CYSTEINE (abbreviated as CYS or C) is a semi-essential proteinogenic 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 oxidization 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.
It can be seen as serine , but with one of the oxygen atoms replaced with sulfur ; replacing said atom with selenium gives selenocysteine .
* 1 Sources
* 1.1 Dietary sources * 1.2 Industrial sources * 1.3 Biosynthesis
* 2 Biological functions
* 2.1 Precursor to the antioxidant glutathione * 2.2 Precursor to iron-sulfur clusters * 2.3 Metal ion binding * 2.4 Roles in protein structure
* 3 Applications
* 3.1 Reducing toxic effects of alcohol * 3.2 N-Acetylcysteine
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
* Animal sources: meat (including pork and poultry ), eggs , dairy ;
The majority of L-cysteine is obtained industrially by hydrolysis of animal materials, such as poultry feathers or hog hair. Despite widespread belief otherwise, there is little evidence that human hair is used as a source material and its use is explicitly banned in the European Union. Synthetically produced L-cysteine, compliant with Jewish kosher and Muslim halal laws, is also available, albeit at a higher price. The synthetic route involves fermentation using a mutant of E. coli . Degussa introduced a route from substituted thiazolines . Following this technology, L-cysteine is produced by the hydrolysis of racemic 2-amino-Δ2-thiazoline-4-carboxylic acid using Pseudomonas thiazolinophilum.
In animals, biosynthesis begins with the amino acid serine . The sulfur is derived from methionine , which is converted to homocysteine through the intermediate S-adenosylmethionine . Cystathionine beta-synthase then combines homocysteine and serine to form the asymmetrical thioether cystathionine . The enzyme cystathionine gamma-lyase converts the cystathionine into cysteine and alpha-ketobutyrate . In plants and bacteria , cysteine biosynthesis also starts from serine, which is converted to O-acetylserine by the enzyme serine transacetylase. The enzyme O-acetylserine (thiol)-lyase, using sulfide sources, converts this ester into cysteine, releasing acetate.
The cysteine thiol group is nucleophilic and easily oxidized. The reactivity is enhanced when the thiol is ionized, and cysteine residues in proteins have pKa values close to neutrality, so are often in their reactive thiolate form in the cell. Because of its high reactivity, the thiol group of cysteine has numerous biological functions.
PRECURSOR TO THE ANTIOXIDANT GLUTATHIONE
Due to the ability of thiols to undergo redox reactions, cysteine has
antioxidant properties. Cysteine's antioxidant properties are
typically expressed in the tripeptide glutathione , which occurs in
humans as well as other organisms. The systemic availability of oral
glutathione (GSH) is negligible; so it must be biosynthesized from its
constituent amino acids, cysteine, glycine , and glutamic acid .
PRECURSOR TO IRON-SULFUR CLUSTERS
METAL ION BINDING
Beyond the iron-sulfur proteins, many other metal cofactors in enzymes are bound to the thiolate substituent of cysteinyl residues. Examples include zinc in zinc fingers and alcohol dehydrogenase , copper in the blue copper proteins , iron in cytochrome P450 , and nickel in the -hydrogenases . The thiol group also has a high affinity for heavy metals , so that proteins containing cysteine, such as metallothionein , will bind metals such as mercury, lead, and cadmium tightly.
ROLES IN PROTEIN STRUCTURE
In the translation of messenger RNA molecules to produce polypeptides, cysteine is coded for by the UGU and UGC codons .
While free cysteine residues do occur in proteins, most are
covalently bonded to other cysteine residues to form disulfide bonds .
Disulfide bonds play an important role in the folding and stability of
some proteins, usually proteins secreted to the extracellular medium.
Since most cellular compartments are reducing environments , disulfide
bonds are generally unstable in the cytosol with some exceptions as
noted below. Figure 2:
Disulfide bonds in proteins are formed by oxidation of the thiol
groups of cysteine residues. The other sulfur-containing amino acid,
methionine , cannot form disulfide bonds. More aggressive oxidants
convert cysteine to the corresponding sulfinic acid and sulfonic acid
Aside from its oxidation to cystine, cysteine participates in numerous posttranslational modifications . The nucleophilic thiol group allows cysteine to conjugate to other groups, e.g., in prenylation . Ubiquitin ligases transfer ubiquitin to its pendant, proteins, and caspases , which engage in proteolysis in the apoptotic cycle. Inteins often function with the help of a catalytic cysteine. These roles are typically limited to the intracellular milieu, where the environment is reducing, and cysteine is not oxidized to cystine.
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.
In the field of personal care, cysteine is used for permanent wave applications, predominantly in Asia. Again, the cysteine is used for breaking up the disulfide bonds in the hair 's keratin .
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; or increasing the beneficial antioxidant glutathione (which is diminished in smokers).
REDUCING TOXIC EFFECTS OF ALCOHOL
N-Acetyl-L-cysteine is a derivative of cysteine wherein an acetyl group is attached to the nitrogen atom. This compound is sold as a dietary supplement, and used as an antidote in cases of acetaminophen overdose, and obsessive compulsive disorders such as trichotillomania .
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. To avoid this problem, L-cysteine can also be sourced from microbial or other synthetic processes.
Wikimedia Commons has media related to CYSTEINE .
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