Cysteine

Some important facts about Cysteine
It was first isolated from kidney stones. It is a sulfur-containing amino acid. It is unique amongst the twenty natural amino acids as it contains a thiol group. The proton of the thiol of cysteine is much more acid than the hydroxylic proton of serine, making the nucleophilic thiol(ate) much more reactive than the hydroxyl of serine. Thiol groups can undergo oxidation/reduction (redox) reactions; when cysteine is oxidised it can form cystine, which is two cysteine residues joined by a disulfide bond. The cysteine thiol group is also a nucleophile and can undergo addition and substitution reactions. Cysteine when oxidized to cystine no longer functions as a nucleophile.
Inteins often function with the help of a catalytic cysteine.
Cysteine also plays a key role in stabilizing extracellular proteins by forming disulphides eg.the digestive enzymes of the small intestine.
It is often involved in electron-transfer reactions, and help the enzyme catalyse its reaction. It has antioxidant properties and is also part of the antioxidant glutathione. The sulfhydryl (thiol) group (SH) of cysteine that serves as proton-donor and is responsible for the biological activity of glutathione. N-acetyl-L-cysteine (NAC) is sold as a dietary supplement 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
Cysteine absorbed during digestion as cystine (two cysteine molecules linked by a disulfide bond) in the gastrointestinal tract is more stable than the free amino acid cysteine. Cystine travels safely through the GI tract and blood plasma and is promptly reduced to the two cysteine molecules upon cell entry
Some important cysteine-derived nucleophiles include ubiquitin ligases, which transfer ubiquitin to its pendant proteins, and caspases, which engage in proteolysis in the apoptotic cycle.

Benefits
It counteracts the poisonous effects of acetaldehyde, a particularly toxic by-product of alcohol in the human body. Cysteine attracts the toxin, breaking it down into the non-toxic acetate, a substance similar to vinegar and may be effective in preventing hangovers, as well as preventing liver and brain damage.
The cysteine derivative N-acetyl cysteine (NAC) is often used as a cough medicine as it breaks up the disulfide bonds in the mucus and thus liquefies it. It is used for breaking up the disulfide bonds in the hair's keratin.
Cysteine is a very popular target for site-directed labeling experiments. It is found in beta-keratin, the main protein in nails, skin as well as hair. It not only is important in collagen production but also assists in skin elasticity and texture. It is used in cosmetics as a skin moisturizer.
People suffering from AIDS/HIV may benefit from cysteine in proper amounts, as low levels are normally reported in people with this problem.

Cystine is required for
Proper vitamin B6 utilization and is also helpful in the healing of burns and wounds. It also assists in the supply of insulin to the pancreas, which is needed for the assimilation of sugars and starches.

Metabolism
It can be synthesized by the human body under normal physiological conditions from methionine. Cysteine is also critical to the metabolism of a number of essential biochemicals including coenzyme A, heparin, biotin, lipoid acid, and glutathione.

Dosage
The dosage listed is the Recommended Daily Allowance (RDA), but be aware that this dosage is the minimum that you require per day, to ward off serious deficiency of this particular nutrient. In the therapeutic use of this nutrient, the dosage is usually increased considerably, but the toxicity level must be kept in mind.

Toxicity and symptoms of high intake
People suffering from diabetes should be careful when taking supplementation, as it could inactivate insulin

Source
Found in meat, red peppers, garlic, onions, broccoli, brussel sprouts, oats, milk, whey protein, and wheat germ. A source of bonded cysteine (cystine) is undenatured bovine whey protein