Collagen is a protein made up of amino acids, which are in turn built of carbon, oxygen and hydrogen. Collagen contains specific amino acids – glycine, proline, hydroxyproline and arginine. It makes up approximately 30% of the proteins within the body, which are tough and strong structures found all over the person in bones, tendons and ligaments.
Collagen, one of the most predominant proteins, is also found in nature, exclusively in animals, especially in the flesh and connective tissues of mammals, and is a part of the tissue that helps in firmness, suppleness and constant renewal of skin cells.
Collagen, vital for skin elasticity, is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25-35% of the whole-body protein content.
In tanning
The tanning process involves the isolation of collagen, being the main leather-making protein component of rawhide.
The process of tannage mainly involves a relative change in the physico-chemical properties of collagen, making it then useful as leather. The extent of tannage is relatively but quantitatively described by the technical term ‘hydrothermal shrinkage temperature’ (HST). This is the temperature when shrinkage occurs with the fully water-saturated leather sample immersed in water being heated. Using these shrinkage temperature values, the extent of collagen stabilisation can be stated for different leathers.
In addition to what is mentioned above, collagen helps to give strength to various structures of the body, and also protects structures like the skin by preventing absorption and spreading of pathogenic substances, environmental toxins, micro-organisms and cancerous cells. Essentially, collagen protein is the cement that holds everything together.
Collagen is also present in all the smooth muscle tissues, blood vessels digestive tract, heart, gallbladder, kidneys and bladder holding the cells and tissues together.
Structure of collagen
Collagen microscopically occurs in elongated fibrils. It is mostly found in fibrous tissues such as tendon, ligament and skin, and is also abundant in tendons, ligaments, corneas and intervertebral discs.
In muscle tissue, collagen serves as a major component of endomysium. One to 2% of the muscles are formed of collagen and around 6% of the total weight of muscle is formed of collagen.
In the mid-1930s, collagen was first discovered to have a molecular structure. Nobel laureates Crick, Pauling, Rich and Yonath and others including Brodsky, Berman and Ramachandran have been researching the structure of collagen and their possible functions.
After several speculations of individual peptide chain, the final model that has been developed is the ‘Madras’ model, which provided an essentially correct model of the molecule’s quaternary structure – although this model still required some refinement. It is a triple-helical structure.
Collagen is further packed into fibrillar collagen types with hexagional or quasi-hexagonal shapes. The packing may be ‘sheet-like’ or microfibrillar. The microfibrillar structure of collagen fibrils in tendons, ligaments and corneas has been directly imaged by electron microscopy. The collagen molecule, also known as the ‘tropocollagen", is part of larger collagen aggregates such as fibrils. The whole molecule is approximately 300nm long and 1.5nm in diameter.
Individually, there are three polypeptide strands. These are called alpha chains and each of them has conformation of a left-handed helix. An alpha helix is a different structure with a right-handed conformation. Furthermore, the three left-handed helices are twisted together into a right-handed coil, forming a triple helix or ‘super helix’. The final cooperative quaternary structure stabilised by numerous hydrogen bonds.
In type I collagen, and possibly all fibrillar collagens if not all collagens, each of the triple helices forms a right-handed super-super-coil that is referred to as the collagen microfibril. Thereafter, each of the microfibril is interdigitated or intercalated with its neighbouring microfibrils. This strengthens the structure of the individual molecules.
Arrangement of amino acids
Collagen contains specific amino acids – glycine, proline, hydroxyproline and arginine. These amino acids have a regular arrangement in each of the three chains of these collagen subunits. The sequence often follows the pattern gly-pro-x or gly-x-hyp, where X may be any of various other amino acid residues. Proline or hydroxyproline constitute about one sixth of the total sequence.
Glycine accounts for a third of the sequence, meaning that approximately half of the collagen sequence is not glycine, proline or hydroxyproline. In addition, the regular repetition and high glycine content is found in only a few other fibrous proteins, such as silk fibroin.
Collagens do not contain chemically reactive side groups unlike in enzymes and transport proteins, and collagen determines cell phenotype, cell adhesion, tissue regulation and infrastructure and its non-proline rich regions have cell or matrix association/regulation roles.
The tensile strength of collagen depends on the formation of covalent intermolecular cross-links between the individual protein subunits. The fibril-containing collagens in higher vertebrates (types I, II, III, V and XI) are cross-linked through a mechanism based on the reactions of aldehydes generated enzymically from lysine (or hydroxylysine) side-chains by lysyl oxidase.
Certain other collagen types (collagen type IX of cartilage, for example) are also cross-linked by the lysyl oxidase mechanism.
