Cystine

Not to be confused with cytosine, cysteine, cytisine, or cytidine.
Cystine
Identifiers
56-89-3 Yes
ChEBI CHEBI:35492 Yes
ChEMBL ChEMBL366563 Yes
ChemSpider 575 Yes
Jmol-3D images Image
KEGG C01420 Yes
PubChem 67678
UNII 48TCX9A1VT Yes
Properties
Molecular formula
 C6H12N2O4S2
Molar mass 240.30 g·mol−1
Hazards
MSDS External MSDS
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Cystine is the amino acid formed by the oxidation of two cysteine molecules that covalently link via a disulfide bond. This organosulfur compound has the formula (SCH2CH(NH2)CO2H)2. It is a white solid that is slightly soluble in water. Human hair and skin contain approximately 10-14% cystine by mass. It was discovered in 1810 by William Hyde Wollaston but was not recognized as being derived of proteins until it was isolated from the horn of a cow in 1899.[1]

Properties and nutritional aspects

Disulfide bonding, along with hydrogen bonding and hydrophobic interactions is partially responsible for the formation of the gluten matrix in bread.[2]

The disulfide link is readily reduced to give the corresponding thiol cysteine. Typical thiols for this reaction is 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 cysteine. Disulfide bonds cleave more rapidly at higher temperatures.[3]

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.[4] In humans the excretion of high levels of cystine crystals can be indicative of cystinosis, a rare genetic disease.

Cystine formation reaction

The cystine formation reaction starting from cysteine is the following one:

formation reaction of cystine starting from cysteine

Cystine is formed from the oxidation of two cysteine molecules forming a covalent disulfide bond between the two cysteine R groups, releasing 2 protons and 2 electrons.[5] In cell biology, cystine (found in proteins) can only exist in non-reductive (oxidative) organelles, such as the secretory pathway (ER, Golgi, Lysosomes, Vesicles and ECM). Meaning that in reductive conditions (Cytoplasm, Nucleus, etc.) cysteine is favorably found.[6] This reductive nature of the cytosol is in part caused by high Glutathione levels.[7]

Biological transport

Cystine serves as a substrate for the cystine-glutamate antiporter. This transport system, which is highly specific for cystine and glutamate, is used to increase 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, increase glutamate release into the extracellular space.

See also

References

  1. ↑ "cystine." Encyclopædia Britannica. 2007. Encyclopædia Britannica Online. 27 July 2007 www.britannica.com/eb/article-9028437/cystine
  2. ↑ Gortner, R. A.; W. F. Hoffman, W. F. (1941). "l-Cystine". Org. Synth.; Coll. Vol. 1, p. 194
  3. ↑ M.A. Aslaksena, O.H. Romarheima, T. Storebakkena and A. Skrede (28 June 2006). "Evaluation of content and digestibility of disulfide bonds and free thiols in unextruded and extruded diets containing fish meal and soybean protein sources". Animal Feed Science and Technology 128 (3–4): 320–330. doi:10.1016/j.anifeedsci.2005.11.008.
  4. ↑ Gahl WA, Thoene JG, Schneider JA. "Cystinosis" New England Journal of Medicine 2002, vol. 347. pp. 111-121.
  5. ↑ Department of Pharmaceutical Sciences, University of Kentucky. Journal of Pharmaceutical Science. February 2005. “Kinetics and mechanism of the reaction of cysteine and hydrogen peroxide in aqueous solution.”
  6. ↑ On the cysteine and cystine content of proteins
  7. ↑ On the cysteine and cystine content of proteins