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Structural formula
Structure of L- cysteine, the naturally occurring enantiomer
Surname Cysteine
other names
Molecular formula C 3 H 7 NO 2 S
Brief description

colorless solid with a characteristic odor 

External identifiers / databases
CAS number
  • 52-90-4 ( L - enantiomer )
  • 921-01-7 ( D -enantiomer)
  • 3374-22-9 ( DL -cysteine)
  • 52-89-1 ( L- cysteine hydrochloride )
  • 207121-46-8 ( D -cysteine · hydrochloride · mono hydrate )
  • 7048-04-6 ( L -cysteine · hydrochloride · mono hydrate )
EC number 200-158-2
ECHA InfoCard 100,000,145
PubChem 5862
ChemSpider 5653
DrugBank DB00151
Wikidata Q186474
Molar mass 121.16 g mol −1
Physical state


Melting point

220-228 ° C 

pK s value
  • p K COOH : 1.91 (25 ° C)
  • p K NH 2 : 10.28 (25 ° C)
  • p K side chain : 8.14 (25 ° C)
  • easily soluble in water: 280 g l −1 (20 ° C) 
  • good in alcohol, acetic acid, not ether and benzene
safety instructions
GHS labeling of hazardous substances
07 - Warning


H and P phrases H: 302
P: no P-phrases
Toxicological data

1890 mg kg −1 ( LD 50ratoral )

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Cysteine (pronounced: Cyste-ín), abbreviated to Cys or C , is an α - amino acid with the side chain –CH 2 –SH that contains sulfur . Only the naturally occurring enantiomeric form L - cysteine ​​[synonym: ( R ) -cysteine] is a proteinogenic amino acid; in adults it can be formed in the liver from the sulfur-containing amino acid L - methionine .

By oxidation of the sulfhydryl groups , two cysteine ​​residues can form a disulfide bridge with one another , which creates cystine . Such disulfide bridges stabilize the tertiary and quaternary structure of numerous proteins and are important for the formation and maintenance of functional conformations .


Cysteine ​​can exist in the enantiomeric forms D and L , with only the L form occurring in proteins . Since sulfur is assigned a higher priority than oxygen according to the CIP nomenclature , L- cysteine ​​- like the disulfide L - cystine and L - selenocysteine - is a proteinogenic amino acid with an ( R ) configuration .

In this article, the information on physiology concerns L- cysteine alone . If cysteine is spoken of without any addition , L- cysteine is generally meant. The racemic DL- cysteine ​​[synonym: ( RS ) -cysteine] and enantiomerically pure D- cysteine ​​[synonym: ( S ) -cysteine] are synthetically accessible and are of little practical importance.

The racemization of L- amino acids can be used for amino acid dating - an age determination for fossil bone material.

Enantiomers of cysteine
Surname L- cysteine D- cysteine
other names ( R ) -cysteine ( S ) -cysteine
Structural formula L-cysteine D-cysteine
CAS number 52-90-4 921-01-7
3374-22-9 (racemate)
EC number 200-158-2 213-062-0
222-160-2 (racemate)
ECHA info card 100,000,145 100,011,875
100.020.147 (racemate)
PubChem 5862 92851
594 (racemate)
DrugBank DB00151 DB03201
- (racemate)
Wikidata Q186474 Q16633812
Q27089394 (racemate)


L- cysteine ​​was first isolated from kidney stones as cystine in 1810 by the English scientist William Hyde Wollaston , from which the name ( ancient Greek κύστις küstis , German “bladder” , “ urinary bladder ”) is derived. Wollaston initially called the new substance "cystic oxide" before Jöns Jakob Berzelius later named it cystine. The Swedish chemist Count Mörner succeeded in isolating it from proteins for the first time in 1899. Before that, the Freiburg professor Eugen Baumann had succeeded for the first time in obtaining the actual amino acid cysteine ​​by reducing cystine. Emil Fischer was finally able to explain the structural formula of cysteine ​​beyond any doubt.


L- cysteine ​​is found in proteins, but not all proteins contain cysteine. Computational analysis of 207 unrelated proteins resulted in an average mass fraction of 2.6% cysteine; in the same analysis, 1.7% cysteine ​​was determined for whey protein .

Chemical formula of L- cystine with the disulfide bridge marked in blue . L- cystine should not be confused with L- cysteine.

High L- cysteine ​​content (and thus high stability) can be found e.g. B. in keratin : feather keratin contains about 7%, wool keratin 11 to 17% cysteine. But also very small sterically stabilized proteins such as snake toxins ( myotoxin , neurotoxin etc .; about 40 to 70 amino acids) contain 10 to 14% cysteine ​​in the form of cystine (disulfide bridges).


The following examples give an overview of the cysteine ​​content and each relate to 100 g of the food; the percentage of cysteine ​​in the total protein is also given.

Food Total protein Cysteine proportion of
Pork, raw 20.95 g 242 mg 1.2%
Chicken breast fillet, raw 21.23 g 222 mg 1.0%
Salmon, raw 20.42 g 219 mg 1.1%
Chicken egg 12.57 g 272 mg 2.2%
Cow's milk, 3.7% fat 0 3.28 g 0 30 mg 0.9%
Sunflower seeds 20.78 g 451 mg 2.2%
Walnuts 15.23 g 208 mg 1.4%
Wholemeal wheat flour 13.70 g 317 mg 2.3%
Wholemeal corn flour 0 6.93 g 125 mg 1.8%
Rice, unpeeled 0 7.94 g 0 96 mg 1.2%
Soybeans, dried 36.49 g 655 mg 1.8%
Peas, dried 24.55 g 373 mg 1.5%

Cysteine ​​is one of the non-essential amino acids. At least for adults, it is certain that the body can synthesize all of its cysteine ​​requirements from the essential amino acid methionine , provided that the diet contains enough of it. Whether cysteine ​​in turn is able to replace part of the methionine is still the subject of research. Sometimes cysteine ​​and methionine are grouped under the term sulfur-containing amino acids and a common requirement is stated. It should be noted that this is not a real combined requirement, but only the methionine requirement for a cysteine-free diet.

The terms cysteine ​​and cystine are often used synonymously in the literature and nutrient databases when specifying the cysteine ​​content. Strictly speaking, this is not correct, as cysteine ​​denotes the monomer and cystine denotes the dimer formed by a sulfur bridge. However, many common analytical methods do not quantify the two compounds separately.

Biochemical significance

Various functions of cysteine ​​in the organism are derived from the relative reactivity of its free thiol group . During protein folding, a disulfide bridge (–S – S–) can form between cysteine ​​residues of the same polypeptide chain , which come into spatial proximity to one another during the folding process. The bridging by an additional covalent bond between amino acids at non-adjacent positions in the chain increases the stability of their spatial arrangement, the tertiary structure . In the folding protein molecule, under the action of protein disulfide isomerases, a disulfide bridge, also known as a cystine bridge, can possibly be shifted to other cysteine ​​residues. The cross- bridges formed via the S atom of cysteine ​​residues play an essential role in the folding of the native protein and stabilize its tertiary structure through covalent bonds. Stabilizing disulfide bridges occur in many, mainly secretory and extracellular proteins, for example in insulin . They can also stabilize the quaternary structure of a protein complex made up of several polypeptide chains or link chains with one another, for example in the case of antibodies , and link linked chains to form bundles, as in the case of keratins . In the case of short peptides with cysteine ​​at the chain end, such as the nonapeptide oxytocin , a proteohormone , the formation of a disulfide bridge creates a ring-shaped structure, which makes unspecific degradation by peptidases difficult.

A larger group of enzymes has iron-sulfur clusters coordinated by cysteine ​​residues . The relatively reactive thiol group of cysteine ​​can also be directly involved in the catalytic mechanism, as in glyceraldehyde-3-phosphate dehydrogenase , where cysteine ​​binds the substrate to the active site.

Cysteine ​​is also a starting material in the biosynthesis of compounds such as glutathione , coenzyme A and taurine .


Cysteine ​​is mainly present as an "inner salt" or zwitterion , the formation of which can be explained by the fact that the proton of the carboxy group migrates to the lone pair of electrons on the nitrogen atom of the amino group :

Zwitterions of L- cysteine ​​(left) or D- cysteine ​​(right)

The zwitterion does not migrate in the electric field because it is uncharged as a whole. Strictly speaking, this is the case at the isoelectric point (at a certain pH value), at which the cysteine ​​also has its lowest solubility in water. The isoelectric point of cysteine ​​is at pH 5.02.

Cysteine ​​could be counted among the non- essential amino acids as it can be produced by the body. However, the essential amino acid methionine is required for this. Therefore, cysteine ​​is usually considered to be semi-essential. As a component of many proteins and enzymes , it is often involved in the catalytic mechanism.

In a neutral to alkaline aqueous solution, when air is admitted, oxidation to cystine takes place . When exposed to stronger oxidizing agents, cysteic acid is formed.

Technical extraction

Like almost all other amino acids , L- cystine can be obtained by hydrolysis through the action of hydrochloric acid on proteins such as keratin (mostly from keratin-rich tissues such as human or animal hair or feathers) . The L- cystine obtained in this way can then be converted into L- cysteine by electrochemical reduction .

Since today there is a clear trend away from animal products to plant-based alternatives among consumers, cysteine, which was obtained from plants, is now also being used. This is made by fermentation from raw materials on a vegan basis and inorganic trace elements. The L -cysteine is used in the baking industry as Teigweichmacher.

For some time now, the representation has also been made by fermentation with bacteria , e.g. B. Escherichia coli , also possible using genetically modified organisms (see illustration of tryptophan ).

Racemic cysteine ​​( DL- cysteine) can be obtained fully synthetically from 2-chloroacetaldehyde , sodium hydrogen sulfide , ammonia and acetone via the intermediate product 2,2-dimethyl-3-thiazoline obtained from the Asinger reaction . Then, hydrogen cyanide annealed and acid hydrolysis.

Biosynthesis and metabolism

Cysteine ​​is biosynthetically formed in the liver from serine , which provides the basic structure, and methionine via homocysteine , which contributes the SH group . The enzymes cystathionine synthetase and cystathionase are required for this. Serine or methionine deficiencies consequently inhibit cysteine ​​synthesis.

The amino acid can be broken down by α, β - elimination . This creates amino acrylate and hydrogen sulfide (H 2 S). H 2 S is oxidized to sulfate (SO 4 2− ). Amino acrylate isomerizes to iminopropionate , which hydrolytically splits off its amino group and thus becomes pyruvate .

It can also become β-mercaptopyruvate through transamination . The sulfite transferase transfers sulfite to the thiol group, thereby converting it into a thiosulfate . After hydrolysis of the carbon-sulfur bond, pyruvate is then released; the thiosulfate (S 2 O 3 2− ) is oxidized to the sulfate. Cysteine ​​can also be oxidized at the SH group and then decarboxylated to taurine .

By genetic defects in Cystintransporter tract stomach and a resumption, after recording in the kidney in cystinuria arise. The mutation in the rBAT gene also affects the metabolism of the amino acids lysine , arginine and ornithine , i.e. the polyamino-amino acids.

Therapeutic functions

Active pharmaceutical ingredients are produced from L- cysteine ​​on an industrial scale, e.g. B. ( R ) - S - carboxymethylcysteine and ( R ) - N - acetylcysteine (ACC and NAC, respectively). These two active pharmaceutical ingredients are used as oral mucolytic agents to liquefy the often viscous bronchial mucus in chronic bronchitis and chronic obstructive pulmonary disease. When cysteine ​​is given, the increased bronchial mucus that forms in the course of these diseases becomes thinner and can therefore be coughed up more easily. Cysteine ​​also increases a number of lymphocyte functions, such as cytotoxic T cell activity. Cysteine ​​and glutathione prevent the expression of NF-AT , the nuclear transcription factor, in stimulated T-cell lines. In vitro studies show that the stimulating effect of TNF ( tumor necrosis factor ), induced by free radicals, on HIV replication in monocytes can be inhibited by sulfur-containing antioxidants. These fundamental studies suggest that treating inflammatory diseases and AIDS with cysteine ​​could potentially be useful with it.

Cysteine ​​can complex heavy metal ions . It is therefore used, among other things, as a therapeutic agent for silver poisoning. Since it binds free radicals to the thiol group , cysteine ​​is also used to prevent radiation damage. In fetuses, premature babies and newborns, as well as in liver cirrhosis , the activity of the enzyme cystathionase is either absent or severely restricted. In these cases, an exogenous supply of cysteine ​​is necessary. It is a radical scavenger that renders cell-damaging substances harmless and for which recent studies postulate a certain preventive function against neurodegenerative diseases .

In the very rare neurodegeneration with brain iron accumulation causes a mutation in the enzyme pantothenate kinase encoding PANK2 gene that there will be an accumulation of cysteine iron - complexes in the brain - especially in the globus pallidus and substantia nigra pars reticulata is -. This in turn leads to an increase in free radicals and ultimately to oxidative damage to the nerve cells of the brain.

Cysteine ​​is a component of amino acid infusion solutions for parenteral nutrition.

Food additive

L- cysteine ​​is used in the form of the hydrochloride as a flour treatment agent and baking agent in the manufacture of baked goods . It softens the glue by depolymerizing the molecules of the glutenin fraction through thiol-disulfide exchange with the intermolecular disulfide bonds ( i.e. breaking the bonds that hold the long chain molecules together). As a result, the dough becomes more elastic and develops faster. With high-gluten flour , a higher volume can be achieved because the propellant gas (such as the carbon dioxide formed by the yeast ) can loosen the dough more easily. Cysteine ​​hydrochloride can also be added to the production of pasta in order to speed up dough production (an addition of 0.01% reduces the mixing or kneading time by 15–20%). It inhibits the formation of melanoidins in non-enzymatic tanning and thus counteracts discoloration. In addition to these uses, like other amino acids, cysteine ​​acts as a flavor , flavor enhancer, and nutrient .

L- cysteine ​​hydrochloride or hydrochloride monohydrate is approved under European food law as an additive without maximum quantity restrictions ( quantum satis ) under the number E 920 . In principle, it must be declared, but not if it is no longer technologically effective in the labeled product in accordance with Article 20 of the Food Information Ordinance and Section 9, Paragraph 8, No. 1 of the Additive Authorization Ordinance . In the opinion of the Backmittelinstitut (an institution of the Association of Baking Ingredients ), the technological effectiveness of cysteine, which is used as a flour treatment agent (i.e. was added to the flour before doughing), does not extend to the finished baked goods, but to the dough , if it is used as a Semi-finished product is offered. As a result, it does not need to be labeled for finished baked goods. The Federation of German Consumer Organizations does not share this legal opinion.

Further areas of application

In Japanese hairdressing salons, cysteine, which is able to break disulfide bonds in keratin, replaces the strong smelling thioglycolic acid common in Europe when it comes to preparing hair for permanent waves . Cysteine ​​is also used in other cosmetic products.

Web links

Wiktionary: Cysteine  - explanations of meanings, word origins, synonyms, translations

Individual evidence

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