Cysteine

Not to be confused with cytosine, cystine, cytisine, cytidine, or Sistine.

"Cys" redirects here. For other uses, see Cys (disambiguation).

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).

l-Cysteine

Skeletal formula of L-cysteine
Ball-and-stick model Space-filling model
Names
IUPAC name Cysteine
Other names 2-Amino-3-sulfhydrylpropanoic acid
Identifiers
CAS Number	52-90-4^Y 52-89-1 (hydrochloride)^Y
3D model (JSmol)	Interactive image Zwitterion: Interactive image
Abbreviations	Cys, C
ChEBI	CHEBI:15356^Y
ChEMBL	ChEMBL54943^Y
ChemSpider	574 (Racemic)^Y 5653 (L-form)^Y
ECHA InfoCard	100.000.145
EC Number	200-158-2
E number	E920 (glazing agents, ...)
IUPHAR/BPS	4782
KEGG	D00026^Y
PubChem CID	5862
UNII	K848JZ4886^Y A9U1687S1S (hydrochloride)^Y
CompTox Dashboard (EPA)	DTXSID8022876
InChI InChI=1S/C3H7NO2S/c4-2(1-7)3(5)6/h2,7H,1,4H2,(H,5,6)^Y Key: XUJNEKJLAYXESH-UHFFFAOYSA-N^Y InChI=1/C3H7NO2S/c4-2(1-7)3(5)6/h2,7H,1,4H2,(H,5,6)/t2-/m0/s1 Key: XUJNEKJLAYXESH-REOHCLBHBU InChI=1/C3H7NO2S/c4-2(1-7)3(5)6/h2,7H,1,4H2,(H,5,6) Key: XUJNEKJLAYXESH-UHFFFAOYAC
SMILES C([C@@H](C(=O)O)N)S Zwitterion: C([C@@H](C(=O)[O-])[NH3+])S
Properties^[3]
Chemical formula	C3H7NO2S
Molar mass	121.15 g·mol^−1
Appearance	white crystals or powder
Melting point	240 °C (464 °F; 513 K) decomposes
Solubility in water	277g/L (at 25 °C) ^[1]
Solubility	1.5g/100g ethanol 19 °C ^[2]
Chiral rotation ([α]D)	+9.4° (H2O, c = 1.3)
Supplementary data page
Cysteine (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is ^YN ?) Infobox references

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.^[7][8] The deprotonated form can generally be described by the symbol Cym as well.^[8][9]

When used as a food additive, cysteine has the E number E920.

Cysteine is encoded by the codons UGU and UGC.

Structure

Like other amino acids (not as a residue of a protein), cysteine exists as a zwitterion. Cysteine has l chirality in the older d/l notation based on homology to d- and l-glyceraldehyde. In the newer R/S system of designating chirality, based on the atomic numbers of atoms near the asymmetric carbon, cysteine (and selenocysteine) have R chirality, because of the presence of sulfur (or selenium) as a second neighbor to the asymmetric carbon atom. The remaining chiral amino acids, having lighter atoms in that position, have S chirality. Replacing sulfur with selenium gives selenocysteine.

 

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

Dietary sources

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.^[10] 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

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.^[11][12] Synthetically produced l-cysteine, compliant with Jewish kosher and Muslim halal laws, is also available, albeit at a higher price.^[13] The synthetic route involves fermentation using a mutant of E. coli. Evonik (formerly Degussa) introduced a route from substituted thiazolines.^[14] Following this technology, l-cysteine is produced by the hydrolysis of racemic 2-amino-Δ²-thiazoline-4-carboxylic acid using Pseudomonas thiazolinophilum.^[15]

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 plants and bacteria, cysteine biosynthesis also starts from serine, which is converted to O-acetylserine by the enzyme serine transacetylase. The enzyme cysteine synthase, using sulfide sources, converts this ester into cysteine, releasing acetate.^[16]

Biological functions

The cysteine sulfhydryl group is nucleophilic and easily oxidized. The reactivity is enhanced when the thiol is ionized, and cysteine residues in proteins have pK_a values close to neutrality, so are often in their reactive thiolate form in the cell.^[17] Because of its high reactivity, the sulfhydryl group of cysteine has numerous biological functions.

Precursor to the antioxidant glutathione

Due to the ability of thiols to undergo redox reactions, cysteine and cysteinyl residues have antioxidant properties. Its antioxidant properties are typically expressed in the tripeptide glutathione, which occurs in humans and 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. While glutamic acid is usually sufficient because amino acid nitrogen is recycled through glutamate as an intermediary, dietary cysteine and glycine supplementation can improve synthesis of glutathione.^[18]

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.^[19]

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.^[20] The sulfhydryl 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.^[21]

Roles in protein structure

In the translation of messenger RNA molecules to produce polypeptides, cysteine is coded for by the UGU and UGC codons.

Cysteine has traditionally been considered to be a hydrophilic amino acid, based largely on the chemical parallel between its sulfhydryl group and the hydroxyl groups in the side chains of other polar amino acids. However, the cysteine side chain has been shown to stabilize hydrophobic interactions in micelles to a greater degree than the side chain in the nonpolar amino acid glycine and the polar amino acid serine.^[22] In a statistical analysis of the frequency with which amino acids appear in various proteins, cysteine residues were found to associate with hydrophobic regions of proteins. Their hydrophobic tendency was equivalent to that of known nonpolar amino acids such as methionine and tyrosine (tyrosine is polar aromatic but also hydrophobic^[23]), those of which were much greater than that of known polar amino acids such as serine and threonine.^[24] Hydrophobicity scales, which rank amino acids from most hydrophobic to most hydrophilic, consistently place cysteine towards the hydrophobic end of the spectrum, even when they are based on methods that are not influenced by the tendency of cysteines to form disulfide bonds in proteins. Therefore, cysteine is now often grouped among the hydrophobic amino acids,^[25][26] though it is sometimes also classified as slightly polar,^[27] or polar.^[6]

Most cysteine residues are covalently bonded to other cysteine residues to form disulfide bonds, which play an important role in the folding and stability of some proteins, usually proteins secreted to the extracellular medium.^[28] 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), two cysteines bound together by a disulfide bond

Disulfide bonds in proteins are formed by oxidation of the sulfhydryl group 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 tertiary 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 oxidizes the environment. In this environment, cysteines are, in general, oxidized to cystine and are no longer functional as a nucleophiles.

Aside from its oxidation to cystine, cysteine participates in numerous post-translational modifications. The nucleophilic sulfhydryl 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.^[29] l-Cysteine is also used as a processing aid for baking.^[30]

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 selectively attach to cysteine using a covalent Michael addition. Site-directed spin labeling for EPR or paramagnetic relaxation-enhanced NMR also uses cysteine extensively.

Reducing toxic effects of alcohol

Cysteine has been proposed as a preventive or antidote for some of the negative effects of alcohol, including liver damage and hangover. It counteracts the poisonous effects of acetaldehyde.^[31] Cysteine supports the next step in metabolism, which turns acetaldehyde into acetic acid^{[citation needed]}.

In a rat study, test animals received an LD₉₀ dose of acetaldehyde. Those that received cysteine had an 80% survival rate; when both cysteine and thiamine were administered, all animals survived. The control group had a 10% survival rate.^[32]

In 2020 an article was published that suggests L-cysteine might also work in humans.^[33]

N-Acetylcysteine

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.^[34]

Sheep

Cysteine is required by sheep to produce wool. It is an essential amino acid that is taken in from their feed. As a consequence, during drought conditions, sheep produce less wool; however, transgenic sheep that can make their own cysteine have been developed.^[35]

Chemical reactions

Being multifunctional, cysteine undergoes a variety of reactions. Much attention has focused on protecting the sulfhydryl group.^[36] 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.^[37]

Safety

Relative to most other amino acids, cysteine is much more toxic.^[38]

Dietary restrictions

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.^[39] To avoid this problem, l-cysteine can also be sourced from microbial or other synthetic processes.

History

In 1884 German chemist Eugen Baumann found that when cystine was treated with a reducing agent, cystine revealed itself to be a dimer of a monomer which he named "cysteïne".^[40]

See also

 

Wikimedia Commons has media related to Cysteine.

• Amino acids
• Cysteine metabolism
• Cystinuria
• Saville reaction
• Sullivan reaction

References

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2. Belitz, H.-D; Grosch, Werner; Schieberle, Peter (2009-02-27). Food Chemistry. ISBN 9783540699330.
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10. Larsson, Susanna C.; Håkansson, Niclas; Wolk, Alicja (April 2015). "Dietary Cysteine and Other Amino Acids and Stroke Incidence in Women". Stroke. 46 (4): 922–926. doi:10.1161/STROKEAHA.114.008022. PMID 25669310. S2CID 14895681.
11. "EU Chemical Requirements". Retrieved May 24, 2020. ...L-cysteine hydrochloride or hydrochloride monohydrate. Human hair may not be used as a source for this substance
12. "Regulation (EC) No 1223/2009 of the European Parliament and of the Council of 30 November 2009 on cosmetic products". Retrieved July 28, 2021. ...ANNEX II LIST OF SUBSTANCES PROHIBITED IN COSMETIC PRODUCTS...416 Cells, tissues or products of human origin
13. "Questions About Food Ingredients: What is L-cysteine/cysteine/cystine?". Vegetarian Resource Group.
14. Martens, Jürgen; Offermanns, Heribert; Scherberich, Paul (1981). "Facile Synthesis of Racemic Cysteine". Angewandte Chemie International Edition in English. 20 (8): 668. doi:10.1002/anie.198106681.
15. Drauz, Karlheinz; Grayson, Ian; Kleemann, Axel; Krimmer, Hans-Peter; Leuchtenberger, Wolfgang; Weckbecker, Christoph (2007). "Amino Acids". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a02_057.pub2. ISBN 978-3-527-30673-2.
16. Hell R (1997). "Molecular physiology of plant sulfur metabolism". Planta. 202 (2): 138–48. doi:10.1007/s004250050112. PMID 9202491. S2CID 2539629.
17. Bulaj G, Kortemme T, Goldenberg DP (June 1998). "Ionization-reactivity relationships for cysteine thiols in polypeptides". Biochemistry. 37 (25): 8965–72. doi:10.1021/bi973101r. PMID 9636038.
18. Sekhar, Rajagopal V; Patel, Sanjeet G (2011). "Deficient synthesis of glutathione underlies oxidative stress in aging and can be corrected by dietary cysteine and glycine supplementation". The American Journal of Clinical Nutrition. 94 (3): 847–853. doi:10.3945/ajcn.110.003483. PMC 3155927. PMID 21795440.  
19. Lill R, Mühlenhoff U (2006). "Iron-sulfur protein biogenesis in eukaryotes: components and mechanisms". Annu. Rev. Cell Dev. Biol. 22: 457–86. doi:10.1146/annurev.cellbio.22.010305.104538. PMID 16824008.
20. Lippard, Stephen J.; Berg, Jeremy M. (1994). Principles of Bioinorganic Chemistry. Mill Valley, CA: University Science Books. ISBN 978-0-935702-73-6.^{[page needed]}
21. Baker DH, Czarnecki-Maulden GL (June 1987). "Pharmacologic role of cysteine in ameliorating or exacerbating mineral toxicities". J. Nutr. 117 (6): 1003–10. doi:10.1093/jn/117.6.1003. PMID 3298579.
22. Heitmann P (January 1968). "A model for sulfhydryl groups in proteins. Hydrophobic interactions of the cystein side chain in micelles". Eur. J. Biochem. 3 (3): 346–50. doi:10.1111/j.1432-1033.1968.tb19535.x. PMID 5650851.
23. . Curtin University. Archived from the original on 2015-09-07. Retrieved 2015-09-09.
24. Nagano N, Ota M, Nishikawa K (September 1999). "Strong hydrophobic nature of cysteine residues in proteins". FEBS Lett. 458 (1): 69–71. doi:10.1016/S0014-5793(99)01122-9. PMID 10518936. S2CID 34980474.
25. Betts, M.J.; R.B. Russell (2003). "Hydrophobic amino acids". Amino Acid Properties and Consequences of Substitutions, In: Bioinformatics for Geneticists. Wiley. Retrieved 2012-09-16.
26. Gorga, Frank R. (1998–2001). . Archived from the original on 2012-09-05. Retrieved 2012-09-16.
27. . Elmhurst College. Archived from the original on 2012-10-02. Retrieved 2012-09-16.
28. Sevier CS, Kaiser CA (November 2002). "Formation and transfer of disulphide bonds in living cells". Nat. Rev. Mol. Cell Biol. 3 (11): 836–47. doi:10.1038/nrm954. PMID 12415301. S2CID 2885059.
29. Huang, Tzou-Chi; Ho, Chi-Tang (2001-07-27). Hui, Y. H.; Nip, Wai-Kit; Rogers, Robert (eds.). Meat Science and Applications, ch. Flavors of Meat Products. CRC. pp. 71–102. ISBN 978-0-203-90808-2.
30. . U.S. Food and Drug Administration. November 2004. Archived from the original on 2009-05-12. Retrieved 2009-09-06.
31. Otoyama, Ippo; Hamada, Hironobu; Kimura, Tatsushi; Namba, Haruchi; Sekikawa, Kiyokazu; Kamikawa, Norimichi; Kajiwara, Teruki; Aizawa, Fumiya; Sato, Yoshinobu M. (2019). "L-cysteine improves blood fluidity impaired by acetaldehyde: In vitro evaluation". PLOS ONE. 14 (3): e0214585. Bibcode:2019PLoSO..1414585O. doi:10.1371/journal.pone.0214585. PMC 6440629. PMID 30925182.
32. Sprince H, Parker CM, Smith GG, Gonzales LJ (April 1974). "Protection against acetaldehyde toxicity in the rat by L-cysteine, thiamin and L-2-methylthiazolidine-4-carboxylic acid". Agents Actions. 4 (2): 125–30. doi:10.1007/BF01966822. PMID 4842541. S2CID 5924137.
33. Eriksson, C J Peter; Metsälä, Markus; Möykkynen, Tommi; Mäkisalo, Heikki; Kärkkäinen, Olli; Palmén, Maria; Salminen, Joonas E; Kauhanen, Jussi (20 October 2020). "L-Cysteine Containing Vitamin Supplement Which Prevents or Alleviates Alcohol-related Hangover Symptoms: Nausea, Headache, Stress and Anxiety". Alcohol and Alcoholism. 55 (6): 660–666. doi:10.1093/alcalc/agaa082. hdl:10138/339340. PMID 32808029.
34. Kanter MZ (October 2006). "Comparison of oral and i.v. acetylcysteine in the treatment of acetaminophen poisoning". Am J Health Syst Pharm. 63 (19): 1821–7. doi:10.2146/ajhp060050. PMID 16990628. S2CID 9209528.
35. Powell BC, Walker SK, Bawden CS, Sivaprasad AV, Rogers GE (1994). "Transgenic sheep and wool growth: possibilities and current status". Reprod. Fertil. Dev. 6 (5): 615–23. doi:10.1071/RD9940615. PMID 7569041.
36. Milkowski, John D.; Veber, Daniel F.; Hirschmann, Ralph (1979). "Thiol Protection with the Acetamidomethyl Group: S-Acetamidomethyl-L-Cysteine Hydrochloride". Organic Syntheses. 59: 190. doi:10.15227/orgsyn.059.0190.
37. Arnold, Alan P.; Jackson, W. Gregory (1990). "Stereospecificity in the Synthesis of the Tris((R)-Cysteinato-N,S)- and Tris((R)-Cysteinesulfinato-N,S)cobaltate(III) Ions". Inorganic Chemistry. 29 (18): 3618–3620. doi:10.1021/ic00343a061.
38. Anderson, Mary E.; Meister, Alton (1987). "Intracellular delivery of cysteine". Sulfur and Sulfur Amino Acids. Methods in Enzymology. Vol. 143. pp. 313–325. doi:10.1016/0076-6879(87)43059-0. ISBN 9780121820435. PMID 3309557.
39. "Kosher View of L-Cysteine". kashrut.com. May 2003.
40. Baumann, E. (1884). "Ueber Cystin und Cysteïn" [On cystine and cysteine]. Zeitschrift für physiologische Chemie (in German). 8: 299–305. From pp. 301-302: "Die Analyse der Substanz ergibt Werthe, welche den vom Cystin (C₆H₁₂N₂S₂O₄) verlangten sich nähern, […] nenne ich dieses Reduktionsprodukt des Cystins: Cysteïn." (Analysis of the substance [cysteine] reveals values which approximate those [that are] required by cystine (C₆H₁₂N₂S₂O₄), however the new base [cysteine] can clearly be recognized as a reduction product of cystine, to which the [empirical] formula C₃H₇NSO₂, [which had] previously [been] ascribed to cystine, is [now] ascribed. In order to indicate the relationships of this substance to cystine, I name this reduction product of cystine: "cysteïne".) Note: Baumann's proposed structures for cysteine and cystine (see p.302) are incorrect: for cysteine, he proposed CH₃CNH₂(SH)COOH .

Further reading

• Nagano N, Ota M, Nishikawa K (September 1999). "Strong hydrophobic nature of cysteine residues in proteins". FEBS Lett. 458 (1): 69–71. doi:10.1016/S0014-5793(99)01122-9. PMID 10518936. S2CID 34980474.

External links

• Cysteine MS Spectrum
• International Kidney Stone Institute 2019-05-13 at the Wayback Machine
• http://www.chemie.fu-berlin.de/chemistry/bio/aminoacid/cystein en.html 2016-11-10 at the Wayback Machine
• Kosher View of L-Cysteine