The collagen molecule or the tropocollagen contains subunits that assemble without assistance and has regular ends that form larger arrays in the extracellular spaces of tissues.
Different collagen types form larger fibrillar bundles with the help of several different classes of proteins like glycoproteins and proteoglycans. Collagen fibrils are semi-crystalline aggregates of collagen molecules. These are actually bundles of fibrils.
Each of the tissues has a different arrangement of these fibrils to give it different structure, shape and tensile strength. In bone for example, entire collagen triple helices lie in a parallel, staggered array. The gaps are 40 nm between the ends of the tropocollagen subunits. This serves as the nucleation sites for the deposition of long, hard, fine crystals of the mineral component - hydroxyapatite, Ca10(PO4)6(OH)2 with some phosphate. This turns collagen in cartilage that is softer into hard bone. Type I collagen gives bone its tensile strength.
Staggering and D period
This is called staggering and in the fibrillar collagens, the molecules are staggered from each other by about 67 nm (a unit that is referred to as ‘D’ and changes depending upon the hydration state of the aggregate).
Each of these D periods has 4 and a part of a collagen molecule. This is because 300 nm divided by 67 nm does not give an integer (the length of the collagen molecule divided by the stagger distance D). Thus in each D-period repeat of the microfibril, there is a part containing five molecules in cross-section - called the “overlap” and a part containing only 4 molecules, called the "gap".
Types of collagen
More than 20 genetically distinct collagens exist in animal tissues. Collagen types I, II, III, V and XI self-assemble into D-periodic cross-striated fibrils. Here the D is approximately 67 nm and there is characteristic axial periodicity of collagen. These form the most abundant collagens in vertebrates.
- Type I collagen is found throughout the body except in cartilaginous tissues. It is also synthesized in response to injury and in the fibrous nodules in fibrous diseases.
- Type II collagen is found in cartilage, developing cornea and vitreous humour. These are formed from two or more collagens or co-polymers rather than a single type of collagen.
- Type III collagen is found in the walls of arteries and other hollow organs and usually occurs in the same fibril with type I collagen.
- Type V collagen and type XI collagen are minor components of tissue and occur as fibrils with type I and type II collagen respectively.
The ends of the collagen fibrils are capped by short extrahelical telopeptides. The telopeptides, which do not have a repeating Gly-Xaa-Yaa structure and do not adopt a triple-helical conformation, account for 2% of the molecule and are critical for fibril formation.