Cysteine is an amino acid and belongs to the class which has neutral R-groups. It is hydrophilic. The thiol side chain in cysteine often participates in enzymatic reactions, as a nucleophile. The thiol is susceptible to oxidation to give the disulfide derivative cystine, which serves an important structural role in many proteins.
Together with methionine, cysteine is one of two sulfur-containing proteinogenic amino acids.
Cysteine is an important source of sulfide in human metabolism. The sulfide in iron-sulfur clusters and in nitrogenase is extracted from cysteine, which is converted to alanine in the process.
Cysteine is a precursor in the food, pharmaceutical, and personal-care industries. One of the largest applications is the production of flavors. For example, the reaction of cysteine with sugars in a Maillard reaction yields meat flavors. Cysteine is also used as a processing aid for baking. In the field of personal care, cysteine is used for permanent-wave applications, predominantly in Asia. The keratin in hair has many disulfide bonds - hair-straigntening with an iron is effective in breaking the disulfide bonds, but is accompanied by a sulfur odor .
Two cysteine molecules can bond to form cystine with a disulfide bond. In protein structures, disulfide bonds between cysteines from separate points in the polypeptide can influence the bending and shape of the protein. Insulin is an example of a protein with such cystine crosslinking.
Tillery, Enger and Ross