Cystine is the oxidized derivative of the

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...

cysteine and has the formula (SCH₂CH(NH₂)CO₂H)₂. It is a white solid that is poorly soluble in water. As a residue in proteins, cystine serves two functions: a site of

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

reactions and a mechanical linkage that allows proteins to retain their three-dimensional structure.

Formation and reactions

Structure

Cystine is the

disulfide In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...

derived from the amino acid cysteine. The conversion can be viewed as an oxidation: : Cystine contains a

disulfide bond In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...

, two amine groups, and two carboxylic acid groups. As for other amino acids, the amine and carboxylic acid groups exist is rapid equilibrium with the ammonium-carboxylate

tautomer Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert. The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hyd ...

. The great majority of the literature concerns the ''l,l-''cystine, derived from ''l''-cysteine. Other isomers include ''d,d''-cystine and the

meso isomer A meso compound or meso isomer is a non-optically active member of a set of stereoisomers, at least two of which are optically active. This means that despite containing two or more stereocenters, the molecule is not chiral. A meso compound is "sup ...

d,l-cystine, neither of which is biologically significant.

Occurrence

Cystine is common in many foods such as eggs, meat, dairy products, and whole grains as well as skin, horns and hair. It was not recognized as being derived of proteins until it was isolated from the

horn Horn most often refers to: *Horn (acoustic), a conical or bell shaped aperture used to guide sound ** Horn (instrument), collective name for tube-shaped wind musical instruments *Horn (anatomy), a pointed, bony projection on the head of various ...

of a

cow Cattle (''Bos taurus'') are large, domesticated, cloven-hooved, herbivores. They are a prominent modern member of the subfamily Bovinae and the most widespread species of the genus ''Bos''. Adult females are referred to as cows and adult ma ...

in 1899. Human hair and skin contain approximately 10–14% cystine by mass.

History

Cystine was discovered in 1810 by the English chemist

William Hyde Wollaston William Hyde Wollaston (; 6 August 1766 – 22 December 1828) was an English chemist and physicist who is famous for discovering the chemical elements palladium and rhodium. He also developed a way to process platinum ore into malleable ingot ...

, who called it "cystic oxide". In 1833, the Swedish chemist Jöns Jacob Berzelius named the amino acid "cystine". The Norwegian chemist Christian J. Thaulow determined, in 1838, the

empirical formula In chemistry, the empirical formula of a chemical compound is the simplest whole number ratio of atoms present in a compound. A simple example of this concept is that the empirical formula of sulfur monoxide, or SO, would simply be SO, as is the ...

of cystine. In 1884, the German chemist

Eugen Baumann Eugen Baumann (12 December 1846 – 3 November 1896) was a German chemist. He was one of the first people to create polyvinyl chloride (PVC), and, together with Carl Schotten, he discovered the Schotten-Baumann reaction. Life Baumann was born i ...

found that when cystine was treated with a reducing agent, cystine revealed itself to be a

dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ...

of a

monomer In chemistry, a monomer ( ; '' mono-'', "one" + ''-mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification ...

which he named "cysteïne". In 1899, cystine was first isolated from protein (horn tissue) by the Swedish chemist Karl A. H. Mörner (1855-1917). The chemical structure of cystine was determined by synthesis in 1903 by the German chemist

Emil Erlenmeyer Richard August Carl Emil Erlenmeyer (28 June 182522 January 1909), known simply as Emil Erlenmeyer, was a German chemist known for contributing to the early development of the theory of structure, formulating the Erlenmeyer rule, and designing ...

Redox

It is formed from the oxidation of two cysteine molecules, which results in the formation of a

. In cell biology, cystine residues (found in proteins) only exist in non-reductive (oxidative) organelles, such as the secretory pathway ( endoplasmic reticulum,

Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles ...

lysosome A lysosome () is a membrane-bound organelle found in many animal cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane pr ...

s, and vesicles) and extracellular spaces (e.g.,

extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide s ...

). Under reductive conditions (in the cytoplasm, nucleus, etc.) cysteine is predominant. The disulfide link is readily reduced to give the corresponding

thiol In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl gro ...

cysteine. Typical thiols for this reaction are

mercaptoethanol 2-Mercaptoethanol (also β-mercaptoethanol, BME, 2BME, 2-ME or β-met) is the chemical compound with the formula HOCH2CH2SH. ME or βME, as it is commonly abbreviated, is used to reduce disulfide bonds and can act as a biological antioxidant by ...

and

dithiothreitol Dithiothreitol (DTT) is the common name for a small-molecule redox reagent also known as Cleland's reagent, after W. Wallace Cleland. DTT's formula is C4H10O2S2 and the chemical structure of one of its enantiomers in its reduced form is shown o ...

: :(SCH₂CH(NH₂)CO₂H)₂ + 2 RSH → 2 HSCH₂CH(NH₂)CO₂H + RSSR Because of the facility of the thiol-disulfide exchange, the nutritional benefits and sources of cystine are identical to those for the more-common cysteine. Disulfide bonds cleave more rapidly at higher temperatures.

Cystine-based disorders

The presence of cystine in urine is often indicative of amino acid reabsorption defects.

Cystinuria Cystinuria is an inherited autosomal recessive disease characterized by high concentrations of the amino acid cystine in the urine, leading to the formation of cystine stones in the kidneys, ureters, and bladder. It is a type of aminoaciduri ...

has been reported to occur in dogs. In humans the excretion of high levels of cystine crystals can be indicative of

cystinosis Cystinosis is a lysosomal storage disease characterized by the abnormal accumulation of cystine, the oxidized dimer of the amino acid cysteine. It is a genetic disorder that follows an autosomal recessive inheritance pattern. It is a rare autosom ...

, a rare genetic disease.

Biological transport

Cystine serves as a substrate for the cystine-glutamate antiporter. This transport system, which is highly specific for cystine and glutamate, increases the concentration of cystine inside the cell. In this system, the anionic form of cystine is transported in exchange for glutamate. Cystine is quickly reduced to cysteine. Cysteine prodrugs, e.g. acetylcysteine, induce release of glutamate into the extracellular space.

Nutritional supplements

Cysteine supplements are sometimes marketed as anti-aging products with claims of improved skin elasticity. Cysteine is more easily absorbed by the body than cystine, so most supplements contain cysteine rather than cystine. N-acetyl-cysteine (NAC) is better absorbed than other cysteine or cystine supplements.

References

External links

* {{E number infobox 920-929 Organic disulfides Sulfur amino acids Non-proteinogenic amino acids