cystine

Cystine
Identifiers
CAS number	[56-89-3]
Properties
Molecular formula	C6H12N2O4S2
Molar mass	240.3 g mol−1
Hazards
MSDS	External MSDS
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Cystine is the amino acid dimer formed when a pair of cysteine molecules are joined by a disulfide bond. It is described by the formula (SCH₂CH(NH₂)CO₂H)₂. It is a colorless solid, and melts at 247-249 °C. It was discovered in 1810 by William Hyde Wollaston but was not recognized as a component of proteins until it was isolated from the horn of a cow in 1899.^[1] Through formation of disulfide bonds within and between protein molecules, cystine is a significant determinant of the tertiary structure of most proteins. Disulfide bonding, along with hydrogen bonding and hydrophobic interactions is partially responsible for the formation of the gluten matrix in bread. Human hair contains approximately 5% cystine by mass.^[2]

Properties

The disulfide link is readily reduced to give the corresponding thiol, cysteine. This reaction is typically effected with thiols such as mercaptoethanol or dithiothreitol.

(SCH₂CH(NH₂)CO₂H)₂ + 2 RSH → 2 HSCH₂CH(NH₂)CO₂H + RSSR

Nutritional sources

Supplemental N-acetyl cysteine is claimed to be a source of cystine, but the dose of this supplement is limited by side effects. One of the richest nutritional sources of cystine in the diet is undenatured whey proteins from milk.^{[citation needed]} The disulfide-bonded cystine is not digested or significantly hydrolyzed by the stomach, but is transported by the blood stream to the tissues of the body.^{[citation needed]} Here, within the cells of the body, the weak disulfide bond is cleaved to give cysteine, from which glutathione can be synthesized.^{[citation needed]}

In animal feed

Disulfide bonds can be broken at temperatures above about 150 °C, especially at low moisture levels (below about 20%)^[3].

Side effects

Nutritional sources of cystine are virtually free of the toxic side effects associated with the single molecule of cysteine, N-acetyl cysteine. The greatest dietary source of cystine is bio-active, unpasteurized or low-heat pasteurized undenatured whey proteins.^{[citation needed]}

See also

• Cystinuria
• Cysteine

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: 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.