Cysteine (; symbol Cys or C) is a semiessential proteinogenic amino acid with the formula HS−CH2−CH(NH2)−COOH. The thiol side chain in cysteine enables the formation of disulfide bonds, and often participates in enzymatic reactions as a nucleophile. Cysteine is chiral, but both D and L-cysteine are found in nature. L‑Cysteine is a protein...
