All proteins in our body are built from amino acids. There are a lot of proteins in the body, and there are only 20 building blocks - the amino acids of which they are composed. Thus, proteins differ from each other in a set of amino acids and their sequence. Cysteine is one of the 20 amino acids.
Cysteine - what is it?
Cysteine is an aliphatic sulfur-containing amino acid. Aliphatic - containing only saturated bonds. Like any amino acid, the formula of cysteine includes a carboxyl (-COOH) and amino group (-NH2), as well as a unique thiol (-SH). The thiol (another name is sulfhydryl) group includes a sulfur atom and a hydrogen atom.
The molecular chemical formula of cysteine is C3H7NO2S. Molecular weight - 121.
Formula of the amino acid cysteine
To depict the structure of amino acids, different formulas are used. Below are several options for writing the structural formula of cysteine.
All amino acids have amino and carboxyl groups attached to the α-carbon atom, and differ only in the structure of the radical attached to the same carbon atom. For example, below are the structural formulas of alanine, cysteine and glycine, serine and cystine.
All amino acids have the same backbone and different radicals. It is the structure of the radical that underlies the qualification of amino acids and determines the properties of the molecule itself. In cysteine, the radical formula is CH2-SH. This radical belongs to the group of polar, uncharged, hydrophilic radicals. This means that sections of the protein containing cysteine can add water (hydrate) and interact with other sections of the protein, also containing amino acids with hydrophilic groups, using hydrogen bonds.
Cysteine contains a unique thiol group
Cysteine is a unique amino acid. It is the only one among the 20 natural amino acids that contains a thiol (-HS) group. Thiol groups can undergo oxidative and reduction reactions. When the thiol group of cysteine is oxidized, cystine is formed - an amino acid that consists of two cysteine residues connected by a disulfide bond. The reaction is reversible - the restoration of the disulfide bond regenerates two cysteine molecules. Cystine disulfide bonds are critical to determining the structures of many proteins.
Oxidation of the thiol group of cysteine leads to the formation of a disulfide bond with anotherthiol, during further oxidation, sulfinic and sulfonic acids are formed.
Due to its ability to enter into redox reactions, cysteine has antioxidant properties.
Cysteine is a component of proteins
Amino acids that make up proteins are called proteinogenic. As already mentioned, there are 20 of them, and cysteine is one of them. To form the primary structure of a protein, amino acids are joined together in a long chain. The connection occurs due to the groups of the skeleton of amino acids, radicals do not participate in this. The bond between amino acids is formed by the carboxyl group of one amino acid and the amino group of another amino acid. A bond formed in this way between two amino acids is called a peptide bond.
The figure shows the formula of the tripeptide alanine cysteine phenylalanine and the scheme of its formation.
The smallest peptide in the body is glutathione, which consists of only two amino acids, including cysteine. Two amino acids linked together are called a dipeptide, three are called a tripeptide. Here is another formula of a tripeptide of alanine, lysine and cysteine.
Substances containing from 10 to 40 amino acids are called polypeptides. Proteins themselves contain more than 40 amino acid residues. Cysteine is a component of many peptides and proteins, such as insulin.
Sources of cysteine
Every day a person should consume 4.1 mg of cysteine per 1 kg of body weight. That is, in the human bodyweighing 70 kg should receive 287 mg of this amino acid per day.
Part of cysteine can be synthesized in the body, part comes from food. The following is a list of foods that contain the maximum amount of the amino acid.
| Cysteine content in products | |
| Product | Cysteine content per 100 g of product, mg |
| Soy products | 638 |
| Beef and lamb | 460 |
| Seeds (sunflower, watermelon, sesame, flax, pumpkin) and nuts (pistachio, pine) | 451 |
| Chicken meat | 423 |
| Oats and oat bran | 408 |
| Pork | 388 |
| Fish (tuna, salmon, perch, mackerel, halibut) and shellfish (mussels, shrimp) | 335 |
| Cheese, dairy and eggs | 292 |
| Beans (chickpeas, beans, beans, lentils) | 127 |
| Cereals (buckwheat, barley, rice) | 120 |
In addition, cysteine is found in red peppers, garlic, onions, dark leafy vegetables - Brussels sprouts, broccoli.
Produce food supplements, such as L-cysteine hydrochloride, N-acetylcysteine. The second is more soluble and is easier for the body to absorb.
In industry, L-cysteine is obtained by hydrolysis from bird feathers, bristles and human hair. A more expensive synthetic L-cysteine is produced, suitable for Muslim and Jewish food regulations (in accordance withreligious aspects).
Cysteine synthesis in the body
Cysteine, along with tyrosine, is a conditionally essential amino acid. This means that they can be synthesized in the body, but only from essential amino acids: cysteine from methionine, tyrosine from phenylalanine.
For the synthesis of cysteine, two amino acids are needed - the essential methionine and the nonessential serine. Methionine is a sulfur atom donor. Cysteine is synthesized from homocysteine in two reactions catalyzed by pyridoxal phosphate. Genetic disorders, as well as a lack of vitamins B9 (folic acid), B6 and B12 lead to disrupt the use of the enzyme, homocysteine is converted not to cysteine, but to homocystin. This substance accumulates in the body, causing a disease accompanied by cataracts, osteoporosis, mental retardation.
Synthesis in the body may be deficient in the elderly and infants, persons with certain metabolic diseases, suffering from malabsorption syndrome.
Cysteine synthesis reactions
In the animal body, cysteine is synthesized directly from serine, and methionine is the source of sulfur. Methionine is converted to homocysteine via the intermediates S-AM and S-AG. S-adenosylmethionine - the active form of methionine, is formed by the combination of ATP and methionine. Acts as a donor of the methyl group in the synthesis of various compounds: cysteine, adrenaline, acetylcholine, lecithin, carnitine.
As a result of transmethylation, S-AM is converted to S-adenosylhomocysteine (S-AH). Last during hydrolysisforms adenosine and homocysteine. Homocysteine combines with serine with the participation of the enzyme cystathionine-β-synthase with the formation of thioether cystathionine. Cystathionine is converted to cysteine and α-ketobutyrate by the enzyme cystathionine γ-lyase.
In plants and bacteria, synthesis occurs differently. Various substances, even hydrogen sulfide, can serve as a source of sulfur for the synthesis of cysteine.
The biological role of cysteine
Due to the thiol group (-HS) in the formula of cysteine, disulfide bonds are formed in proteins, called disulfide bridges. Disulfide bonds are covalent, strong. They are formed between two cysteine molecules in the protein. Intrachain bridges can form within a single polypeptide chain, and interchain bridges between individual protein chains. For example, both types of bridges take place in the structure of insulin. These bonds maintain the tertiary and quaternary structure of the protein.
Disulfide bonds contain mostly extracellular proteins. For example, this type of connection is of great importance in stabilizing the structure of insulin, immunoglobulins and digestive enzymes. Proteins containing many disulfide bridges are more resistant to heat denaturation, allowing them to maintain their activity under more extreme conditions.
Features of the cysteine formula provide it with antioxidant properties. Cysteine plays the role of an antioxidant, entering into oxidation-reduction reactions. The thiol group has a high affinity for heavy metals, thereforeproteins containing cysteine bind metals such as mercury, lead and cadmium. The pK of cysteine in the protein is such that it ensures that the amino acid is in the reactive thiolate form, that is, cysteine easily donates the HS anion.
Cysteine is an important source of sulfur in metabolism.
Functions of cysteine
Due to the presence of a thiol group that easily reacts, cysteine is involved in various processes in the body and performs many functions.
- Possesses antioxidant properties.
- Participates in the synthesis of glutathione.
- Participates in the synthesis of taurine, biotin, coenzyme A, heparin.
- Participates in the formation of lymphocytes.
- It is part of β-keratin, which is involved in the formation of tissues of the skin, hair, mucous membrane of the digestive system.
- Promotes the neutralization of some toxic substances.
Use of cysteine
Cysteine has found wide application in the medical, pharmaceutical, food industries.
Cysteine is often used in the treatment of various diseases:
- For bronchitis and emphysema as it thins mucus.
- For rheumatoid arthritis, vein disease and cancer.
- For heavy metal poisoning.
In addition, cysteine accelerates recovery after operations and burns, activates leukocytes.
Cysteine accelerates fat burning and muscle building, so it is often used by athletes.
Amino acid is used as a flavoring agent. Cysteine is a registered food additive E920.
