Collagens are a family of structural proteins that play a crucial role in the extracellular matrix (ECM) of connective tissues in animals. They are the most abundant proteins in the human body, accounting for approximately 25-30% of the total protein content. Collagens provide strength, support, and organization to various tissues such as skin, bones, tendons, ligaments, cartilage, blood vessels, and the cornea of the eye.
There are at least 28 different types of collagens identified, each encoded by a specific gene. These collagen types can be categorized into several classes based on their structure and function:
- Fibrillar collagens: These are the most abundant and well-known collagens, which form rope-like fibrils with high tensile strength. They include types I, II, III, V, XI, XXIV, and XXVII. For example, type I collagen is the primary collagen found in skin, tendons, and bones, while type II collagen is the major component of cartilage.
- Network-forming collagens: These collagens form sheet-like networks and are mainly found in basement membranes, which provide support and separation between different tissue layers. Types IV, VIII, and X belong to this group.
- Facit (Fibril-Associated Collagens with Interrupted Triple helices) collagens: These collagens do not form fibrils themselves but associate with fibrillar collagens, helping to organize and stabilize the collagen network. Examples include types IX, XII, XIV, XVI, XIX, XX, and XXI.
- Beaded filament-forming collagens: This group includes type VI collagen, which forms beaded filaments that interact with other ECM components to provide structural support.
- Anchoring fibril collagens: Type VII collagen belongs to this group and forms anchoring fibrils that connect the basement membrane to the underlying connective tissue, providing stability and resistance to mechanical stress.
- Transmembrane collagens: These collagens have a transmembrane domain and play roles in cell adhesion, signaling, and tissue organization. Types XIII, XVII, XXIII, and XXV are examples of transmembrane collagens.
- Other collagens: Some collagens have unique structures and functions that do not fit into the above categories. For instance, type XV collagen is a basement membrane-associated collagen, and type XVIII collagen is a multiplexin that can interact with various ECM components.
Collagens are synthesized as precursor molecules called procollagens, which undergo post-translational modifications, such as hydroxylation and glycosylation, and are then assembled into triple-helical structures. Once secreted into the extracellular space, procollagens are processed by specific enzymes that cleave their N- and C-terminal propeptide regions, resulting in the formation of mature collagen molecules. These molecules then self-assemble into fibrils, which are further organized into collagen fibers, depending on the tissue type and functional requirements.
Dysfunction or abnormalities in collagen synthesis, processing, or assembly can lead to various connective tissue disorders, such as osteogenesis imperfecta, Ehlers-Danlos syndrome, and scurvy. Additionally, the production of collagen decreases with age, contributing to the development of wrinkles, joint stiffness, and weakened tendons and ligaments.