.1016/s1074-5521(03)00174-1 |pmid=12954327 |issn=1074-5521}}</ref> These properties contribute to the depletion of cysteine from [[Electron transport chain|respiratory chain]] complexes, such as [[Respiratory complex I|Complexes I]] and [[Complex IV|IV]],<ref>{{Cite journal |last1=Moosmann |first1=Bernd |last2=Behl |first2=Christian |date=February 2008 |title=Mitochondrially encoded cysteine predicts animal lifespan |journal=Aging Cell |language=en |volume=7 |issue=1 |pages=32–46 |doi=10.1111/j.1474-9726.2007.00349.x |pmid=18028257 |issn=1474-9718|doi-access=free }}</ref> since reactive oxygen species ([[Reactive oxygen species|ROS]]) produced by the respiratory chain can react with the cysteine residues in these complexes, leading to dysfunctional proteins and potentially contributing to aging. The primary response of a protein to ROS is the oxidation of cysteine and the loss of free thiol groups,<ref>{{Cite journal |last=Sohal |first=Rajindar S |date=2002-07-01 |title=Role of oxidative stress and protein oxidation in the aging process1, 2 |url=https://www.sciencedirect.com/science/article/pii/S0891584902008560 |journal=Free Radical Biology and Medicine |volume=33 |issue=1 |pages=37–44 |doi=10.1016/S0891-5849(02)00856-0 |pmid=12086680 |issn=0891-5849}}</ref> resulting in increased [[Thiyl radical|thiyl radicals]] and associated protein cross-linking.<ref>{{Cite journal |last1=Jacob |first1=Claus |last2=Giles |first2=Gregory I. |last3=Giles |first3=Niroshini M. |last4=Sies |first4=Helmut |date=2003-10-13 |title=Sulfur and Selenium: The Role of Oxidation State in Protein Structure and Function |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.200300573 |journal=Angewandte Chemie International Edition |language=en |volume=42 |issue=39 |pages=4742–4758 |doi=10.1002/anie.200300573 |pmid=14562341 |issn=1433-7851}}</ref><ref>{{Cite journal |last1=Nauser |first1=Thomas |last2=Pelling |first2=Jill |last3=Schöneich |first3=Christian |date=2004-10-01 |title=Thiyl Radical Reaction with Amino Acid Side Chains: Rate Constants for Hydrogen Transfer and Relevance for Posttranslational Protein Modification |url=https://pubs.acs.org/doi/10.1021/tx049856y |journal=Chemical Research in Toxicology |language=en |volume=17 |issue=10 |pages=1323–1328 |doi=10.1021/tx049856y |pmid=15487892 |issn=0893-228X}}</ref> In contrast, another sulfur-containing, redox-active amino acid, methionine, does not exhibit these biochemical properties and its content is relatively upregulated in [[Mitochondrion|mitochondrially]] encoded proteins.<ref>{{citation |last1=Moosmann |first1=Bernd |first2=Parvana |last2=Hajieva |first3=Christian |last3=Behl |title=The antioxidant function of protein methionine explains about the evolution of a non-standard genetic code in mitochondria |journal=Free Radical Biology and Medicine |volume=41 |article-number=402 |doi=10.1016/j.freeradbiomed.2006.10.015 |page=S149–S150 |date=2006}}</ref>

Cysteine, mainly the {{sm|l}}-[[enantiomer]], is a [[precursor (chemistry)|precursor]] in the food, pharmaceutical, and personal-care industries. One of the largest applications is the production of flavors. For example, the reaction of cysteine with sugars in a [[Maillard reaction]] yields meat flavors.<ref>{{cite book |first1=Tzou-Chi |last1=Huang |first2=Chi-Tang |last2=Ho |url=https://books.google.com/books?id=651Zv5hUzyIC&pg=PA71 |pages=71–102 |editor1-first=Y. H. |editor1-last=Hui |editor2-first=Wai-Kit |editor2-last=Nip |editor3-first=Robert |editor3-last=Rogers |title=Meat Science and Applications, ch. Flavors of Meat Products|publisher=CRC |isbn=978-0-203-90808-2|date=2001-07-27 }}</ref> {{sm|l}}-Cysteine is also used as a [[improving agent|processing aid]] for baking.<ref>{{Cite web|url=http://www.cfsan.fda.gov/~dms/foodic.html |title=Food Ingredients and Colors |date=November 2004 |publisher=U.S. Food and Drug Administration |access-date=2009-09-06 |url-status=dead |archive-url=https://web.archive.org/web/20090512090657/https://www.cfsan.fda.gov/~dms/foodic.html |archive-date=2009-05-12 }}</ref>

In the field of personal care, cysteine is used for [[permanent wave|permanent-wave]] applications, predominantly in Asia. Again, the cysteine is used for breaking up the disulfide bonds in the [[hair]]'s [[keratin]].

Cysteine is a very popular target for site-directed labeling experiments to investigate biomolecular structure and dynamics. [[Maleimide]]s selectively attach to cysteine using a covalent [[Michael addition]]. [[Site-directed spin labeling]] for EPR or paramagnetic relaxation-enhanced NMR also uses cysteine extensively.

===Reducing toxic effects of alcohol===

Cysteine has been proposed as a preventive or antidote for some of the negative effects of alcohol, including liver damage and [[hangover]]. It counteracts the poisonous effects of [[acetaldehyde]].<ref>{{cite journal |last1=Otoyama |first1=Ippo |last2=Hamada |first2=Hironobu |last3=Kimura |first3=Tatsushi |last4=Namba |first4=Haruchi |last5=Sekikawa |first5=Kiyokazu |last6=Kamikawa |first6=Norimichi |last7=Kajiwara |first7=Teruki |last8=Aizawa |first8=Fumiya |last9=Sato |first9=Yoshinobu M. |title=L-cysteine improves blood fluidity impaired by acetaldehyde: In vitro evaluation |journal=PLOS ONE |date=2019 |volume=14 |issue=3 |pages=e0214585 |doi=10.1371/journal.pone.0214585 |pmid=30925182 |pmc=6440629 |bibcode=2019PLoSO..1414585O |doi-access=free }}</ref> It binds to acetaldehyde to form the low-toxicity heterocycle methyl[[thioproline]].<ref>{{cite thesis|title=Interaction of alcohol and smoking in the pathogenesis of upper digestive tract cancers: possible chemoprevention with cysteine|url=https://helda.helsinki.fi/items/8d28d7d1-e6f4-4a2c-89e9-9a25bdffdf93|id=URN:ISBN:952-10-3056-9<!--Not a real ISBN, per WorldCat, but urn.fi recognizes the URN-->|hdl=10138/22689|last=Salaspuro|first=Ville|institution=[[University of Helsinki]]|year=2006|type=Academic dissertation|pages=41–44}}</ref>

In a [[rat]] study, test animals received an [[Lethal dose|LD₉₀]] dose of acetaldehyde. Those that received cysteine had an 80% survival rate; when both cysteine and [[thiamine]] were administered, all animals survived. The [[control group]] had a 10% survival rate.<ref>{{cite journal |vauthors=Sprince H, Parker CM, Smith GG, Gonzales LJ |title=Protection against acetaldehyde toxicity in the rat by L-cysteine, thiamin and L-2-methylthiazolidine-4-carboxylic acid |journal=Agents Actions |volume=4 |issue=2 |pages=125–30 |date=April 1974 |pmid=4842541 |doi=10.1007/BF01966822|s2cid=5924137 }}</ref>

In 2020 an article was published that suggests L-cysteine might also work in humans.<ref>{{cite journal |last1=Eriksson |first1=C J Peter |last2=Metsälä |first2=Markus |last3=Möykkynen |first3=Tommi |last4=Mäkisalo |first4=Heikki |last5=Kärkkäinen |first5=Olli |last6=Palmén |first6=Maria |last7=Salminen |first7=Joonas E |last8=Kauhanen |first8=Jussi |title=L-Cysteine Containing Vitamin Supplement Which Prevents or Alleviates Alcohol-related Hangover Symptoms: Nausea, Headache, Stress and Anxiety |journal=Alcohol and Alcoholism |date=20 October 2020 |volume=55 |issue=6 |pages=660–666 |doi=10.1093/alcalc/agaa082 |pmid=32808029 |hdl=10138/339340 |hdl-access=free }}</ref>

===''N''-Acetylcysteine===

[[N-Acetylcysteine|''N''-Acetyl-{{sm|l}}-cysteine]] is a derivative of cysteine wherein an [[acetyl group]] is attached to the nitrogen atom. This compound is sold as a dietary supplement, and used as an [[antidote]] in cases of [[acetaminophen]] overdose.<ref>{{cite journal |author=Kanter MZ |s2cid=9209528 |title=Comparison of oral and i.v. acetylcysteine in the treatment of acetaminophen poisoning |journal=Am J Health Syst Pharm |volume=63 |issue=19 |pages=1821–7 |date=October 2006 |pmid=16990628 |doi=10.2146/ajhp060050}}</ref>

===Sheep===

Cysteine is required by [[sheep]] to produce wool. It is an essential amino acid that is taken in from their feed. As a consequence, during drought conditions, sheep produce less wool; however, [[transgenic]] sheep that can make their own cysteine have been developed.<ref name="Transgenic sheep">{{cite journal |vauthors=Powell BC, Walker SK, Bawden CS, Sivaprasad AV, Rogers GE |title=Transgenic sheep and wool growth: possibilities and current status |journal=Reprod. Fertil. Dev. |volume=6 |issue=5 |pages=615–23 |year=1994 |pmid=7569041 |doi=10.1071/RD9940615}}</ref>

==Chemical reactions==

Being multifunctional, cysteine undergoes a variety of reactions. Much attention has focused on protecting the sulfhydryl group.<ref>{{cite journal |doi=10.15227/orgsyn.059.0190 |title=Thiol Protection with the Acetamidomethyl Group: S-Acetamidomethyl-L-Cysteine Hydrochloride |journal=Organic Syntheses |date=1979 |volume=59 |page=190|first1=John D. |last1=Milkowski|first2=Daniel F. |last2=Veber|first3=Ralph|last3=Hirschmann }}</ref> [[Methylation]] of cysteine gives [[S-Methylcysteine|S-methylcysteine]]. Treatment with formaldehyde gives the [[thiazolidine]] [[thioproline]]. Cysteine forms a variety of [[coordination complex]]es upon treatment with metal ions.<ref>{{cite journal |doi=10.1021/ic00343a061|title=Stereospecificity in the Synthesis of the Tris((''R'')-Cysteinato-''N,S'')- and Tris((''R'')-Cysteinesulfinato-N,S)cobaltate(III) Ions |year=1990 |last1=Arnold |first1=Alan P. |last2=Jackson |first2=W. Gregory |journal=Inorganic Chemistry |volume=29 |issue=18 |pages=3618–3620 }}</ref>

==Safety==

Relative to most other amino acids, cysteine is much more toxic.<ref>{{cite book |doi=10.1016/0076-6879(87)43059-0 |chapter=Intracellular delivery of cysteine |title=Sulfur and Sulfur Amino Acids |series=Methods in Enzymology |year=1987 |last1=Anderson |first1=Mary E. |last2=Meister |first2=Alton |volume=143 |pages=313–325 |pmid=3309557 |isbn=9780121820435 }}</ref>

==History==

{{Main|Cystine}}

{{section-stub|date=January 2023}}

In 1884 German chemist [[Eugen Baumann]] found that when cystine was treated with a reducing agent, cystine revealed itself to be a [[Dimer (chemistry)|dimer]] of a [[monomer]] which he named "cysteïne".<ref>{{cite journal |last1=Baumann |first1=E. |title=Ueber Cystin und Cysteïn |journal=Zeitschrift für physiologische Chemie |date=1884 |volume=8 |pages=299–305 |url=https://babel.hathitrust.org/cgi/pt?id=coo.31924078260563&view=1up&seq=309 |trans-title=On cystine and cysteine |language=German}} From pp. 301-302: ''"Die Analyse der Substanz ergibt Werthe, welche den vom Cystin (C₆H₁₂N₂S₂O₄) verlangten sich nähern, […] nenne ich dieses Reduktionsprodukt des Cystins: Cysteïn."'' (Analysis of the substance [cysteine] reveals values which approximate those [that are] required by cystine (C₆H₁₂N₂S₂O₄), however the new base [cysteine] can clearly be recognized as a reduction product of cystine, to which the [empirical] formula C₃H₇NSO₂, [which had] previously [been] ascribed to cystine, is [now] ascribed. In order to indicate the relationships of this substance to cystine, I name this reduction product of cystine: "cysteïne".) Note: Baumann's proposed structures for cysteine and cystine (see p.302) are incorrect: for cysteine, he proposed CH₃CNH₂(SH)COOH .</ref>

{{Commons category|Cysteine}}

* [[Saville reaction]]

* [[Sullivan reaction]]

{{Reflist|30em}}

==Further reading==

* {{cite journal |vauthors=Nagano N, Ota M, Nishikawa K |title=Strong hydrophobic nature of cysteine residues in proteins |journal=FEBS Lett. |volume=458 |issue=1 |pages=69–71 |date=September 1999 |pmid=10518936 |doi=10.1016/S0014-5793(99)01122-9|s2cid=34980474 |doi-access=free |bibcode=1999FEBSL.458...69N }}

* Holly (2005). ''[https://web.archive.org/web/20130521211422/http://cystinuriaclearinghouse.com/index.html Cystinuria Clearinghouse]''

* [http://gmd.mpimp-golm.mpg.de/Spectrums/90eac3d9-ee59-4f02-a245-631bde944b66.aspx Cysteine MS Spectrum]

* [http://www.iksi.org International Kidney Stone Institute] {{Webarchive|url=https://web.archive.org/web/20190513095547/http://www.iksi.org/ |date=2019-05-13 }}

{{Cough and cold preparations}}

{{Antidotes}}

{{Amino acids}}

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[[Category:Alpha-Amino acids]]

[[Category:Food additives]]

[[Category:E-number additives]]

[[Category:Excitatory amino acids]]