Living reference work entry
Cystine (Fig. 1) is formed by the oxidative condensation of two cysteine molecules. Cysteine, one of the 20 protein amino acids, has a thiol (─SH) group. In aqueous solution, two thiol groups readily oxidize to form a disulfide bond (─S─S─). Thus, cystine rather than cysteine is usually determined when the amino acid composition of protein hydrolysates is analyzed. It can be easily reduced to give two cysteine molecules. In protein molecules, cysteine residues make intramolecular disulfide bonds, which stabilize protein tertiary structure.
KeywordsAqueous Solution Acid Composition Bioorganic Chemistry Disulfide Bond Cysteine Residue
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