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L-Cystein - L-Cysteine.svg
IUPAC name Cysteine
Other names 2-Amino-3-sulfhydrylpropanoic acid
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

Cysteine (abbreviated as Cys or C)[1] is an α-amino acid. It has the chemical formula HO2CCH(NH2)CH2SH.

Cysteine is a semi-essential amino acid, which means that humans can make it.[2] The codons UGU and UGC code for cysteine. The thiol side chain in cysteine often does enzyme reactions as a nucleophile. The thiol oxidises to give the disulfide derivative cystine, which is important in many protein structures. When used as a food additive, cysteine has the E number E920.


  1. Nomenclature and symbolism for amino acids and peptides (IUPAC-IUB Recommendations 1983). Pure Appl. Chem. 56 (5), 1984: 595–624.
  2. "The primary structure of proteins is the amino acid sequence". The Microbial World. University of Wisconsin-Madison Bacteriology Department. Retrieved 16 September 2012.