All the proteins in our body are built from amino acids. There are a lot of proteins in the body, and the bricks - the amino acids of which they are composed, are only 20. Thus, proteins differ from each other in a set of amino acids and their sequence. Cysteine is one of these 20 amino acids.
Cysteine - what is it?
Cysteine is an aliphatic sulfur-containing amino acid. Aliphatic - containing only saturated bonds. As with any amino acid, the cysteine formula includes a carboxylic acid (-COOH) and an amino group (-NH 2 ), as well as a unique thiol (-SH). The thiol group (another name is sulfhydryl) includes a sulfur atom and a hydrogen atom.
The molecular chemical formula of cysteine is C 3 H 7 NO 2 S. The molecular weight is 121.
Cysteine Amino Acid Formula
Different formulas are used to depict the structure of amino acids . Below are a few 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, the structural formulas of alanine, cysteine and glycine, serine and cystine are presented below.
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 formula of the radical is CH 2 —SH. This radical belongs to the group of polar uncharged, hydrophilic. This means that sections of the protein containing cysteine can attach water (hydrate) and interact via hydrogen bonds with other sections of the protein that also contain amino acids with hydrophilic groups.
Cysteine contains a unique thiol group
Cysteine is a unique amino acid. It is the only one among 20 natural amino acids that contains a thiol (-HS) group. Thiol groups can undergo oxidative and reduction reactions. During the oxidation of the thiol group of cysteine, cystine is formed - an amino acid that is 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 for determining the structures of many proteins.
Oxidation of the thiol group of cysteine leads to the formation of a disulfide bond with another thiol; in the course of further oxidation, sulfinic and sulfonic acids are formed.
Due to the ability to enter into redox reactions, cysteine has antioxidant properties.
Cysteine - 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. For the formation of the primary structure of the protein, amino acids are linked together in a long chain. The connection is due to the groups of the backbone of amino acids, radicals are not involved in this. The bond between amino acids is formed due to the carboxyl group of one amino acid and the amino group of another amino acid. The bond formed in this way between two amino acids is called a peptide.
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 contains only two amino acids, including cysteine. Two amino acids, interconnected, are called a dipeptide, three - a tripeptide. We also give the formula of a tripeptide from alanine, lysine and cysteine.
Substances containing from 10 to 40 amino acids are called polypeptides. The composition of the proteins themselves includes more than 40 amino acid residues. Cysteine is part of many peptides and proteins, for example, insulin.
Sources of Cysteine
Every day, a person should consume 4.1 mg of cysteine per 1 kg of weight. That is, 287 mg of this amino acid per day should enter the human body with a mass of 70 kg.
Part of cysteine can be synthesized in the body, part comes with food. The following is a list of foods containing the maximum amount of amino acid.
| Cysteine Content in Products |
| Product | The cysteine content in 100 g of the product, mg |
| Soya products | 638 |
| Beef and lamb | 460 |
| Seeds (sunflower, watermelon, sesame, flax, pumpkin) and nuts (pistachios, cedar) | 451 |
| Chicken meat | 423 |
| Oats and oat bran | 408 |
| Pork | 388 |
| Fish (tuna, salmon, perch, mackerel, halibut) and shellfish (mussels, shrimps) | 335 |
| Cheese, Dairy and Eggs | 292 |
| Legumes (chickpeas, beans, beans, lentils) | 127 |
| Groats (buckwheat, barley, rice) | 120 |
In addition, cysteine is found in red pepper, garlic, onions, in dark leafy vegetables - Brussels sprouts, broccoli.
Food additives are produced, for example, L-cysteine hydrochloride, N-acetylcysteine. The second has greater solubility and is more easily absorbed by the body.
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 rules (in accordance with religious aspects).
The synthesis of cysteine in the body
Cysteine, along with tyrosine, refers to conditionally replaceable amino acids. This means that they can be synthesized in the body, but only from essential amino acids: cysteine from methionine, tyrosine from phenylalanine.
Two amino acids are required for the synthesis of cysteine - the essential methionine and the replaceable serine. Methionine is a sulfur atom donor. Cysteine is synthesized from homocysteine in two reactions catalyzed by pyridoxalphosphate. Genetic disorders, as well as a lack of vitamins B 9 (folic acid), B 6 and B 12 lead to impaired use of the enzyme, homocysteine is converted not to cysteine, but to homocystine. This substance accumulates in the body, causing a disease accompanied by cataracts, osteoporosis, and mental retardation.
Synthesis in the body may be insufficient in the elderly and infants, persons with some metabolic diseases, suffering from malabsorption syndrome.
Cysteine synthesis reactions
In the animal’s body, cysteine is synthesized directly from serine, and the source of sulfur is methionine. Methionine is converted to homocysteine through the intermediates S-AM and S-AG. S-adenosylmethionine is an active form of methionine, formed by the combination of ATP and methionine. He acts as a donor of the methyl group in the synthesis reactions of various compounds: cysteine, adrenaline, acetylcholine, lecithin, carnitine.
As a result of transmethylation, S-AM is converted to S-adenosyl homocysteine (S-AG). The latter forms adenosine and homocysteine during hydrolysis. 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 action of the enzyme cystathionine-γ-lyase.
In plants and bacteria, synthesis occurs differently. Various substances can act as a source of sulfur for the synthesis of cysteine, even hydrogen sulfide.
The biological role of cysteine
Due to the thiol group (-HS) in the cysteine formula, disulfide bonds called disulfide bridges are formed in the proteins. Disulfide bonds are covalent, strong. They are formed between two cysteine molecules in a protein. Intrachain bridges may form within the same polypeptide chain, and interchain bridges between individual protein chains. For example, both types of bridges occur in the structure of insulin. These bonds provide maintenance of the tertiary and quaternary structure of the protein.
Disulfide bonds contain mainly 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 thermal denaturation, which allows 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, so proteins containing cysteine bind metals such as mercury, lead and cadmium. The composition of the pK protein of cysteine is such that it ensures that the amino acid is in reactive thiolate form, i.e., cysteine readily gives off the HS anion.
Cysteine is an important source of sulfur in metabolism.
Cysteine function
Due to the presence of a thiol group that easily reacts, cysteine is involved in various processes in the body and performs many functions.
- It has 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 skin tissues, hair, and the digestive system mucosa.
- Helps neutralize certain toxic substances.
Cysteine use
Cysteine is widely used in the medical, pharmaceutical, food industry.
Cysteine is often used in the treatment of various diseases:
- With bronchitis and emphysema, as it dilutes the mucus.
- With rheumatoid arthritis, diseases of the veins and cancer.
- In case of heavy metal poisoning.
In addition, cysteine accelerates recovery after operations and burns, activates white blood cells.
Cysteine accelerates fat burning and building muscle tissue, which is why athletes often use it.
Amino acid is used as a flavoring. Cysteine is a registered food supplement E920.