Tropocollagen is the basic structural unit of collagen, the most abundant protein in the extracellular matrix (ECM) of connective tissues in animals, including humans. Collagen provides strength, support, and elasticity to various tissues such as skin, tendons, ligaments, bones, cartilage, and blood vessels.
Tropocollagen is a rod-shaped molecule made up of three polypeptide chains, called alpha chains, that are twisted together into a triple helix structure. Each alpha chain has a repeating amino acid sequence of glycine-proline-X or glycine-X-hydroxyproline, where X can be any other amino acid. The presence of glycine, the smallest amino acid, at every third position allows for the tight packing of the three chains into the triple helix.
The tropocollagen molecules then align and assemble into larger structures called collagen fibrils, which are stabilized by covalent cross-links. These fibrils further aggregate to form collagen fibers, which are the functional units in connective tissues. There are several types of collagen (at least 28 types), each with a unique composition and function. The most common types are type I, II, and III collagen, found in various tissues such as skin, bone, and cartilage.
Collagen synthesis and assembly are complex processes that involve various enzymes, including prolyl and lysyl hydroxylases and collagen peptidases. Genetic mutations or deficiencies in these enzymes can lead to collagen-related diseases such as Ehlers-Danlos syndrome and osteogenesis imperfecta.