collagens

Collagens are a family of extracellular matrix proteins that provide structural support in connective tissues. They are the most abundant proteins in animals and form a major component of skin, bone, tendon, cartilage, and blood vessels. The defining feature of collagens is a triple-helix structure: three polypeptide chains wind around one another, stabilized by a high content of glycine every third residue and by hydroxyproline and hydroxylysine residues added after translation.

Most collagens are synthesized as procollagens with N- and C-terminal propeptides, secreted into the extracellular space, and then processed to mature collagen. Individual collagen molecules assemble into fibrils and networks, which are then cross-linked by enzymes such as lysyl oxidase to increase tensile strength. Collagens are categorized into several types; among the best known are the fibrillar types I, II, and III, which provide resistance to tension in skin, bone, tendons (type I), cartilage (type II), and vessel walls (type III). Other types form networks (type IV in basement membranes) or have supporting roles in connective tissues (types V, IX, XII, XIV, etc.).

In addition to providing structural support, collagens influence cell adhesion, differentiation, and migration and play a crucial role in wound healing and development. Collagen turnover is mediated by matrix metalloproteinases and other proteases and is tightly regulated.

Clinical relevance includes scurvy, caused by vitamin C deficiency that impairs collagen hydroxylation, and heritable disorders such as osteogenesis imperfecta and Ehlers-Danlos syndromes. Collagen-derived biomaterials and hydrolyzed collagen are used in medicine and cosmetics, though evidence for certain therapeutic claims varies.