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The major fibrous protein in animals, present in all types of multicellular animals and probably the most abundant animal protein in nature. It is estimated that collagen accounts for about 30% of the total human body protein. Collagen is located in the extracellular matrix of connective tissues. It is part of the interacting network of proteoglycans and proteins that provides a structural framework for both soft and calcified connective tissues. By self-associating into fibrils and by binding to proteoglycans and other matrix components, collagen contributes to tissue integrity and mechanical properties. Collagen interacts with cells through the integrin cell receptors and mediates cellular adhesion and migration. Important roles for collagen have been identified in development, wound healing, platelet aggregation, and aging. Its commercial importance in leather and the production of gelatin and glue have long been recognized. More recently, it is being used as a basis for biomaterials. Examples of its biomedical applications include injectable collagen to lessen facial wrinkles and defects; surgical collagen sponges to increase blood clotting; and artificial skin for the treatment of burns.
The classification of an extracellular matrix protein as a collagen is based on the presence of a domain with a distinctive triple-helical conformation. The collagen triple helix consists of three polypeptide chains supercoiled about a common axis and linked by hydrogen bonds. At least 19 distinct molecules have been classified as collagens, and specific types are associated with particular tissues. The most prevalent and well-studied collagens belong to the fibril-forming or interstitial collagen family. The molecules in a fibril are covalently cross-linked by an enzymatic mechanism to strengthen and stabilize them. Inhibition of the enzyme involved in cross-linking results in a dramatic decrease in the tensile strength of tissues, a condition known as lathyrism.
Type I is the most common fibril-forming collagen. Its fibrils make up the mineralized matrix in bone, the strong parallel bundles of fibers in tendon, and the plywoodlike alternating layers in the transparent cornea. Type II is the major fibril-forming collagen in cartilage, while type III is found in blood vessels and skin, together with type I. Basement membranes, which serve to separate cell layers and act as filtration barriers, contain a distinctive group of collagens, denoted as type IV collagens, which are organized into a network or meshlike sheet structure. In the kidney glomerulus, the network based on type IV collagen acts as a filter to determine which molecules will pass from the blood into the urine. See Bone, Connective tissue
An orderly breakdown of collagen is necessary during development and tissue remodeling. For instance, following childbirth, the uterus reduces in size, which involves a massive degradation of collagen. An abnormal increase in the degradation of cartilage collagen is seen in osteoarthritis. Collagen breakdown also appears to be essential for tumor metastases. A number of hereditary diseases have been shown to be due to mutations in specific collagen genes. Osteogenesis imperfecta (brittle bone) disease is characterized by fragile bones and is due to mutations in type I collagen. Some cartilage disorders are caused by mutations in type II collagen. Ruptured arteries are found in Ehlers-Danlos syndrome type IV, which arises from mutations in type III collagen.
a fibrillar protein of the scleroprotein group; the principal component of the collagenous fibers of the connective tissues in animals.
Collagen molecules (length, approximately 3,000 Å; width, 15 Å) consist of three interwined helical polypeptide chains. The primary structure of the collagen molecule is characterized by frequent repetition of a glycyl-prolyl-oxyprolyl sequence (about one-third of all amino acid residues), which influences the configuration of the chains. The collagen in mature fibers is insoluble in water and organic solvents and soluble in 10-percent alkaline solution. A part of the collagen of immature fibers (procollagen, or tropocollagen) is soluble in weak acid. Collagen fibers undergo marked contraction when heated in water; with prolonged heating, they are denatured and turn to gelatin.
Collagen constitutes about a third of all protein in the body. It is an important structural component of connective tissue, tendons, ligaments, cartilage, skin, bones, and fish scales; its function is chiefly supportive. There is no collagen in plants. Tannins increase the resistance of collagen fibers to chemical, physical, and bacterial influences—the basis for tanning hides and furs.
REFERENCESFinean, J. Biologicheskie ul’trastruktury. Moscow, 1970. (Translated from English.)
Treatise on Collagen, vols. 1–2. Edited by G. N. Ramachandran and B. Gould. New York, 1967–68.
V. O. SHPIKITER