Proteoglycans consist of a core protein to which glycosaminoglycan (GAG) side chains are attached. The collagenous backbone dictates the tissue architecture, shape, and organization. The resulting collagens then assemble into fibrils via covalent cross-links formed between lysine residues of two collagen chains by a process catalyzed by extracellular enzyme lysyl oxidases (LOX). The fibrillar procollagens, following processing, are secreted into the extracellular space where their propeptides are removed. Selected lysine residues are also hydroxylated by lysyl hydroxylase. Specific proline residues in collagens are hydroxylated by prolyl 4-hydroxylase and prolyl 3-hydroxylase. The FACITs do not assemble into fibrils by themselves, but are associated with collagen fibrils. Nonfibril supramolecular structures, such as the networks of collagen IV in basement membranes and beaded filaments, are formed by nonfibrillar collagens. These noncollagenous domains are proteolytically removed and triple helices formed are associated laterally into fibrils. The fibril-forming collagens contain continuous triple-helix-forming domains flanked by amino- and carboxyl-terminal noncollagenous domains. In vertebrates, 46 distinct collagen chains assemble to form 28 collagen types 2, 4 that are categorized into fibril-forming collagens (e.g., types I, II, III), network-forming collagens (e.g., the basement membrane collagen type IV), fibril-associated collagens with interruptions in their triple helices, or FACITs (e.g., types IX, XII), and others (e.g., type VI). Major components include collagens, proteoglycans, elastin, and cell-binding glycoproteins, each with distinct physical and biochemical properties.Ĭollagen is composed of 3 polypeptide α chains that form a triple helical structure.
The “core matrisome” 3 comprises approximately 300 proteins.