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Collagen is the main protein of connective tissue. It has great tensile strength, and is the main component of ligaments and tendons. It is responsible for skin elasticity, and its degradation leads to wrinkles that accompany aging. Collagen also fills out the cornea where it is present in crystalline form. Collagen is the most abundant protein in mammals. Collagen has an unusual amino acid composition. It contains large amounts of glycine and proline, as well as two amino acids that are not inserted directly by ribosomes – hydroxyproline and hydroxylysine – the former composing a rather large percentage of the total amino acids. They are derivatised from proline and lysine in enzymatic processes of post translational modification, for which vitamin C is required. This is related to why vitamin C deficiencies can cause scurvy, a disease that leads to loss of teeth and easy bruising caused by a reduction in strength of connective tissue due to, a lack of collagen, or defective collagen. The white collagen that makes up the matrix of most connective tissue in mammals consists of inter-woven fibres of the protein collagen. The collagen fibres consist of globular units of the collagen sub-unit tropocollagen. Tropocollagen sub-units spontaneously arrange themselves under physiological conditions into staggered array structures stabilised by numerous hydrogen and covalent bonds. Tropocollagen sub-units are left-handed triple helices where each strand is, further, a right-handed helix itself. Thus, tropocollagen may be considered to be a coiled coil. Another rare feature of collagen is its regular arrangement of amino acids in each of the alpha chains of the collagen sub-units. The sequence generally follows the pattern Gly-X-Y, where Gly for glycine, X for proline, and Y for proline or hydroxyproline. There are very few other proteins with such regularity. The inordinate number of Gly residues allows the otherwise sterically disallowed, tight coiling of each of the alpha chain subunits of tropocollagen, where there is a rise per turn of just 0.3 nm as opposed to the .36 nm of a regular Alpha helical coil. Hydroxylysine and hydroxyproline play important roles in the stabilisation of the tropocollagen globular structure as well as the final fibre shaped structure by forming covalent bonds. The resulting structure is called a collagen helix.
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