Cystine is the oxidized derivative of the

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

cysteine Cysteine (; symbol Cys or C) is a semiessential proteinogenic amino acid with the chemical formula, formula . The thiol side chain in cysteine enables the formation of Disulfide, disulfide bonds, and often participates in enzymatic reactions as ...

and has the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...

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

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

Formation and reactions

Structure

Cystine is the

disulfide In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inorg ...

derived from the amino acid

. The conversion can be viewed as an oxidation: : Cystine contains a

disulfide bond In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inor ...

, two amine groups, and two carboxylic acid groups. As for other amino acids, the amine and carboxylic acid groups exist in rapid equilibrium with the ammonium-carboxylate tautomer. 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 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 Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, re ...

until it was isolated from the horn of a cow 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 i ...

, who called it "cystic oxide". In 1833, the Swedish chemist

Jöns Jacob Berzelius Baron Jöns Jacob Berzelius (; 20 August 1779 – 7 August 1848) was a Swedish chemist. Berzelius is considered, along with Robert Boyle, John Dalton, and Antoine Lavoisier, to be one of the founders of modern chemistry. Berzelius became a memb ...

named the amino acid "cystine". The Norwegian chemist Christian J. Thaulow determined, in 1838, the empirical formula of cystine. In 1884, the German chemist Eugen Baumann found that when cystine was treated with a reducing agent, cystine revealed itself to be a dimer of a

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

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 1825 – 22 January 1909), known simply as Emil Erlenmeyer, was a German chemist known for contributing to the early development of the theory of chemical structure and formulating the Erlenmeyer rul ...

. The history of cystine and

is complicated by the dimer-monomer relationship of the two. The cysteine monomer was proposed as the actual unit by Embden in 1901. The sulfur within the structure of cysteine and cystine has been subject of historical interest. In 1902, Osborne partially succeeded in analysing cystine content via lead compounds. An improved colorimetric method was developed in 1922 by Folin and Looney. An iodometric analysis method was developed by Okuda in 1925.

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 The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryote, eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for ...

Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic Cell (biology), cells. Part of the endomembrane system in the cytoplasm, it protein targeting, packages proteins ...

lysosome A lysosome () is a membrane-bound organelle that is found in all mammalian cells, with the exception of red blood cells (erythrocytes). There are normally hundreds of lysosomes in the cytosol, where they function as the cell’s degradation cent ...

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

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

). 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 grou ...

. Typical thiols for this reaction are mercaptoethanol and dithiothreitol: :(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

. 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 has been reported to occur in dogs. In humans the excretion of high levels of cystine crystals can be indicative of cystinosis, a rare genetic disease. Cystine stones account for about 1-2% of

kidney stone disease Kidney stone disease (known as nephrolithiasis, renal calculus disease, or urolithiasis) is a crystallopathy and occurs when there are too many minerals in the urine and not enough liquid or hydration. This imbalance causes tiny pieces of cry ...

in adults.

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 Alpha-Amino acids Organic disulfides Sulfur amino acids Non-proteinogenic amino acids