There are more than 20 known types of collagen and they are classified by a Roman numeral based on their time of discovery. They are found in all connective tissues such as skin, bone, cartilage, vasculature, tendons and ligaments where their function is to provide support. All collagen molecules consist of three polypeptide chains folded into a left-handed helical conformation, and the three helical chains are then wrapped around each other into a right-handed triple helix. Homotrimer collagen types have 3 identical chains, whereas heterotimer collagen types contain two or three different chains. Everythird amino acid is glycine in the repeating Gly-X-Y sequence in each of the chains. Glycine allows for the triple helix confirmation where the three chains come together and X is usually proline and Y, hydroxyproline.
Types I, II, III, V and XI are considered the fibril-forming collagens as they form long fibers consisting of a triple-helical domain and are similar in structure. They are characterized by 67 nm banded fibrils that are quarter -staggered. The length of these collagens is approximately 300 nm and 1.5nm in diameter and consist of 1000 Gly-X-Y repeating residues. Collagens are synthesized as precursor molecules called procollagens that contain amino (N) and carboxy (C) terminal propeptide domains which consist of collagen and non-collagen-like sequences. The N and C terminal propeptides are linked to the main triple-helical domain by short, non-collagenous sequences, called telopeptides. The telopeptides are the primary sites for intermolecular cross-linking, which is important for the stabilization of the collagen fibers.
The procollagen molecules are processed to mature collagen molecules by cleavage of the N and C propeptides by specific N and C proteinases.