A typical collagen molecule is long, stiff, extracellular structure in which three polypeptides are wound around one another in a rope like triple helix. The chains are held together by hydrogen bonds. Variations in the amino acid sequence of the alpha chain result in collagen molecules with slightly different properties.
Type I,II and III collagens are fibrillar, a and are found in skin, tendon, bone cornea and blood vessels. Type IX and XII are fibril associated and are found in cartilage, tendon and ligaments. Type IV and VII from networks in basement membrane and beneath stratified squamous epithelia.
Collagen is rich in glycine and proline. The glycine residues are part of a repeating sequence-Gly-X-Y-where X is frequently proline and Y is often hydroxyproline and hydroxyllysine.
Hydroxyproline and hydroxyline result from the hydroxylation by specific hydroxylases of proline and lysine residues after their incorporation into alpha chains. The enzymes require ascorbic acid as a cofactor. The hydroxyl group of the hydroxylysine residues of collagen may be enzymatically glycosylated.
The precursors of collagen alpha chains are formed in fibroblasts, osteoblasts and chondrobalsts, and travel via the endoplasmic reticulam and Golgi to the extracellular matrix. There, the N terminal and C terminal properties are removed by procollagen peptides. In some collagen, the collagen molecules self assemble into fibrils in which the adjacent triple helices are arranged in a staged pattern, each overlap ping its neighbor by a length approximately three quarters of a molecules. The triple helices are then cross linked, giving the fibrillar array great tensile strength.