ABSTRACT
Collagen is among the most abundant structural proteins that give rise to various mechanical properties that tissues need. From the biomechanical point of view, of the most interest are the fibrillar collagens, including several variants such as type I, II, III, V, and XI (Hulmes 2002; Kadler et al. 2007). Among them, type I, II, and III constitute at least 80%∼90% of collagen in the human body (Myllyharju and Kivirikko 2004). Type I collagen is the most widely occurring collagen found in skin, tendon, bone, cornea, lung and the vasculature. Type II is limited essentially to cartilage. Type III is found in relatively elastic tissues such as embryonic skin, lung, blood vessels, and reticular tissues. As in 116all collagens, each fibrillar collagen molecule consists of three polypeptide chains, called α chains. Molecules can be homotrimeric, consisting of three identical α chains, as in collagens II and III, or heterotypic, consisting of up to three genetically distinct α chains, as in collagen I, V and XI. Fibrillar collagen molecules are characterized by a long central triple-helical region in each α chain, consisting of a continuous (Gly-X-Y)n repeat, where n is 337–343 (depending on collagen type) and X and Y can be any amino acid. For collagen type I, X and Y are often proline and hydroxyproline. These long (∼300 nm) and thin (1.5 nm diameter) (Buehler 2006) triple-helical molecules flank each other in a staggered fashion with a 64–67 nm spacing between ends, forming fibrils with sizes from 50 to 500 nm. These fibrils are further packed into thicker fiber bundles and tissue structures, which fulfils specific biomechanical functions. Figure 6.1 illustrates this hierarchical structure that type I collagen builds in a tendon (read the caption for more details) (Fratzl 2003).
