International Scientific Vocabulary, from cystine + -ein
First Known Use: 1884
Medical Definition of CYSTEINE
: a sulfur-containing amino acid C3H7NO2S occurring in many proteins and glutathione and readily oxidizable to cystine—abbreviation Cys
Sulfur-containing nonessential amino acid. In peptides and proteins, the sulfur atoms of two cysteine molecules are bonded to each other to make cystine, another amino acid. The bonded sulfur atoms form a disulfide bridge, a principal factor in the shape and function of skeletal and connective tissue proteins and in the great stability of structural proteins such as keratin.