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Collagen is an extracellular protein organized as soluble fibers with high tensile strength. Real and natural collagen is always animal, and can never appear as natural vegan collagen. A molecule of type I collagen consists of 3 polypeptide chains. It has the shape of a "rod". If it had the thickness of a pencil, it would have a length of 1.5 m. This "rod" is reinforced by crosslinks.
A single chain of collagen is defined as an alpha chain. Each collagen molecule consists of three alpha chains that are usually identical. The only known exception is type I collagen. Type I collagen consists of two identical chains (a1) and one non-identical chain (a2) designated as [a1(I)]2a2. It is the only heteropolymer among all collagens. Index I is used because the chains in particular collagen types differ slightly in their amino acid composition.
The amino acid sequence is a typical property of protein, which determines its structure as a whole. Collagen, contains 19 amino acids, two of which do not occur in other proteins, these unique amino acids are hydroxyproline and hydroxylysine . Collagen also contains more glycine than most other proteins, but it does not contain cysteine, cystine - dimer of cysteine (with the exception of collagen III), and tryptophan.
The unique shape and properties of the collagen molecule depend on its amino acid composition and sequence. Collagen has a distinct amino acid composition and sequence: Gly-XY (Glycine, X is often proline and Y is often 4-hydroxyproline - with some 3-hydroxyproline and some 5-hydroxylysine). Hydroxyproline provides stability to collagen, probably through intramolecular hydrogen bonds that may involve bridging water molecules.
Pro residues are converted to hydroxyproline in a reaction catalyzed by prolyl hydroxylase. If collagen is synthesized under conditions that inactivate prolyl hydroxylase, it loses its natural conformation (denaturation) at 240°C, whereas normal collagen denatures at 390°C (denatured collagen is known as gelatin). Prolyl hydroxylase requires ascorbic acid (vitamin C) to maintain activity. If there is a lack of vitamin C, the disease scurvy occurs, because collagen fibers cannot be manufactured properly, this results in skin damage and poor wound healing.
1. The number of glycine residues amounts to 1/3 of all amino acid residues. After the amino acids have created a peptide, the former amino acids are called amino acid residues. In a peptide, the amino acid residue that has an amino group that is not attached to anything is called the N-terminal. The amino acid residue with a carboxyl group that is not attached to anything is called the C-terminal.
2. The number of imino acid residues is 1/5 of all amino acid residues in mammals and birds. An imino acid is an organic compound that contains both an imine group and a carboxyl group. Imino acids are related to amino acids, which contain both an amino group and a carboxyl group. Amino acids that contain a secondary amine group are sometimes called imino acids
3. The presence of two specific hydroxyamino acids: hydroxyproline, hydroxylysine.
4. The presence of a certain amount of aldehyde groups (participating in crosslinking bonds).
5. The presence of hexoses attached to protein side chains.
6. The presence of characteristic hydrophilic and hydrophobic space groupings in a chain.
7. The average molecular weight of a residue is 90.7 Daltons.
8. The number of amino acids in a chain amounts to approximately 1,000 on average.
9. The average molecular weight of a chain is approximately 90,000 Daltons.
The collagen has a known sequence. Details of this sequence are given in monographs.
1. The alpha chain of collagen consists of a central helical portion containing 1011-1047 amino acid residues of which every third must be glycine.
2. The helical part contains 20% imino acids in the second or third position, if we divide the molecule into tripeptides each starting with glycine (GXY). In mammals, about 2/3 of the imino acids are hydroxylated in collagen and they are always in the Y position (4-hydroxyproline). The only exception is 3-hydroxyproline which occurs in the X position but only once or twice in the chain.
3. The non-helical extensions are relatively rich in hydrophobic amino acids and contain a lysine residue that can be oxidized enzymatically and serves as a functional group for the formation of intra- and intermolecular cross-links.
4. Hydroxylysine occurs exclusively in collagen. It is the only amino acid that is glycosylated at several sites but not all residues in the chain. Lysine, like proline, is only hydroxylated when it is in the Y position.
5. The average content of proline plus hydroxyproline (Hyp) is equal throughout the chain, except at the C-terminus, which ends with 5 consecutive three peptides Gly-Pro-Hyp. This suggests an exceptional stability of the C-terminal helical region of the molecule.
X-ray studies show that collagen's three polypeptide chains are parallel and wrap around each other with a gentle, right-handed "repeel-like" twist to form a triple-helical structure. Every third residue of each polypeptide chain passes through the center of the triple helix, which is so tight that only one glycine side chain can fit there. The three polypeptide chains are also staggered so that glycine, X and Y residues from the three chains occur at similar levels. The displaced peptide groups are oriented so that the NH of each Glycine forms a strong H-bond with the carbonyl oxygen of an X residue on an adjacent chain. The bulky and relatively inflexible Pro and Hyp residues add stiffness to the entire assembly.
As with the twisted fibers of a rope, the elongated and twisted polypeptide chains of collagen convert a longitudinal tensile force into a more easily supported lateral compressive force on the nearly incompressible triple helix. This is because the opposite twisted directions of collagen's polypeptide chains and triple helix prevent the twists from pulling out under tension.
The repetitive sequence in collagen called the helical region consists of an infinite set of points, which lie on a helical line and are separated by a constant axial translation.
Collagen contains covalently bound carbohydrates in amounts ranging from 0.4-12% by weight depending on the collagen's tissue of origin. The carbohydrates consisting mostly of glucose, galactose and their disaccharides are covalently attached to collagen at its 5-hydroxylysyl residues by specific enzymes. They are located in the "hole" regions of the collagen fibrils.
The presumed existence of an ester-type linkage, via hexose residues, probably stems from the fact that saccharide units have been found in collagen, which are attached to hydroxylysine by glycosidic linkage in the helical region of the molecule, either as galactosyl-hydroxylysine or glucosyl-galactosyl-hydroxylysine.
Type I and type II collagens contain about 0.4% carbohydrates and type II contains about 4%. The main sites of glycosylation are those involved in the intramolecular cross-linking. So far, no experimental evidence has been made to show the function of these carbohydrates. It has been thought that they may regulate the formation of cross-links and aggregation of collagen molecules into the quartz-displacement arrangement.
The insolubility of collagen in solvents is explained by the observation that it is both intramolecularly and intermolecularly covalently cross-linked. The cross-links cannot be disulfide links, as in keratin , because collagen is almost devoid of Cysteine residues. Rather, they are derived from lysine and histidine side chains. Up to four side chains can be covalently linked to each other. The crosslinks do not form randomly but tend to occur near the N and C termini of the collagen molecules. The aspects of cross-linking are closely related to the aging of molecules. The degree of cross-linking increases with the age of the animal (meat from older animals is tougher).
In early postnatal tissues, the amount of reducible crosslinks is high and decreases as physical maturation progresses. The stable crosslinks that replace the reducible ones have not yet been determined with certainty. Changes in the physical and chemical properties of collagen fibers due to aging are very evident. The fibers become increasingly insoluble, their ability to swell in acidic solution decreases and likewise their sensitivity to enzyme attack, while their mechanical strength and stiffness increase. Stiffness increases throughout life, creating brittleness resulting in reduced tensile strength.
