Collagen is the most abundant protein in the human body. A typical collagen molecule is a long, rigid structure in which three polypeptides (referred to as “α chains”) are wound around one another in a rope-like triple helix (Figure 4.1). Although these molecules are found throughout the body, their types and organization are dictated by the structural role collagen plays in a particular organ. In some tissues, collagen may be dispersed as a gel that gives support to the structure, as in the extracellular matrix or the vitreous humor of the eye. In other tissues, collagen may be bundled in tight, parallel fibers that provide great strength, as in tendons. In the cornea of the eye, collagen is stacked so as to transmit light with a minimum of scattering. Collagen of bone occurs as fibers arranged at an angle to each other so as to resist mechanical shear from any direction. A. Types of collagen The collagen superfamily of proteins includes more than 25 collagen types, as well as additional proteins that have collagen-like domains. The three polypeptide α chains are held together by hydrogen bonds between the chains. Variations in the amino acid sequence of the α chains result in structural components that are about the same size (approximately 1,000 amino acids long), but with slightly different properties. These α chains are combined to form the various types of collagen found in the tissues. For example, the most common collagen, type I, contains two chains called α1 and one chain called α2 (α12α2), whereas type II collagen contains three α1 chains (α13)
Fibril-forming collagens: Types I, II, and III are the fibrillar collagens, and have the rope-like structure described above for a typical collagen molecule. In the electron microscope, these linear polymers of fibrils have characteristic banding patterns, reflecting the regular staggered packing of the individual collagen mol ecules in the fibril (Figure 4.3). Type I collagen fibers are found in supporting elements of high tensile strength (for example, tendon and cornea), whereas fibers formed from type II collagen molecules are restricted to cartilaginous structures. The fibers derived from type III collagen are prevalent in more distensible tissues, such as blood vessels. 2. Network-forming collagens: Types IV and VII form a three-dimensional mesh, rather than distinct fibrils (Figure 4.4). For example, type IV molecules assemble into a sheet or meshwork that constitutes a major part of basement membranes. Basement membranes are thin, sheet-like structures that provide mechanical support for adjacent cells, and function as a semipermeable filtration barrier to macromolecules in organs such as the kidney and the lung.