Cysteine

Cysteine is a semiessential <a href="/facts/Proteinogenic_amino_acid/xoXITvm0">proteinogenic amino acid</a> with the <a href="/facts/Chemical_formula/hghUtthl">formula</a> HS−CH2−CH(NH2)−COOH. The <a href="/facts/Thiol/VemKEDP9">thiol</a> side chain in cysteine enables the formation of <a href="/facts/Disulfide/eDx5Xtlg">disulfide bonds</a>, and often participates in <a href="/facts/Enzymatic/DSI1Fh7y">enzymatic</a> reactions as a <a href="/facts/Nucleophile/xYccoaJQ">nucleophile</a>. Cysteine is chiral, but both D and L-cysteine are found in nature. L‑Cysteine is a protein monomer in all biota, and D-cysteine acts as a signaling molecule in mammalian nervous systems. Cysteine is named after its discovery in urine, which comes from the urinary bladder or cyst, from <a href="/facts/Ancient_Greek/LIK7NlUh">Greek</a> κύστις kýstis, "bladder". 
The thiol is susceptible to oxidation to give the <a href="/facts/Disulfide_bond/eDx5Xtlg">disulfide</a> derivative <a href="/facts/Cystine/MmwEHkkI">cystine</a>, which serves an important structural role in many <a href="/facts/Protein/Jxmagu3E">proteins</a>. In this case, the symbol Cyx is sometimes used. The deprotonated form can generally be described by the symbol Cym as well.
When used as a food additive, cysteine has the <a href="/facts/E_number/n3VSY29I">E number</a> E920.
Cysteine is <a href="/facts/Genetic_code/zUK0bY1B">encoded</a> by the <a href="/facts/Codon/zUK0bY1B">codons</a> UGU and UGC.

Cysteine open-in-new

Cysteine