Cysteinyl

L-Cysteine

Names
IUPAC name Cysteine
Other names 2-Amino-3-sulfhydrylpropanoic acid
Identifiers
CAS Number	52-90-4 Y 52-89-1 (hydrochloride) N
3D model (JSmol)	Interactive image Interactive image
ChEBI	CHEBI:15356 Y
ChEMBL	ChEMBL54943 Y
ChemSpider	574 (Racemic) Y 5653 (L-form) Y
ECHA InfoCard	100.000.145
EC Number	200-158-2
E number	E920 (glazing agents, ...)
IUPHAR/BPS	4782
KEGG	D00026 Y
PubChem CID	5862
UNII	K848JZ4886 Y
InChI InChI=1S/C3H7NO2S/c4-2(1-7)3(5)6/h2,7H,1,4H2,(H,5,6) Y Key: XUJNEKJLAYXESH-UHFFFAOYSA-N Y 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 C(C(C(=O)O)N)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	soluble
Solubility	1.5g/100g ethanol 19 degC
Chiral rotation ([α]D)	+9.4° (H2O, c = 1.3)
Supplementary data page
Structure and properties	Refractive index (n), Dielectric constant (εr), etc.
Thermodynamic data	Phase behaviour solid–liquid–gas
Spectral data	UV, IR, NMR, MS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N  (what is YN ?)
Infobox references

Cysteine (symbol Cys or C) (/ˈsɪstɪiːn/) is a semi-essentialproteinogenic amino acid with the formula HO₂CCH(NH₂)CH₂SH. It is encoded by the codons UGU and UGC. The thiol side chain in cysteine often participates in enzymatic reactions, as a nucleophile. The thiol is susceptible to oxidation to give the disulfide derivative cystine, which serves an important structural role in many proteins. When used as a food additive, it has the E number E920.

Cysteine has the same structure as serine, but with one of its oxygen atoms replaced by sulfur; replacing it with selenium gives selenocysteine. (Like other natural proteinogenic amino acids cysteine has (L) chirality in the older D/L notation based on homology to D and 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 (resp. selenium) as a second neighbor to the asymmetric carbon. The remaining chiral amino acids, having lighter atoms in that position, have S chirality.)

Although classified as a non-essential amino acid, in rare cases, cysteine may be essential for infants, the elderly, and individuals with certain metabolic disease or who suffer from malabsorption syndromes. Cysteine can usually be synthesized by the human body under normal physiological conditions if a sufficient quantity of methionine is available. Cysteine is catabolized in the gastrointestinal tract and blood plasma. In contrast, cystine travels safely through the GI tract and blood plasma and is promptly reduced to the two cysteine molecules upon cell entry.