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22-AUGUST-2008 06:13:22 - Cysteine Not to be confused with cystine, its oxidized dimer. Skeletal structure of L-cysteine 3D model of the amino acid cysteine Space-filling model of the amino acid cysteine Cysteine Systematic IUPAC name 2R-2-amino-3-sulfanyl-propanoic acid Identifiers CAS number 52-90-4 PubChem 5862 Chemical data Formula C3H7NO2S Molar mass 121.16 g/mol SMILES NC@@HSCO=O Complete data Cysteine abbreviated as Cys or C1 is an α-amino acid with the chemical formula HO2CCHNH2CH2SH. It is a non-essential amino acid, which means that humans can synthesize it. Its codons are UGU and UGC. With a thiol side chain, cysteine is classified as a hydrophilic amino acid. Because of the high reactivity of this thiol, cysteine is an important structural and functional component of many proteins and enzymes. Cysteine is named after cystine, its oxidized dimer. Contents 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 Oxidation to cystine linkages 2.3 Precursor to iron-sulfur clusters 2.4 Metal ion binding 2.5 Post-translational modifications 3 Applications 4 Sheep 5 Reducing Toxic Effects of Alcohol 5.1 N-acetylcysteine NAC 6 See also 7 References 8 External links 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 and is catabolized in the gastrointestinal tract and blood plasma. In contrast, cysteine is absorbed during digestion as cystine, which is more stable in the gastrointestinal tract. 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 Animal vegetarian sources: eggs, milk, whey protein, ricotta, cottage cheese, yogurt Vegan sources: red peppers, garlic, onions, broccoli, brussels sprouts, oats, granola, wheat germ Industrial sources See also Food safety in China#Soy sauce made from human hair. At the present time, the cheapest source of material from which food-grade L-cysteine may be purified in high yield is by hydrolysis of human hair. Other sources include feathers and pig bristles.citations neededThe companies producing cysteine by hydrolysis are located mainly in China. There is some debate as to whether or not consuming L-cysteine derived from human hair constitutes cannibalism. Although many other amino acids were accessible via fermentation for some years, L-cysteine was unavailable until 2001 when German company Wacker Chemie introduced a production route via fermentation non-human, non-animal origin. Biosynthesis 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.2 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 pKa values close to neutrality, so are often in their reactive thiolate form in the cell.3 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. Oxidation to cystine linkages 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. 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 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.4 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.5 The thiol group also has a high affinity for heavy metals, so that proteins containing cysteine will bind metals such as mercury, lead, and cadmium tightly.6 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.citation needed 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 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.7 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 Cystine, showing disulfide bond Cystine, showing disulfide bond 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. 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 acetaldehyde8 , 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 .9 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. 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. NAC is also used as a dietary supplement as already indicated above, as well as a specific antidote in cases of acetaminophen overdose. See also Selenocysteine Amino acids Thiols Cysteine metabolism Cystinuria References ^ IUPAC-IUBMB Joint Commission on Biochemical Nomenclature. Nomenclature and Symbolism for Amino Acids and Peptides. Recommendations on Organic Biochemical Nomenclature, Symbols Terminology etc. Retrieved on 2007-05-17. ^ Hell, R. 1997. Molecular physiology of plant sulfur metabolism Planta 202:138-148. PMID: 9202491 ^ Bulaj G, Kortemme T, Goldenberg D 1998. Ionization-reactivity relationships for cysteine thiols in polypeptides.. Biochemistry 37 25: 8965-72. doi:10.1021/bi973101r. PMID 9636038. ^ Roland Lill, Ulrich Mühlenhoff Iron-Sulfur Protein Biogenesis in Eukaryotes: Components and Mechanisms Annual Review of Cell and Developmental Biology, 2006, Volume 22, pp. 457-486. doi:10.1146/annurev.cellbio.22.010305.104538. ^ S. J. Lippard, J. M. Berg Principles of Bioinorganic Chemistry University Science Books: Mill Valley, CA; 1994. ISBN 0-935702-73-3. ^ Baker D, Czarnecki-Maulden G 1987. Pharmacologic role of cysteine in ameliorating or exacerbating mineral toxicities.. J Nutr 117 6: 1003-10. doi:10.1126/science.2237411.br inactive 2008-06-25. PMID 3298579. ^ http://quitsmoking.about.com/cs/nicotineinhaler/a/cigingredients.htm ^ http://www.lef.org/protocols/prtcl-004.shtml ^ Effects of cysteine on acetaldehyde lethality http://www.springerlink.com/content/w307w62037125v33/ 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. Interaction of alcohol and smoking in the pathogenesis of upper digestive tract cancers - possible chemoprevention with cysteine Cystine Kidney Stones v d e The 20 Common Amino Acids dp = data page Branched-chain amino acids Isoleucine dp | Leucine dp | Valine dp Non Branch-chain Alanine dp | Arginine dp | Asparagine dp | Aspartic acid dp | Cysteine dp | Glutamic acid dp | Glutamine dp | Glycine dp | Histidine dp | Lysine dp | Methionine dp | Phenylalanine dp | Proline dp | Serine dp | Threonine dp | Tryptophan dp | Tyrosine dp Other classifications Essential amino acids | Ketogenic amino acid | Glucogenic amino acid Major families of biochemicals Saccharides | Carbohydrates | Glycosides | | Amino acids | Peptides | Proteins | Glycoproteins | | Lipids | Terpenes | Steroids | Carotenoids Alkaloids | Nucleobases | Nucleic acids | | Enzyme cofactors | Flavonoids | Polyketides | Tetrapyrroles v d e E numbers Colours E100-199 Preservatives E200-299 Antioxidants Acidity regulators E300-399 Thickeners, stabilisers emulsifiers E400-499 pH regulators anti-caking agents E500-599 Flavour enhancers E600-699 Miscellaneous E900-999 Additional chemicals E1100-1599 Waxes E900-909 Synthetic glazes E910-919 Improving agents E920-929 Packaging gases E930-949 Sweeteners E950-969 Foaming agents E990-999 L-cysteine E920 L-cystine E921 Potassium persulfate E922 Ammonium persulfate E923 Potassium bromate E924 Chlorine E925 Chlorine dioxide E926 Azodicarbonamide E927 Carbamide E927b Benzoyl peroxide E928 Retrieved from http://en..org/wiki/Cysteine Categories: Proteinogenic amino acids | Glucogenic amino acids | Sulfur amino acids | ThiolsHidden categories: Pages with DOIs broken since 2008 | All articles with statements | Articles with statements since February 2008 | Articles with statements since June 2007 Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages العربية Català Česky Deutsch Español Esperanto Français í•œêµì–´ Bahasa Indonesia Italiano עברית LatvieÅ¡u Lëtzebuergesch Lietuvių Magyar Nederlands 日本語 ‪Norsk bokmÃ¥l‬ Occitan Polski Português РуÑ?Ñ?кий Suomi Svenska УкраїнÑ?ька ä¸æ–‡ This page was last modified on 22 August 2008, at 03:41. of the GNU Free Documentation License. ® , Inc., a U.S.
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