Cysteine is an a-amino acid with the chemical formula HO2CCH(NH2)CH2SH. It is not an essential amino acid, which means that humans can synthesize it. Its codons are UGU and UGC. Cysteine is classified as a hydrophilic amino acid with a thiol side chain. The high reactivity of this thiol makes cysteine an important structural and functional component of many proteins and enzymes.