Cysteine

Cysteine is a naturally occurring hydrophilic ("water loving") amino acid which has a sulfhydryl (SH) group and is found in most proteins. When it is exposed to air it oxidizes to form cystine, which is two cysteine molecules joined by a disulfide bond. One molecule of water (H2O) is the byproduct from the creation of each molecule of cystine.

What does cysteine do?

Cysteine is a nonessential amino acid (protein building block), meaning that cysteine can be made in the human body. Cysteine is one of the few amino acids that contains sulfur. This allows cysteine to bond in a special way and maintain the structure of proteins in the body. Cysteine is a component of the antioxidantglutathione. The body also uses cysteine to produce taurine, another amino acid.

Cysteine can also be converted into glucose and used as a source of energy. Cysteine strengthens the protective lining of the stomach and intestines, which may help prevent damage caused by aspirin and similar drugs.1 In addition, cysteine may play an important role in the communication between immune system cells.2 Cysteine is rarely used as a dietary supplement. N-acetyl cysteine (NAC), which contains cysteine, is more commonly used as a supplement.

Where is cysteine found?

The body can synthesize cysteine from methionine and other building blocks. Cysteine, the amino acid from which NAC is derived, is found in most high-protein foods.

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Structure of Amino Acids
Amino Acid Properties

Classes of Amino Acids
Amino Acid Disorders Screening
Functions and Non-functions in proteins