Dietary supplement, nutrient. Dough conditioner. Cysteine (abbreviated as Cys or C) is an ?-amino acid with the chemical formula HO2CCH(NH2)CH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is nonpolar and thus cysteine is usually classified as a hydrophobic amino acid. The thiol side chain often participates in enzymatic reactions, serving as a nucleophile. The thiol is susceptible to oxidization to give the disulfide derivative cystine, which serves an important structural role in many proteins. Cysteine is named after cystine.; Cysteine is a very popular target for site-directed labeling experiments to investigate biomolecular structure and dynamics. Maleimides will selectively attach to cysteine using a covalent Michael addition. Site-directed spin labeling for EPR or paramagnetic relaxation enhanced NMR also uses cysteine extensively.; Cysteine is an important source of sulfide in human metabolism. The sulfide in iron-sulfur clusters and in nitrogenase is extracted from cysteine, which is converted to alanine in the process.; Cysteine is required by sheep in order to produce wool: it is an essential amino acid which must be taken in as food from grass. As a consequence, during drought conditions, sheep stop producing wool; however, transgenic sheep which can make their own cysteine have been developed.[citation needed]; Cysteine, mainly the L-enantiomer, is a precursor in the food, pharmaceutical, and personal care industries. One of the largest applications is the production of flavors. For example, the reaction of cysteine with sugars in a Maillard reaction yields meat flavors. L-cysteine is also used as a processing aid for baking. Small quantities (in the tens of ppm range) help to soften the dough and thus reduce processing time. http://www.cfsan.fda.gov/~dms/foodic.html; High levels of Cysteine, due to its reducing capacity, have been shown to inactivate insulin under certain conditions. This is because insulin contains three disulfide bonds, one of which can be reduced by cysteine. If this happens, insulin looses its characteristic structure and thus looses functionality. During a hypoglycemia attack (where there is too much insulin in the blood causing an unsafe drop in blood sugar) Cysteine can be used to inactivate insulin, allowing blood sugar levels to normalize. In some cases, the use of Thiamine, vitamin C, and Cysteine have been successful in treating severe cases of hypoglycemia. Additionally, due to its interaction with insulin, diabetics should avoid supplements or medications that contain cysteine or have the potential to increase cysteine levels.; In a 1994 report released by five top cigarette companies, cysteine is one of the 599 additives to cigarettes. Like most cigarette additives, however, its use or purpose is unknown. Its inclusion in cigarettes could offer two benefits: Acting as an expectorant, since smoking increases mucus production in the lungs; and increasing the beneficial antioxidant glutathione (which is diminished in smokers).; N-acetyl-L-cysteine (NAC) is a derivative of cysteine wherein an acetyl group is attached to the nitrogen atom. This compound is sometimes considered as a dietary supplement, although it is not an ideal source since it is catabolized in the gut.[citation needed] NAC is often used as a cough medicine because it breaks up the disulfide bonds in the mucus and thus liquefies it, making it easier to cough up. It is also this action of breaking disulfide bonds that makes it useful in thinning the abnormally thick mucus in Cystic Fibrosis patients. NAC is also used as a specific antidote in cases of acetaminophen overdose.; Oxidation of cysteine produces the disulfide cystine. More aggressive oxidants convert cysteine to the corresponding sulfinic acid and sulfonic acid. Cysteine residues play a valuable role by crosslinking proteins, which increases the protein stability in the harsh extracellular environment, and also functions to confer proteolytic resistance (since protein export is a costly process, minimizing its necessity is advantageous). Inside the cell, disulfide bridges between cysteine residues within a polypeptide support the protein's secondary structure. Insulin is an example of a protein with cystine crosslinking, wherein two separate peptide chains are connected by a pair of disulfide bonds.