Cysteine

• (R )-2 -amino-3 -mercapto- propanoic acid
• IUPAC: 2- amino-3- sulfanylpropansäure (without stereochemistry)
• (R)- Thioserin
• α -amino- β -mercaptopropionic acid
• Abbreviations: Cys ( Three letter code)
• C ( letter code)

• 52-90-4 (L- enantiomer)
• 921-01-7 (D - enantiomer)
• 3374-22-9 (DL- cysteine)
• 52-89-1 (L -Cysteine ​​· hydrochloride)
• 3374-22-9 (DL- cysteine)
• 207121-46-8 (D -cysteine ​​· hydrochloride monohydrate · )
• 7048-04-6 (L- cysteine ​​· hydrochloride monohydrate · )

Colorless solid with a characteristic odor

Fixed

220-228 ° C

• PKCOOH: 1.91 (25 ° C)
• PKNH2: 10.28 (25 ° C)
• PKSeitenkette: 8.14 (25 ° C)

• Readily soluble in water: 280 g · l-1 (20 ° C)
• Well in alcohol, acetic acid, not in ether and benzene

Attention

1890 mg · kg -1 ( LD50, rat, oral)

Template: Infobox chemical / molecular formula search available

Cysteine ​​(pronounced cyst - ín ), abbreviated as Cys or C, is a sulfur-containing proteinogenic α - amino acid with side chain - CH2 -SH, which can be formed in the adult liver. Cysteine ​​may be present in the enantiomeric forms of D and L, where in proteins, only the L-form [ Synonym: (R ) -cysteine ​​] is included. Since sulfur after the CIP nomenclature has a higher priority than oxygen, is L-cysteine ​​- in addition to the disulfide L-cystine - and L- selenocysteine ​​the only proteinogenic amino acid with (R )-configuration.

In this article, the information concerning the physiology alone, the L- cysteine. When it is mentioned in this article and in the scientific literature without any addition " cysteine ​​" is always meant L -cysteine. The racemic DL -cysteine ​​[ Synonym: ( RS)- cysteine ​​] and enantiomerically pure D -cysteine ​​[ Synonyms: ( S)- cysteine ​​] are synthetically accessible and only have little practical significance. The racemization of L- amino acids can be used for amino acid dating - are used - an age determination for fossil bone material.

L- cysteine ​​was first isolated from kidney stones, from which the name (Greek κύστις küstis, bubble ',' bladder ' ) is derived.

By oxidation of sulfhydryl groups, two cysteine ​​residues form a disulfide bond with each other, where cystine is formed. Disulfide bonds are responsible besides hydrogen bonds, ionic bonds and van der Waals forces for the formation and maintenance of the tertiary and quaternary structure of proteins.

Occurrence

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

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

Food

The following examples provide an overview of Cysteingehalte and are each based on 100 g of food, in addition, the percentage of cysteine ​​is indicated at the total protein.

Cysteine ​​is one of the nonessential amino acids. At least for adults, it is well established that the body can synthesize all the needs of cysteine ​​also from the essential amino acid methionine, provided that the food contains enough of it. If cysteine ​​in turn is able to replace a portion of the methionine, is still the subject of research. Sometimes cysteine ​​and methionine are sulfur-containing amino acids summarized under the term and given a common need. It should be noted that this is but is not a true combined demand, but only to the Methioninbedarf in cysteine-free diet.

→ figures see main article methionine

Often in the literature and the nutrient databases, the terms cysteine ​​and cystine used interchangeably when specifying the Cysteingehaltes. Strictly speaking this is not correct, since cysteine ​​and cystine referred to the monomer, the resulting dimer through a sulfur bridge. Many common methods of analysis to quantify the two compounds but not separated.

Biochemical importance

Multiple functions of the cysteine ​​in the body are derived from the relative reactivity of its thiol group. So can get yourself in protein folding between cysteine ​​residues that are located in different polypeptide chains or different sites of the same polypeptide chain and by the convolution in spatial proximity, forming a disulfide bond (-S -S-). The process is regulated by the protein disulfide isomerase. Covalent binding increases the stability of the protein structure, and also occurs in many extracellular proteins, such as keratin and insulin. Should also be mentioned that a larger group of enzymes of cysteine ​​residues has coordinated iron -sulfur cluster. The relatively reactive thiol group of cysteine ​​may also be involved directly in the catalytic mechanism, as in the glyceraldehyde -3-phosphate dehydrogenase, cysteine ​​where the substrate binds to the active site.

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

Properties

Cysteine ​​is present predominantly as the "inner salt " or zwitterion, whose formation can be explained by the fact that the proton of the carboxyl group migrates to the lone pair of the nitrogen atom of the amino group:

In an electric field, the zwitterion migrates not because it is not loaded as a whole. Strictly speaking, this is at the isoelectric point ( at a certain pH value) of the case, wherein the cysteine ​​has its lowest solubility in water. The isoelectric point of cysteine ​​is at a pH of 5.02.

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

• Side chain: hydrophilic
• Van der Waals volume of 86
• Hydrophobicity: 2.5

In neutral to alkaline aqueous solution is carried out on exposure to air oxidation to cystine. Upon exposure of stronger oxidants the cysteic acid is formed.

Technical recovery

L- cysteine ​​, as almost all other amino acids, by the action of hydrochloric acid on proteins such as keratin (mostly from keratinreichen tissues such as human or animal hair or feathers ) are obtained by hydrolysis. For some time the representation is also by fermentation with bacteria, such as Escherichia coli, including the use of genetically modified organisms (see illustration tryptophan). Cysteine ​​racemic (DL- cysteine) can be obtained fully synthetically from 2- chloroacetaldehyde, sodium hydrogen sulfide, ammonia, and acetone through the recovered after Asinger intermediate reaction product, 2,2- dimethyl-3 -thiazoline. Then hydrocyanic acid is deposited and subjected to acid hydrolysis.

Biosynthesis and metabolism

Cysteine ​​biosynthetically formed by serine, which provides the basic structure, and methionine homocysteine ​​, which contributes to the SH group in the liver. For this, the enzymes cystathionine synthetase and cystathionase are required. Consequently, serine or Methioninmangel inhibit cysteine ​​synthesis.

The amino acid can be degraded by α, β - elimination. This aminoacrylate and hydrogen sulfide formed ( H2S). H2S is oxidized to sulfate ( SO42 - ). Aminoacrylate isomerizes to Iminopropionat, the hydrolytically cleaves its amino group and thus becomes pyruvate.

By transamination, it can also be a β - Mercaptopyruvat. The sulfite - transferase transfers sulfite to the thiol group and thereby converts it into a thiosulfate to. After hydrolysis of the carbon -sulfur bond pyruvate is then free; the thiosulfate ( S2O32 - ) is oxidized to sulfate. Cysteine ​​can be oxidized to the SH group and then decarboxylated to taurine.

Through genetic defects in Cystintransporter a cystinuria can occur after ingestion in the gastrointestinal tract and reuptake in the kidney. The mutation in the gene rBAT also relates to the metabolism of the amino acids lysine, arginine and ornithine, that is, the polyamino- amino acids.

Therapeutic functions

From L- cysteine, pharmaceutical active ingredients in an industrial scale, are prepared, for example, (R )-S- carboxymethylcysteine ​​and (R )-N- acetylcysteine ​​( NAC or ACC ). These two active pharmaceutical ingredients to liquefy as oral mucolytics the often tenacious bronchial secretions in chronic bronchitis and chronic obstructive pulmonary disease. Under the administration of cysteine ​​increased the bronchial mucus formed in the course of these diseases is less viscous and can be coughed up more easily. Cysteine ​​increases a number of functions of lymphocytes such as cytotoxic T- cell activity. Cysteine ​​and glutathione preventing the expression of NF -AT, the nuclear transcription factor in stimulated T-cell lines. In vitro studies show that the stimulatory effect of TNF ( Tumor Necrosis Factor ) induced by free radicals, can be inhibited on the HIV replication in monocytes by sulfur-containing antioxidants. These basic studies suggest that the treatment of inflammatory diseases and AIDS with cysteine ​​thus possibly might be useful.

Cysteine ​​can form complexes with heavy metal ions. It is therefore used among other things as therapeutic agent for silver poisoning. As it binds free radicals to the thiol group of cysteine ​​is also used for the prevention of radiation damage. In fetuses, premature and newborn infants, as well as in liver cirrhosis, the activity of the enzyme cystathionase is absent or severely restricted. In these cases, an exogenous Cysteinzufuhr necessary. It is a radical scavenger, which makes the cell-damaging substances harmless and for the more recent studies a certain Vorbeugefunktion is postulated against neurodegenerative diseases.

In the very rare neurodegeneration with brain iron accumulation, a mutation causes the gene coding for the enzyme pantothenate kinase PANK2 gene, making it an enrichment of cysteine ​​- iron complexes in the brain - especially in the globus pallidus and substantia nigra pars reticulata - comes. This in turn leads to an increase in free radicals and, ultimately, to an oxidative damage to the neurons of the brain.

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

Food additive

Cysteine ​​is used as a flour treatment agent. It contributes to the accelerated maturation of flour, which is particularly important in automated baking operations, such as biscuits, of great importance. It is added to doughs containing gluten from strong ( strong gluten ) flours, because it changes the adhesive properties of flour, by minimizing the resistance to extension by reduction of disulfide bonds of gluten proteins. In addition, cysteine ​​is used in diet formulations, feed, pharmaceuticals and cosmetics. It is also a raw material for the production of flavor, especially for meat and roasted aromas. As a semi-essential amino acid is cysteine ​​/ cystine added to an adapted milk. L- cysteine ​​is approved for food as a flour treatment agent in the EU as a food additive (E 920 ) with no maximum limit ( quantum satis ). A declaration in finished products is not required.

Other areas of application

In Japanese hairdressers replaced cysteine, can break the disulfide bonds in keratin, which commonly used in Europe, the pungent thioglycolic acid, when it comes to prepare hair for perm. Also in other cosmetic products cysteine ​​is used.