Cysteine (symbol Cys or C;[5] /ˈsɪstɪiːn/)[6] is a semiessential[7] proteinogenic amino acid with the formula HOOC−CH(−NH2)−CH2−SH. The thiol side chain in cysteine 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.[8] Cysteine is named after its discovery in urine, which comes from the urinary bladder or cyst, from Greek κύστη kýsti, "bladder".[9] 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.[10][11] The deprotonated form can generally be described by the symbol Cym as well.[11][12] When used as a food additive, cysteine has the E number E920. Cysteine is encoded by the codons UGU and UGC. Structure Like other amino acids (not as a residue of a protein), cysteine exists as a zwitterion. 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 (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. (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.[13] Although classified as a nonessential amino acid, [14] in rare cases, cysteine may be essential for infants, the elderly, and individuals with certain metabolic diseases 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. Industrial sources The majority of l-cysteine is obtained industrially by hydrolysis of animal materials, such as poultry feathers or hog hair. Despite widespread rumor, [15] human hair is rarely a source material.[16] Indeed, food additive or cosmetic product manufactures may not legally source from human hair in the European Union.[17][18] Some animal-originating sources of l-cysteine as a food additive contravene kosher, halal, vegan, or vegetarian diets.[15] To avoid this problem, synthetic l-cysteine, compliant with Jewish kosher and Muslim halal laws, is also available, albeit at a higher price.[19] The typical synthetic route involves fermentation with an artificial E. coli strain.[20] Alternatively, Evonik (formerly Degussa) introduced a route from substituted thiazolines.[21] Pseudomonas thiazolinophilum hydrolyzes racemic 2‑amino-Δ2‑thiazoline-4‑carboxylic acid to l‑cysteine.[20]