cysteine

Not to be confused with cystine, its oxidized dimer.

L-Cysteine
IUPAC name	Cysteine
Other names	2-Amino-3-mercaptopropanoic acid
Identifiers
CAS number	52-90-4 Y, 52-89-1 (hydrochloride)
PubChem	5862
EC-number	200-158-2
SMILES	C([C@@H](C(=O)O)N)S
InChI	1/C3H7NO2S/c4-2(1-7)3(5)6/h2,7H,1,4H2,(H,5,6)/t2-/m0/s1
ChemSpider ID	5653
Properties[1]
Molecular formula	C3H7NO2S
Molar mass	121.16 g mol−1
Appearance	white crystals or powder
Melting point	240 ºC decomp.
Solubility in water	soluble
Chiral rotation [α]D	+9.4º (H2O, c = 1.3)
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Cysteine (abbreviated as Cys or C)^[2] is an α-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.

Sources

Dietary sources

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 potentially toxic^{[citation needed]} and 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.

Cysteine is found in most high-protein foods, including:

• Animal sources: pork, sausage meat, chicken, turkey, duck, luncheon meat, eggs, milk, whey protein, ricotta, cottage cheese, yogurt
• Vegan sources: red peppers, garlic, onions, broccoli, brussels sprouts, oats, granola, wheat germ

As other amino acids, cysteine has an amphoteric character.

(R)-Cysteine (left) and (S)-Cysteine (right) in zwitterionic form at neutral pH

Industrial sources

L-Cysteine was once obtained industrially by hydrolysis of hair and keratin. The main contemporary route involves fermentation utilizing a mutant of E. coli. Wacker Chemie introduced a route from substituted thiazolines.^[3] Following this technology, L-cysteine is produced by the hydrolysis of racemic 2-amino-Δ²-thiazoline-4-carboxylic acid using Pseudomonas thiazolinophilum.^[4]

Biosynthesis

Cysteine synthesis. Cystathionine beta synthase catalyzes the upper reaction and cystathionine gamma-lyase catalyzes the lower reaction.

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 bacteria, cysteine biosynthesis again 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.^[5]

Biological functions

The cysteine thiol group is nucleophilic and easily oxidized. The reactivity is enhanced when the thiol ionized, and cysteine residues in proteins have pK_a values close to neutrality, so are often in their reactive thiolate form in the cell.^[6] 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. Glutamic acid and glycine are readily available in most Western diets, but the availability of cysteine can be the limiting substrate.^{[citation needed]}

Disulfide bonds

Disulfide bonds play an important role in the folding and stability of some proteins, usually proteins secreted to the extracellular medium.^[7] Since most cellular compartments are reducing environments, disulfide bonds are generally unstable in the cytosol with some exceptions as noted below.

Figure 2: Cystine (shown here in its neutral form) is derived from two molecules of cysteine. It features a disulfide bond.

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. Cysteine residues play a valuable role by crosslinking proteins, which increases the rigidity of proteins 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.

Protein disulfide isomerases catalyze the proper formation of disulfide bonds; the cell transfers dehydroascorbic acid to the endoplasmic reticulum, which oxidises the environment. In this environment, cysteines are, in general, oxidized to cystine and are no longer functional as a nucleophiles.

Precursor to iron-sulfur clusters

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.^[8]

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 [NiFe]-hydrogenases.^[9] The thiol group also has a high affinity for heavy metals, so that proteins containing cysteine, such as such as metallothionein, will bind metals such as mercury, lead, and cadmium tightly.^[10]

Post-translational modifications

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.

Applications

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.^[11] L-cysteine is also used as a processing aid for baking.^[12]

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.

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.

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.^[13] 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).

Sheep

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]}

Reducing toxic effects of alcohol

Cysteine has been proposed as a preventative or antidote for some of the negative effects of alcohol, including liver damage and hangover. It counteracts the poisonous effects of acetaldehyde, which is the major by-product of alcohol metabolism and is responsible for most of the negative aftereffects and long-term damage associated with alcohol use (but not the immediate effects of drunkenness). Cysteine supports the next step in metabolism, which turns acetaldehyde into the relatively harmless acetic acid. In a rat study, test animals received a LD50 dose of acetaldehyde (the amount which normally kills half of all animals). Those that received cysteine had an 80% survival rate; when thiamine was added, all animals survived.^[14] There is not yet direct evidence for or against its effectiveness in humans who consume alcohol at normal levels.

N-acetylcysteine (NAC)

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.

See also

• Selenocysteine
• Amino acids
• Thiols
• Cysteine metabolism
• Cystinuria

References

External links

• Computational Chemistry Wiki
• International Kidney Stone Institute
• http://www.chemie.fu-berlin.de/chemistry/bio/aminoacid/cystein en.html
• On the hydrophobic nature of cysteine.
• 952-10-3056-9 Interaction of alcohol and smoking in the pathogenesis of upper digestive tract cancers - possible chemoprevention with cysteine
• Cystine Kidney Stones