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Collagens are prominent structural proteins which occur as rigid rods found in a almost all organs of the body. They are constituents of bones, cartilage, skin, cornea, etc. In bones, collagen fibres are arranged at an angle to each other to afford mechanical strength, while in cartiage the collagen fibres impart the right toughness. The cornea of the eye has a high proportion of collagen fibres arranged in rows.
The collagen is generally a triple stranded helix one-third of which is composed of glycine residues, and about 25% is made from proline and hydroxy-proline. Amino acid composition of collagens may be summarized as
35    per cent glycine
25    per cent proline plus hydrolyproline, of
    Which 6 to 13 per cent proline and 9 to 16
    Per cent hydroxyproline
The remaining 18 amino acid residues are found in fewer numbers.
    The collagen molecules are made up of three polypeptide chains that are twisted in a rope like fashion. Every third residue in the collagen fibril is glycine which enables twisting in such a manner that the glycine residues come to lie inside. Each fibril constisting of three polypeptide chains is in a left-handed superhelix. The stabilizing forces come from the hydrogen bonding between peptide backbones.
A great deal of heterogeneity is found in collagen molecules, and consequently the composition of collagens from different source is variable. This variation has a functional role. There are two unusual features of the amino acid composition of α-chains. These are:(a) hydroxylation of many proline and lysine residues due to which stability of α-chains of the triple helix is maintained, and (b) hydroxylation of proline and lysine is an enzymic process that takes place after protein synthesis has been accomplished, i.e., a post-translational event.

structure of collagen

Different types of collagens distributed in various tissues

types of collagens distributed in various tissues1

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