What are the amino acids of collagen?
INTRODUCTION
A protein is defined as collagen if it contains the triple collagen helix predominantly in its molecular structure forming an extracellular aggregate with a predominantly structural function.
Collagen is an insoluble fibrous protein that is characterized by containing large amounts of a regular structure forming a cylinder of great length. Collagen is found in all tissues in which it serves as a support frame. Its importance corresponds to its high percentage: for example, it means 4% of the liver, 10% of the lungs, 50% of the cartilage and 70% of the skin.
Collagen is composed of three chains that form a triple helix ( * ) Each chain has about 1400 amino acids of which one in three is a glycine. At other regular intervals, there are other amino acids, proline and hydroxyproline, which are rare in other proteins. The presence of these particular amino acids allows the three chains to coil around each other forming a very resistant fiber. In addition, hydrogen bonds are established between the chains, which give the collagen great stability.
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At least 12 types of collagen are known, numbered I to XII ( * ). The simplest, type I contains a long strand of triple helix that ends in the so-called telopeptides (each of which ends in a -COOH or -NH2 terminal), which are small segments that no longer have superhelical structure. Type I collagen molecules are associated side by side with a reaction catalyzed by a specific enzyme, the lysyl oxidase which binds the hydroxyproline of one chain with a lysine residue of another chain. In this way, long fibers are formed. In other types of collagen, the chains end up with more or less globular structures. For example, the basement membrane that supports the skin is a type IV collagen ( * ) that has a globular end or head and an extra tail. In the basal membrane, the heads are joined to one another while the tails are associated four to four forming complexes in the form of X. In this way a reticulum is formed in which other molecules (in this case laminin and other proteoglycans) ) intersect forming a dense sheet ( * ). Type I collagen is encoded by the COL1A1 and COL1A2 genes .
Type II collagen ( * ), abundant in the hyaline cartilage, in the vitreous of the eye and in the nucleus pulposus of the intervertebral discs, is made up of much thicker fibers.
Collagen type III is encoded by COL1A3 . It is found in almost all tissues in which type I appears as exceptions, bones, tendons and cornea.
Type VI collagen ( * ) is found in many tissues, including the aorta, tendons and skin. It is produced by fibroblasts.
The synthesis of collagen begins in the cytoplasm, forming isolated chains that are taken to the endoplasmic reticulum where the lysine and proline residues are hydroxylated, by means of enzymes that require Fe + 3 and vitamin C as cofactors ( * ). The hydroxylation of proline makes the protein thermostable, while the hydroxylation of lysine will allow the crosslinking of several triple helices. At this point, the glycosyl transferases of the endoplasmic reticulum glycosylate some hydroxylysine residues. The triple helix is then assembled with the ends remaining as free polypeptides, which can be folded to form the globular structures. The triple helices are transported to the Golgi apparatus where they are modified by sulphation, some serines being phosphorylated. The resulting resulting procollagen is excreted from the cell through secretory vesicles.
The conversion of procollagen to collagen takes place extracellularly. The terminal telopeptides are hydrolyzed by specific proteases and the triple helices are assembled into fibrils, at which time other connective tissue proteins such as laminin can participate. Some of the hydroxylysine residues are converted to reactive aldehydes by lysyl oxidase, aldehydes that react with other lysine or hydroxylysine residues to form the crosslinks.
COLLAGEN DISEASES
Some details are known of the defects that can occur in the synthesis of collagen and associated diseases:
- Ehlers-Danlos syndrome: this name is given by a set of at least 10 disorders that are clinically, genetically and biochemically different, but all of them are manifested by a structural weakness of the connective tissue.
- Osteogenesis imperfecta: at least 4 diseases with this name characterized by multiple fractures and deformities are known
- Scurvy: results from the deficiency of vitamin C in the diet, which can not form hydroxyproline because the enzyme lysyl oxidase needs this vitamin as a cofactor. Collagen in patients with scrobute is less stable than normal, which explains many of the clinical manifestations of this disease
- Marfan syndrome: a connective tissue disorder that affects mainly the musculoskeletal and cardiovascular systems and the eyes. The patients show an asthenic complexion, with tall stature, long arms and hands and fingers like those of a spider. The musculature is poorly developed, with little subcutaneous fat and great laxity of the joints and ligaments. The disease is due to a mutation of the FBN1 gene that encodes fibrillin-1
Cutis lax
Cutis lax: a very rare disease, acquired or congenital, in which the degeneration of the elastic fibers of the skin makes it loose and pendulous. The disease is due to a defect of lysyl oxidase, a copper-dependent enzyme that catalyzes the cross-linking reactions of elastin
Kuivaniemi et al. (1997) have reviewed data on the nearly 278 different mutations found in the genes encoding type I, II, III, IX, X, and XI collagens of 317 unrelated patients. Most of the mutations (217, 78% of the total) were from a single base, either by exchange of a codon of a critical amino acid or by an abnormal splicing of the RNA.
Mutations of these 6 collagens cause a wide spectrum of diseases of bone, cartilage, and blood vessels, including osteogenesis imperfecta, a wide variety of chondrodysplasias, Ehlers-Danlos syndrome types IV and VII, and, occasionally, rare types of osteoporosis, osteoarthritis. and family aneurysm.
Collagen is used in cosmetic surgery in the so-called collagen replacement therapy, a procedure that involves injecting subcutaneously natural or synthetic collagen in the areas where wrinkles ( * ), scars or other imperfections of the skin are to disappear.
They are also being used as support in cultures of cartilaginous cells to subsequently implant patients who have suffered injuries, with a new technology called tissue engineering.