Elastin is a structural protein involved in building the connective tissue of the lungs, blood vessels, and skin. It is very elastic, unlike collagen, which is also found in connective tissue. Elastin molecules cross-link with each other in the extracellular space.
What is elastin?
All vertebrates contain the fibrous protein elastin. It is a structural protein responsible for shaping such important organs as the lungs, blood vessels, and skin. Together with collagen, it forms the connective tissue of these organs. The properties of elastin and collagen complement each other. Thus, elastin, as its name suggests, is very elastic, unlike collagen. This makes the connective tissue of the skin, lungs and blood vessels stretchable and deformable. The functions of these three organs require constant size adjustment. Elastin is mainly composed of the amino acids alanine, glycine, proline, valine, lysine, leucine and isoleucine. Within the molecule, hydrophobic and hydrophilic domains alternate. In each hydrophobic domain, characteristic units of the four amino acids alanine, proline, glycine and valine are repeated. The hydrophilic domains have mainly lysine. The lysine residue is oxidized to allysine by the enzyme lysyl oxidase. This results in the replacement of the terminal amino group by a carboxyl group. The lysine residues of the different protein chains combine with each other to form a ring-shaped desmosine, thereby crosslinking the different chains together.
Function, action, and tasks
Elastin’s function as a structural protein within connective tissue is to provide shape and elasticity for the lungs, blood vessels, and skin. All three organs depend on the flexibility of connective tissue. They are subject to constant volume change. The connective tissue has collagen as its structural protein for the most part. It is tear-resistant, but would be too rigid as a sole structural element. It is only by combining the properties of elastin and collagen that connective tissue becomes both elastic and tear-resistant. The basic building block of elastin is tropoelastin. Tropoelastin is composed of alternating hydrophobic and hydrophilic domains. It has an approximate molecular mass of 72 kilodaltons. The tropoelastin units cross-link with each other at the lysine residues. While tropoelastin is water soluble due to its many hydrophilic domains, the water solubility of the cross-linked polymer is abolished. Tropoelastin is formed inside the cells and reaches the extracellular region via membrane transport. There, crosslinking of the basic building blocks then occurs, with ring-shaped desmosine units forming at the crosslinking sites. Three allysine residues and one lysine residue always participate in desmosine formation. Since allysine is an oxidation product of lysine, four lysine residues are ultimately linked together. This form of linkage gives elastin its special elasticity. The cross-linking also protects elastin from denaturation and degradation by almost all proteases. However, the enzyme elastase is an exception. It is the only protease capable of degrading elastin. Thus, it also succeeds in degrading elastins ingested through food.
Formation, occurrence, properties, and optimal values
As mentioned above, elastin is a necessary component of the connective tissue of the lungs, blood vessels, and skin. This applies to all vertebrates. The basic building block tropoelastin can hardly be detected in animal tissues. After the conversion of lysine residues to allysine by lysyl oxidase, there is immediate cross-linking of three allysine residues with one lysine residue. Elastin occurs almost exclusively in its cross-linked form. Nevertheless, the detection of tropoelastin in animal experiments has been achieved by inhibiting the synthesis of lysyl oxidase. If this enzyme is absent, the conversion of lysine to allysine and thus elastin formation does not occur. The resistance of elastin to degradation by proteases provides ideal protection for the skin, lungs and blood vessels. The degradative action of elastase is limited by elastase inhibitors.
Diseases and disorders
Mutations in the ELN gene can cause hereditary diseases in which the structure of elastin is altered. In a condition called dermatochalasis, connective tissue changes occur, resulting in inelastic, sagging skin that droops in wrinkles. The disease can be acquired as well as hereditary. Familial accumulations are observed.Among many other symptoms, this connective tissue weakness also occurs in Williams-Beuren syndrome. This is also a hereditary structural abnormality of elastin. The cause of this disease is a mutation on chromosome 7. Furthermore, there is also congenital aortic stenosis, which is based on a disorder of elastin structure. In this case, the main artery of the heart is narrowed. Blood can only flow from the left ventricle into the bloodstream with delay. In the long term, this leads to heart failure. Five to six percent of all congenital heart defects are congenital aortic stenoses. Some forms of Ehlers-Danlos syndrome are also thought to be caused by elastin malformation. This condition is characterized by overstretchable skin, and is referred to as rubber skin. The weakness of the connective tissue affects many organs, including the heart and the digestive tract. The syndrome is usually inherited in an autosomal dominant manner. In the so-called Menkes syndrome, on the other hand, among many other symptoms, there is also a weakness of the connective tissue, the cause of which is to be found in a disturbed elastin synthesis. Menkes syndrome is actually characterized by a disturbance of copper absorption in the body. However, copper is a cofactor for many enzymes. Among them is lysyl oxidase. Without copper, the enzyme is ineffective. The conversion of lysine residues into allysine no longer takes place. As a result, the cross-linking of the lysine residues to desmosine can also no longer function.
