Cysteine, along with methionine, is a sulfur-containing amino acid. It is jointly responsible for the formation of the secondary, tertiary and quaternary structure of proteins. A deficiency of cysteine leads, among other things, to a weakness of the immune system.
What is cysteine?
Cysteine is a sulfur-containing amino acid with a thiol group (-SH). It can form disulfide bridges with another cysteine within a protein chain or between protein chains. In this process, the secondary, tertiary and quaternary structures are formed by folding of the chains. Only the L-cysteine is incorporated into the proteins. Racemates and D-cysteine play no role biologically. Cysteine is a non-essential amino acid. It can be synthesized in the body from methionine. However, since methionine is an essential amino acid and must always be supplied from the outside, cysteine is referred to as a semi-essential amino acid. Cysteine is not contained in all proteins, but is found particularly in structural proteins of the connective tissue, in the keratin of the hair, in the nails and cornea, in insulin and in some enzymes. The structural proteins are formed by the disulfide bridges. In the cysteine-containing enzymes, the thiol group plays an important catalytic role. Free cysteine is very reactive in aerobic environment and is irreversibly degraded. Therefore, it is stored in the form of the tripeptide glutathione for constant supply to the body.
Function, effect, and tasks
Cysteine performs a variety of functions in the organism. These functions are derived from the reactive thiol group. Thus, disulfide bridges form within protein chains, which help determine the secondary and tertiary structure of the protein. Disulfide bridges between cysteine building blocks of different protein chains form the quaternary structure. This results in stable proteins that can also crosslink with each other. However, cysteine is also present in many enzymes. There, cysteine residues form so-called iron–sulfur clusters with iron. Here, the thiol groups can catalytically support important biochemical syntheses. Coenzyme A also contains the amino acid cysteine in addition to ADP and vitamin B5. Here, too, important reactions of energy metabolism are catalyzed by the thiol group. Furthermore, cysteine is also the starting material for the formation of taurine. Taurine is an aminoethanesulfonic acid and fulfills a whole range of functions. It affects signaling in the nervous system and cardiac function by supporting ion transport of sodium, potassium and calcium across the membrane. In addition, taurine is a powerful antioxidant, which protects tissues from severe oxidative damage. A deficiency of taurine can lead to kidney damage and immune system dysfunction. Finally, taurine also has an anti-inflammatory effect. Cysteine is also the starting material for the tripeptide glutathione. Almost all cells contain glutathione in high concentrations. It acts as a storage form for cysteine, because free cysteine is irreversibly degraded very quickly and is therefore no longer available to the body. Furthermore, glutathione is a powerful antioxidant and indispensable for the body. It also plays a major role in the biotransformation of drugs, toxins and foreign substances in the liver. Impaired glutathione production leads to death within a short period of time.
Formation, occurrence, properties, and optimal levels
Cysteine is not an essential amino acid. However, it is produced in the body from the essential amino acid methionine. Methionine must be supplied with food. Therefore, cysteine is also called a semi-essential amino acid. If there is no cysteine in the diet, the requirement of methionine for an adult is 13 to 16 milligrams per kilogram of body mass. If there is an excess of cysteine in the diet, the figures for the requirement of methionine vary between 5 and 13 milligrams per kilogram of body mass. High levels of cysteine are found in meat, fish, milk, nuts, whole-grain flours of wheat and corn, unhusked rice, soybeans or peas. A daily dose of 0.5 to 1.5 grams of L-cysteine is recommended. However, the need for cysteine and methionine increases in chronic diseases such as cancer, Parkinson’s disease or osteoarthritis because of the increased production of free radicals. The metabolization of drugs also takes place more quickly at higher concentrations.
Diseases and disorders
A deficiency of cysteine can lead to a weakening of the immune system. In addition, detoxification mechanisms in the liver no longer function optimally.These effects are mediated via the storage form of cysteine, glutathione. Normally, the body is sufficiently supplied with cysteine. Thus, a balanced diet contains sufficient cysteine and methionine. Deficiency symptoms can occur with an unbalanced diet or an extreme reduction diet. However, there are also some diseases in which there is an increased need for cysteine. In these diseases, the formation of free radicals increases on the one hand, and on the other hand the formation of toxins that have to be broken down in the liver. These diseases include respiratory diseases, liver damage, rheumatoid arthritis, alcoholism, chronic degenerative diseases, Parkinson’s disease, arteriosclerosis and poisoning by drugs, heavy metals, cigarette smoke and other toxins. Additional administration of L-cysteine can strengthen the immune system. It has also been shown to be effective in osteoporosis. In cases of heavy metal poisoning, the reactive thiol groups of cysteine and glutathione form complexes with the heavy metal ions, which are then water-soluble and can be quickly eliminated from the body. In addition to a cysteine deficiency, a cysteine excess can also occur. Cysteine excess can be caused by overdosage as well as by genetically caused delayed cysteine degradation. There is increased excretion of cysteine in the urine (cysteinuria). In approximately 50 percent of patients, kidney stones or urinary calculi form from precipitated cysteine.
Here’s what you can do yourself
Flushing out cystine stones is best accomplished by drinking plenty of tap water. The family doctor will also prescribe various urinary and kidney teas or light medicines that promote urination and thus effectively help to remove the painful stones. This simple measure is supported by a healthy diet and exercise. Regular exercise increases the chances that the stone will loosen and drain through the urethra. Endurance sports such as running, swimming or walking as well as various physiotherapy exercises are particularly effective. Massages and sauna sessions are also recommended. Both promote circulation and thus help to break down cystine stones. Various home remedies such as cranberry juice, warm beer or applications with aniseed also have a beneficial effect on urinary and kidney stones. If the stone has not been flushed out after three to four days at the latest, the symptoms should be taken to a doctor. Possibly the cystine stone is too large and requires surgical removal or there is a chronic disease. In any case with urinary stones, a medical professional should be consulted to prevent complications.
