Cysteine

l-Cysteine
	Skeletal formula of L-cysteine
Ball-and-stick model	Space-filling model
Names
	IUPAC name Cysteine
	Systematic IUPAC name 2-Amino-3-sulfanylpropanoic acid
	Other names 2-Amino-3-sulfhydrylpropanoic acid; 2-Amino-3-mercaptopropanoic acid;
Identifiers
CAS Number	52-90-4 ; 52-89-1 (hydrochloride) ; 921-01-7 D-cysteine;
3D model (JSmol)	Interactive image; Zwitterion: Interactive image;
Abbreviations	Cys, C
ChEBI	CHEBI:15356 ;
ChEMBL	ChEMBL54943 ;
ChemSpider	574 (Racemic) ; 5653 (L-form) ;
ECHA InfoCard	100.000.145
EC Number	200-158-2;
E number	E920 (glazing agents, ...)
IUPHAR/BPS	4782;
KEGG	D00026 ;
PubChem CID	5862;
UNII	K848JZ4886 ; A9U1687S1S (hydrochloride) ;
CompTox Dashboard (EPA)	DTXSID8022876 ;
	InChI InChI=1S/C3H7NO2S/c4-2(1-7)3(5)6/h2,7H,1,4H2,(H,5,6) Key: XUJNEKJLAYXESH-UHFFFAOYSA-N ; InChI=1/C3H7NO2S/c4-2(1-7)3(5)6/h2,7H,1,4H2,(H,5,6)/t2-/m0/s1 Key: XUJNEKJLAYXESH-REOHCLBHBU; InChI=1/C3H7NO2S/c4-2(1-7)3(5)6/h2,7H,1,4H2,(H,5,6) Key: XUJNEKJLAYXESH-UHFFFAOYAC;
	SMILES C([C@@H](C(=O)O)N)S; Zwitterion: C([C@@H](C(=O)[O-])[NH3+])S;
Properties
Chemical formula	C3H7NO2S
Molar mass	121.15 g·mol−1
Appearance	white crystals or powder
Melting point	240 °C (464 °F; 513 K) decomposes
Solubility in water	277g/L (at 25 °C)
Solubility	1.5g/100g ethanol 19 °C
Acidity (pKa)	1.71 (conjugate acid), 8.33 (thiol), 10.78
Chiral rotation ([α]D)	+9.4° (H2O, c = 1.3)
Supplementary data page
	Cysteine (data page)
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

Cysteine (/ˈsɪstɪiːn/; symbol Cys or C) is a semiessential proteinogenic amino acid with the formula HS−CH₂−CH(NH₂)−COOH. The thiol side chain in cysteine enables the formation of disulfide bonds, and often participates in enzymatic reactions as a nucleophile. Cysteine is chiral, but both D and L-cysteine are found in nature. L‑Cysteine is a protein monomer in all biota, and D-cysteine acts as a signaling molecule in mammalian nervous systems. Cysteine is named after its discovery in urine, which comes from the urinary bladder or cyst, from Greek κύστις kýstis, "bladder".

The thiol is susceptible to oxidation to give the disulfide derivative cystine, which serves an important structural role in many proteins. In this case, the symbol Cyx is sometimes used. The deprotonated form can generally be described by the symbol Cym as well.

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

Cysteine is encoded by the codons UGU and UGC.