Cysteine is only incorporated into proteins at the rate of 2.8 percent relative to the other amino acids, but the unique thiol side chain of this amino acid is often heavily involved in the three-dimensional stability of proteins and enzymes. The side chain is also often involved in the chemistry occurring at the active sites of many enzymes. Cysteine is also critical to the metabolism of a number of essential biochemicals including coenzyme A, heparin, biotin, lipoic acid, and glutathione.