Cystine

Cystine is the disulfide-linked dimer formed when two cysteine molecules are oxidized. Its covalent sulfur–sulfur bond can be reduced to regenerate cysteine.

Cysteine: Cysteine is a sulfur-containing amino acid with a thiol group in its side chain. Oxidation joins two cysteine thiols to form cystine.
Protein folding: The process by which a protein adopts its functional three-dimensional structure. Cystine bridges between protein cysteines can stabilize folded structures.
Amino acid: An organic molecule with amino and carboxyl groups; amino acids are the building blocks of proteins. Cystine is derived from two amino-acid molecules rather than being a protein residue itself.
Cysteine sulfinic acid: An oxidized cysteine derivative whose sulfur bears an additional oxygen compared with cysteine. It is an oxidation product of cysteine, but it is not the cysteine-cystine disulfide dimer.
Disulfide bond: A covalent bond between two sulfur atoms, commonly formed by oxidation of thiol groups. Cystine is the small-molecule form of a disulfide-linked pair.
Keratin: A family of structural proteins abundant in hair, nails, and the outer layer of skin. Cystine cross-links help give keratin-rich materials mechanical strength.
Thiol: A functional group containing sulfur bonded to hydrogen, written –SH. Each cysteine molecule contributes a thiol that can be oxidized into cystine’s disulfide bond.
Cysteine sulfonic acid: A strongly oxidized cysteine derivative with sulfur bonded to three oxygen atoms. Further oxidation changes cysteine’s sulfur beyond the disulfide state found in cystine.
Redox reaction: A chemical reaction involving electron transfer and changes in oxidation states. Cystine formation oxidizes cysteine, while bond reduction restores it.
Cystinuria: An inherited disorder in which impaired kidney and intestinal transport causes excess cystine in urine. Poor cystine reabsorption can lead to recurrent urinary stones.

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NephrolithiasisRelated: Inherited failure to reabsorb cystine can cause cystine stones.