Cysteine is a sulfur-containing amino acid and a fundamental building block for proteins. As a semi-essential amino acid, it plays various roles in the body. The body produces cysteine, and it is also obtained from diet, both contributing to overall health.
The Body’s Cysteine Production
The body synthesizes cysteine primarily through the transsulfuration pathway, mainly in the liver. This pathway starts with methionine, an essential amino acid from the diet, and serine, a non-essential amino acid the body produces. Methionine converts to homocysteine, which provides cysteine’s sulfur atom.
Synthesis occurs in two enzymatic steps. First, cystathionine beta-synthase (CBS) combines homocysteine with serine to form cystathionine. Then, cystathionine gamma-lyase (CGL) breaks down cystathionine, yielding cysteine and alpha-ketobutyrate.
The Many Roles of Cysteine
Cysteine’s sulfur-containing side chain enables its participation in biological processes. It is a building block for proteins, especially in forming disulfide bonds. These bonds, created by the oxidation of two cysteine residues, are important for protein structure and stability, as seen in keratin found in skin, hair, and nails.
Cysteine is also a precursor for glutathione, a powerful antioxidant that protects cells from damage by reactive oxygen species, byproducts of metabolism and environmental factors. Cysteine contributes to detoxification processes and provides sulfide for iron-sulfur clusters, components of many metabolic enzymes. It is also a precursor for other sulfur-containing molecules like taurine and coenzyme A.
Dietary Cysteine and Supplementation
Cysteine is a conditionally essential amino acid; the body can synthesize it, but dietary intake is beneficial, especially if precursor amino acids like methionine are limited. Foods rich in cysteine or its precursors include high-protein sources like poultry, beef, eggs, and dairy products. Legumes, nuts, garlic, onions, and cruciferous vegetables like broccoli also contain cysteine.
N-acetylcysteine (NAC) is a common supplement form of cysteine. NAC is used for its antioxidant and anti-inflammatory properties, as it helps produce glutathione. Though found in some foods, it is most commonly consumed as a supplement. Consulting a healthcare professional is advisable before supplementing, as high doses can lead to imbalances.
When Cysteine Levels Are Imbalanced
Imbalances in cysteine synthesis or metabolism can lead to various health issues. One notable condition is homocystinuria, caused by a genetic defect in the cystathionine beta-synthase (CBS) enzyme. This enzyme deficiency leads to accumulation of homocysteine and methionine in the blood and urine, reducing cysteine levels.
Homocystinuria can lead to complications affecting the eyes, bones, brain, and cardiovascular system, including blood clots. Another condition affecting cysteine levels is cystinuria, a rare inherited disorder where kidneys have difficulty reabsorbing cystine, formed from two cysteine units. This causes cystine to build up in urine, forming kidney stones that can cause pain and kidney damage. Maintaining proper cysteine balance is important for overall health.