What is Cysteine?
{{chembox | Watchedfields = changed | verifiedrevid = 443554430 | Name = <small>L</small>-Cysteine | ImageFile_Ref = {{chemboximage|correct|??}} | ImageFile = L-Cystein - L-Cysteine.svg | ImageFileL1 = L-cysteine-3D-balls2.png | ImageFileR1 = L-cysteine-3D-vdW2.png | IUPACName = Cysteine | OtherNames = 2-Amino-3-sulfhydrylpropanoic acid}} [[File:L-Cystein - L-Cysteine.svg|thumb|[[Skeletal formula]] of L-cysteine]] '''Cysteine''' (abbreviated as '''Cys''' or '''C''')<ref>Nomenclature and symbolism for amino acids and peptides (IUPAC-IUB Recommendations 1983). ''Pure Appl. Chem''. '''56''' (5), 1984: 595–624.</ref> is an α-[[amino acid]]. It has the [[chemical formula]] HO<sub>2</sub>CCH(NH<sub>2</sub>)CH<sub>2</sub>SH. Cysteine is a semi-essential amino acid, which means that humans can make it.<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> The [[codon]]s UGU and UGC code for cysteine. The [[thiol]] side chain in cysteine often does [[enzyme]] reactions as a [[nucleophile]]. The thiol oxidises to give the [[disulfide bond|disulfide]] derivative [[cystine]], which is important in many [[protein]] structures. When used as a food additive, cysteine has the [[E&nbsp;number]] '''E920'''. == References == {{reflist}} {{Proteinogenic amino acids}} {{Amino acid metabolism intermediates}} [[Category:Amino acids]]