Cysteine is a naturally occurring hydrophilic ("water loving") amino acid which has a sulfhydryl (SH) group and is found in most proteins. When it is exposed to air it oxidizes to form cystine, which is two cysteine molecules joined by a disulfide bond. One molecule of water (H2O) is the byproduct from the creation of each molecule of cystine.

Symbol: C or Cys
Side chain type: hydrophilic
Mass: 121.16
Isoelectric point: 5.05

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