Cysteine Explained

Cysteine (symbol Cys or C;)[1] is a semiessential[2] proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. Cysteine is chiral, but both D and L-cysteine are found in nature. LCysteine is a protein monomer in all biota, and D-cysteine acts as a signaling molecule in mammalian nervous systems.[3] Cysteine is named after its discovery in urine, which comes from the urinary bladder or cyst, from Greek κύστη kýsti, "bladder".[4]

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.[5] [6] The deprotonated form can generally be described by the symbol Cym as well.[7]

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 chirality in the older / notation based on homology to - and -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.

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.[8] Although classified as a nonessential amino acid,[9] 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 -cysteine is obtained industrially by hydrolysis of animal materials, such as poultry feathers or hog hair. Despite widespread rumor, human hair is rarely a source material. Indeed, food additive or cosmetic product manufactures may not legally source from human hair in the European Union.[10] [11]

Some animal-originating sources of -cysteine as a food additive contravene kosher, halal, vegan, or vegetarian diets.[12] To avoid this problem, synthetic -cysteine, compliant with Jewish kosher and Muslim halal laws, is also available, albeit at a higher price.[13] The typical synthetic route involves fermentation with an artificial E. coli strain.

Alternatively, Evonik (formerly Degussa) introduced a route from substituted thiazolines.[14] Pseudomonas thiazolinophilum hydrolyzes racemic 2amino-Δ2thiazoline-4carboxylic acid to cysteine.[15]

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 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 pKa 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.[2]

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.

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.

Evolutionary role of cysteine

Cysteine is considered a "newcomer" amino acid, being the 17th amino acid incorporated into the genetic code.[29] [30] Similar to other later-added amino acids such as methionine, tyrosine, and tryptophan, cysteine exhibits strong nucleophilic and redox-active properties.[31] [32] These properties contribute to the depletion of cysteine from respiratory chain complexes, such as Complexes I and IV,[33] since reactive oxygen species (ROS) produced by the respiratory chain can react with the cysteine residues in these complexes, leading to dysfunctional proteins and potentially contributing to aging. The primary response of a protein to ROS is the oxidation of cysteine and the loss of free thiol groups,[34] resulting in increased thiyl radicals and associated protein cross-linking.[35] [36] In contrast, another sulfur-containing, redox-active amino acid, methionine, does not exhibit these biochemical properties and its content is relatively upregulated in mitochondrially encoded proteins.

Applications

Cysteine, mainly the -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.[37] -Cysteine is also used as a processing aid for baking.[38]

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.[39] It binds to acetaldehyde to form the low-toxicity heterocycle methylthioproline.[40]

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.[41]

In 2020 an article was published that suggests L-cysteine might also work in humans.[42]

N-Acetylcysteine

N-Acetyl--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.[43]

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.[44]

Chemical reactions

Being multifunctional, cysteine undergoes a variety of reactions. Much attention has focused on protecting the sulfhydryl group.[45] 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.[46]

Safety

Relative to most other amino acids, cysteine is much more toxic.[47]

History

See main article: Cystine. 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".[48]

See also

Further reading

External links

Notes and References

  1. Web site: cysteine - Definition of cysteine in English by Oxford Dictionaries. https://web.archive.org/web/20160925134245/https://en.oxforddictionaries.com/definition/cysteine. dead. September 25, 2016. Oxford Dictionaries - English. 15 April 2018.
  2. Web site: The primary structure of proteins is the amino acid sequence. The Microbial World. University of Wisconsin-Madison Bacteriology Department. 16 September 2012. 25 May 2013. https://web.archive.org/web/20130525095344/http://www.microbiologytext.com/index.php?module=Book&func=displayarticle&art_id=661. dead.
  3. 10.1073/pnas.2110610118. free. D-cysteine is an endogenous regulator of neural progenitor cell dynamics in the mammalian brain. Evan R.. Semenza. Maged M.. Harraz. Efrat. Abramson. Adarsha P.. Malla. Chirag. Vasavda. Moataz M.. Gadalla. Michael D.. Kornberg. Solomon H.. Snyder. Robin. Roychaudhuri. 18 Aug 2021. 23 Sep 2021. PNAS. 118. 39. e2110610118 . 34556581 . 8488618. 2021PNAS..11810610S .
  4. Saffran . M. . April 1998 . Amino acid names and parlor games: from trivial names to a one-letter code, amino acid names have strained students' memories. Is a more rational nomenclature possible? . Biochemical Education . 26 . 2 . 116–118 . 10.1016/s0307-4412(97)00167-2 . 0307-4412.
  5. Web site: Amber Workshop - Tutorial A1 - Section 1: Do some editing of the PDB file . 2022-06-02 . ambermd.org . 2022-05-22 . https://web.archive.org/web/20220522002234/https://ambermd.org/tutorials/advanced/tutorial1_orig/section1.htm . dead .
  6. Lee . Jumin . Hitzenberger . Manuel . Rieger . Manuel . Kern . Nathan R. . Zacharias . Martin . Im . Wonpil . CHARMM-GUI supports the Amber force fields . The Journal of Chemical Physics . 21 July 2020 . 153 . 3 . 035103 . 10.1063/5.0012280 . 32716185 . 220796795 . free .
  7. Web site: Amber Workshop - Tutorial A1 - Section 1: Do some editing of the PDB file . 2022-06-02 . ambermd.org . 2022-05-22 . https://web.archive.org/web/20220522002234/https://ambermd.org/tutorials/advanced/tutorial1_orig/section1.htm . dead .
  8. Larsson . Susanna C. . Håkansson . Niclas . Wolk . Alicja . Dietary Cysteine and Other Amino Acids and Stroke Incidence in Women . Stroke . April 2015 . 46 . 4 . 922–926 . 10.1161/STROKEAHA.114.008022 . 25669310 . 14895681 .
  9. Web site: Cysteine - Health Encyclopedia - University of Rochester Medical Center . 2024-05-13 . www.urmc.rochester.edu.
  10. Web site: EU Chemical Requirements . May 24, 2020 . ...L-cysteine hydrochloride or hydrochloride monohydrate. Human hair may not be used as a source for this substance.
  11. Web site: Regulation (EC) No 1223/2009 of the European Parliament and of the Council of 30 November 2009 on cosmetic products . July 28, 2021 . ...ANNEX II LIST OF SUBSTANCES PROHIBITED IN COSMETIC PRODUCTS...416 Cells, tissues or products of human origin.
  12. See, e.g., News: Like mountains hanging by a hair: Kosher issues in L-cysteine (Commentary on Chagiga I:8). Zushe Yosef. Blech. MK News and Views. IV. 6. May 2003. Montreal Kosher. kashrut.com. Rabbi Blech does not address hog hair-derived cysteine, which is almost certainly treyf.
  13. Web site: Questions About Food Ingredients: What is L-cysteine/cysteine/cystine? . Vegetarian Resource Group .
  14. Martens . Jürgen . Heribert Offermanns . Offermanns . Heribert . Scherberich . Paul . 1981 . Facile Synthesis of Racemic Cysteine . Angewandte Chemie International Edition in English . 20 . 8 . 668 . 10.1002/anie.198106681.
  15. Book: 10.1002/14356007.a02_057.pub2 . Amino Acids . Ullmann's Encyclopedia of Industrial Chemistry . 2007 . Drauz . Karlheinz . Grayson . Ian . Kleemann . Axel . Krimmer . Hans-Peter . Leuchtenberger . Wolfgang . Weckbecker . Christoph . 978-3-527-30673-2.
  16. Hell R . Molecular physiology of plant sulfur metabolism . Planta . 202 . 2 . 138–48 . 1997 . 9202491 . 10.1007/s004250050112. 1997Plant.202..138H . 2539629 .
  17. Bulaj G, Kortemme T, Goldenberg DP . Ionization-reactivity relationships for cysteine thiols in polypeptides . Biochemistry . 37 . 25 . 8965–72 . June 1998 . 9636038 . 10.1021/bi973101r.
  18. Sekhar . Rajagopal V . Patel . Sanjeet G . 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 . 2011 . 94 . 3 . 847–853 . 10.3945/ajcn.110.003483 . 21795440 . 3155927 .
  19. Lill R, Mühlenhoff U . Iron-sulfur protein biogenesis in eukaryotes: components and mechanisms . Annu. Rev. Cell Dev. Biol. . 22 . 457–86 . 2006 . 16824008 . 10.1146/annurev.cellbio.22.010305.104538.
  20. Book: Stephen J. . Lippard . Jeremy M. . Berg . Principles of Bioinorganic Chemistry . University Science Books . Mill Valley, CA . 1994 . 978-0-935702-73-6.
  21. Baker DH, Czarnecki-Maulden GL . Pharmacologic role of cysteine in ameliorating or exacerbating mineral toxicities . J. Nutr. . 117 . 6 . 1003–10 . June 1987 . 3298579 . 10.1093/jn/117.6.1003 . free .
  22. Heitmann P . A model for sulfhydryl groups in proteins. Hydrophobic interactions of the cystein side chain in micelles . Eur. J. Biochem. . 3 . 3 . 346–50 . January 1968 . 5650851 . 10.1111/j.1432-1033.1968.tb19535.x. free .
  23. Web site: A Review of Amino Acids (tutorial). Curtin University. 2015-09-09. https://web.archive.org/web/20150907052410/http://wbiomed.curtin.edu.au/biochem/tutorials/AAs/AA.html. 2015-09-07. dead.
  24. Nagano N, Ota M, Nishikawa K . Strong hydrophobic nature of cysteine residues in proteins . FEBS Lett. . 458 . 1 . 69–71 . September 1999 . 10518936 . 10.1016/S0014-5793(99)01122-9. 34980474 . free . 1999FEBSL.458...69N .
  25. Web site: Hydrophobic amino acids . 2012-09-16 . Betts . M.J. . R.B. Russell . 2003 . Amino Acid Properties and Consequences of Substitutions, In: Bioinformatics for Geneticists . Wiley.
  26. Web site: Introduction to Protein Structure--Non-Polar Amino Acids . 2012-09-16 . Gorga . Frank R. . 1998–2001 . dead . https://web.archive.org/web/20120905162400/http://webhost.bridgew.edu/fgorga/proteins/nonpolar.htm . 2012-09-05 .
  27. Web site: Virtual Chembook--Amino Acid Structure . 2012-09-16 . Elmhurst College . dead . https://web.archive.org/web/20121002050150/http://www.elmhurst.edu/~chm/vchembook/561aminostructure.html . 2012-10-02 .
  28. Sevier CS, Kaiser CA . Formation and transfer of disulphide bonds in living cells . Nat. Rev. Mol. Cell Biol. . 3 . 11 . 836–47 . November 2002 . 12415301 . 10.1038/nrm954. 2885059 . free .
  29. Osawa . S . Jukes . T H . Watanabe . K . Muto . A . March 1992 . Recent evidence for evolution of the genetic code . Microbiological Reviews . en . 56 . 1 . 229–264 . 10.1128/mr.56.1.229-264.1992 . 0146-0749 . 372862 . 1579111.
  30. Trifonov . Edward N. . September 2009 . The origin of the genetic code and of the earliest oligopeptides . Research in Microbiology . en . 160 . 7 . 481–486 . 10.1016/j.resmic.2009.05.004. 19524038 .
  31. Paulsen . Candice E. . Carroll . Kate S. . 2013-07-10 . Cysteine-Mediated Redox Signaling: Chemistry, Biology, and Tools for Discovery . Chemical Reviews . en . 113 . 7 . 4633–4679 . 10.1021/cr300163e . 0009-2665 . 4303468 . 23514336.
  32. Giles . Niroshini M . Watts . Aaron B . Giles . Gregory I . Fry . Fiona H . Littlechild . Jennifer A . Jacob . Claus . August 2003 . Metal and Redox Modulation of Cysteine Protein Function . Chemistry & Biology . 10 . 8 . 677–693 . 10.1016/s1074-5521(03)00174-1 . 12954327 . 1074-5521.
  33. Moosmann . Bernd . Behl . Christian . February 2008 . Mitochondrially encoded cysteine predicts animal lifespan . Aging Cell . en . 7 . 1 . 32–46 . 10.1111/j.1474-9726.2007.00349.x . 18028257 . 1474-9718. free .
  34. Sohal . Rajindar S . 2002-07-01 . Role of oxidative stress and protein oxidation in the aging process1, 2 . Free Radical Biology and Medicine . 33 . 1 . 37–44 . 10.1016/S0891-5849(02)00856-0 . 12086680 . 0891-5849.
  35. Jacob . Claus . Giles . Gregory I. . Giles . Niroshini M. . Sies . Helmut . 2003-10-13 . Sulfur and Selenium: The Role of Oxidation State in Protein Structure and Function . Angewandte Chemie International Edition . en . 42 . 39 . 4742–4758 . 10.1002/anie.200300573 . 14562341 . 1433-7851.
  36. Nauser . Thomas . Pelling . Jill . Schöneich . Christian . 2004-10-01 . Thiyl Radical Reaction with Amino Acid Side Chains: Rate Constants for Hydrogen Transfer and Relevance for Posttranslational Protein Modification . Chemical Research in Toxicology . en . 17 . 10 . 1323–1328 . 10.1021/tx049856y . 15487892 . 0893-228X.
  37. Book: Tzou-Chi . Huang . Chi-Tang . Ho . Meat Science and Applications, ch. Flavors of Meat Products. 71–102 . Y. H. . Hui . Wai-Kit . Nip . Robert . Rogers . CRC . 978-0-203-90808-2. 2001-07-27 .
  38. Web site: Food Ingredients and Colors . November 2004 . U.S. Food and Drug Administration . 2009-09-06 . dead . https://web.archive.org/web/20090512090657/https://www.cfsan.fda.gov/~dms/foodic.html . 2009-05-12 .
  39. Otoyama . Ippo . Hamada . Hironobu . Kimura . Tatsushi . Namba . Haruchi . Sekikawa . Kiyokazu . Kamikawa . Norimichi . Kajiwara . Teruki . Aizawa . Fumiya . Sato . Yoshinobu M. . L-cysteine improves blood fluidity impaired by acetaldehyde: In vitro evaluation . PLOS ONE . 2019 . 14 . 3 . e0214585 . 10.1371/journal.pone.0214585 . 30925182 . 6440629 . 2019PLoSO..1414585O . free .
  40. Interaction of alcohol and smoking in the pathogenesis of upper digestive tract cancers: possible chemoprevention with cysteine. URN:ISBN:952-10-3056-9. 10138/22689. Salaspuro. Ville. University of Helsinki. 2006. Academic dissertation. 41–44.
  41. Sprince H, Parker CM, Smith GG, Gonzales LJ . Protection against acetaldehyde toxicity in the rat by L-cysteine, thiamin and L-2-methylthiazolidine-4-carboxylic acid . Agents Actions . 4 . 2 . 125–30 . April 1974 . 4842541 . 10.1007/BF01966822. 5924137 .
  42. Eriksson . C J Peter . Metsälä . Markus . Möykkynen . Tommi . Mäkisalo . Heikki . Kärkkäinen . Olli . Palmén . Maria . Salminen . Joonas E . Kauhanen . Jussi . L-Cysteine Containing Vitamin Supplement Which Prevents or Alleviates Alcohol-related Hangover Symptoms: Nausea, Headache, Stress and Anxiety . Alcohol and Alcoholism . 20 October 2020 . 55 . 6 . 660–666 . 10.1093/alcalc/agaa082 . 32808029 . 10138/339340 . free .
  43. Kanter MZ . 9209528 . Comparison of oral and i.v. acetylcysteine in the treatment of acetaminophen poisoning . Am J Health Syst Pharm . 63 . 19 . 1821–7 . October 2006 . 16990628 . 10.2146/ajhp060050.
  44. Powell BC, Walker SK, Bawden CS, Sivaprasad AV, Rogers GE . Transgenic sheep and wool growth: possibilities and current status . Reprod. Fertil. Dev. . 6 . 5 . 615–23 . 1994 . 7569041 . 10.1071/RD9940615.
  45. 10.15227/orgsyn.059.0190 . Thiol Protection with the Acetamidomethyl Group: S-Acetamidomethyl-L-Cysteine Hydrochloride . Organic Syntheses . 1979 . 59 . 190. John D. . Milkowski. Daniel F. . Veber. Ralph. Hirschmann .
  46. 10.1021/ic00343a061. Stereospecificity in the Synthesis of the Tris((R)-Cysteinato-N,S)- and Tris((R)-Cysteinesulfinato-N,S)cobaltate(III) Ions . 1990 . Arnold . Alan P. . Jackson . W. Gregory . Inorganic Chemistry . 29 . 18 . 3618–3620 .
  47. Book: 10.1016/0076-6879(87)43059-0 . Intracellular delivery of cysteine . Sulfur and Sulfur Amino Acids . Methods in Enzymology . 1987 . Anderson . Mary E. . Meister . Alton . 143 . 313–325 . 3309557 . 9780121820435 .
  48. Baumann . E. . Ueber Cystin und Cysteïn . Zeitschrift für physiologische Chemie . 1884 . 8 . 299–305 . On cystine and cysteine . German. 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 .

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