Cysteine
Cysteine (symbol Cys or C;[4] /ˈsɪstɪiːn/)[5] is a semiessential[6] 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 interestingly, both D and L-cysteine are found in nature with D-cysteine having been found in developing brain (see Semenza et al., 2021). Cysteine is named after its discovery in urine, which comes from the urinary bladder or cyst, from kystis "bladder".[7]
Not to be confused with cytosine, cystine, cytisine, cytidine, or Sistine.
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.[8][9] The deprotonated form can generally be described by the symbol Cym as well.[9][10]
When used as a food additive, cysteine has the E number E920.
Cysteine is encoded by the codons UGU and UGC.
Dietary sources[edit]
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.[11] Although classified as a nonessential amino acid, 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[edit]
The majority of l-cysteine is obtained industrially by hydrolysis of animal materials, such as poultry feathers or hog hair. Despite widespread belief otherwise, little evidence shows that human hair is used as a source material and its use is explicitly banned for food additives and cosmetic products in the European Union.[12][13] Synthetically produced l-cysteine, compliant with Jewish kosher and Muslim halal laws, is also available, albeit at a higher price.[14] The synthetic route involves fermentation using a mutant of E. coli. Evonik (formerly Degussa) introduced a route from substituted thiazolines.[15] Following this technology, l-cysteine is produced by the hydrolysis of racemic 2-amino-Δ2-thiazoline-4-carboxylic acid using Pseudomonas thiazolinophilum.[16]
Chemical reactions[edit]
Being multifunctional, cysteine undergoes a variety of reactions. Much attention has focused on protecting the sulfhydryl group.[37] Methylation of cysteine gives S-methylcysteine. Treatment with formaldehyde gives the thiazolidine thioproline. Cysteine forms a variety of coordination complexes upon treatment with metal ions.[38]
Dietary restrictions[edit]
The animal-originating sources of l-cysteine as a food additive are a point of contention for people following dietary restrictions such as kosher, halal, vegan, or vegetarian.[40] To avoid this problem, l-cysteine can also be sourced from microbial or other synthetic processes.