Because it performs such a wide variety of functions, collagen is the most abundant single protein in most vertebrates. In large animals, it may make up a third of the total protein mass. Collagen fibers form the matrix, or cement, material in bone, on which the mineral constituents precipitate. These fibers constitute the major portion of tendons. A network of collagen fibers is an important constituent of skin. Basically, collagen holds most animals together.

The basic unit of the collagen fiber is the tropocollagen molecule, a triple helix of three polypeptide chains, each about 1000 residues in length. This 3-fold helical structure, shown schematically in Figure 6.13, is unique to collagen. The individual chains are left-hand helices, with about 3.3 residues/turn. Three of these chains wrap around one another in a right-hand sense, with hydrogen bonds extending between the chains.

Examination of the model reveals that every third residue, which must lie near the center of the triple helix, can be only glycine (Figure 6.13a). Any side chain would be too bulky. Formation of the individual helices of the collagen type is also favored by the presence of or hydroxyproline in the tropocollagen molecule. A repetitive theme in the sequence is of the form Gly - X - Y, where X is often proline and Y is proline or hydroxyproline. However, other residues are sometimes tolerated in these positions. Like silk fibroin, collagen is a good example of how a particular kind of repetitive sequence dictates a particular structure.

Hydroxylysine is a modified amino acid that also occurs in collagen.