Cysteine is a naturally occurring, sulfur-containing amino acid that is found in most proteins, although only in small quantities. Cysteine is unique amongst the twenty natural amino acids as it contains a thiol group. Thiol groups can undergo oxidation/reduction (redox) reactions; when cysteine is oxidized it can form cystine, which is two cysteine residues joined by a disulfide bond. This reaction is reversible since the reduction of this disulphide bond regenerates two cysteine molecules. The disulphide bonds of cystine are crucial to defining the structures of many proteins. Cysteine is often involved in electron-transfer reactions, and help the enzyme catalyze its reaction. Cysteine is also part of the antioxidant glutathione. N-Acetyl-L-cysteine (NAC) is a form of cysteine where an acetyl group is attached to cysteine's nitrogen atom and is sold as a dietary supplement. Cysteine is named after cystine, which comes from the Greek word kustis meaning bladder (cystine was first isolated from kidney stones). Oxidation of cysteine can produce a disulfide bond with another thiol and further oxidation can produce sulphfinic or sulfonic acids. The cysteine thiol group is also a nucleophile and can undergo addition and substitution reactions. Thiol groups become much more reactive when they are ionized, and cysteine residues in proteins have pKa values close to neutrality, so they are often in their reactive thiolate form in the cell. The thiol group also has a high affinity for heavy metals and proteins containing cysteine will bind metals such as mercury, lead, and cadmium tightly. Due to this ability to undergo redox reactions, cysteine has antioxidant properties. Cysteine is an important source of sulfur in human metabolism, and although it is classified as a non-essential amino acid, cysteine may be essential for infants, the elderly, and individuals with certain metabolic disease or who suffer from malabsorption syndromes. Cysteine may at some point be recognized as an essential or conditionally essential amino acid. Cysteine is important in energy metabolism. As cystine, it is a structural component of many tissues and hormones. Cysteine has clinical uses ranging from baldness to psoriasis to preventing smoker's hack. In some cases, oral cysteine therapy has proved excellent for treatment of asthmatics, enabling them to stop theophylline and other medications. Cysteine also enhances the effect of topically applied silver, tin, and zinc salts in preventing dental cavities. In the future, cysteine may play a role in the treatment of cobalt toxicity, diabetes, psychosis, cancer, and seizures (http://www.dcnutrition.com/AminoAcids/). Cysteine has been identified as a uremic toxin according to the European Uremic Toxin Working Group.