Collagen is an inextensible fibrous protein that is found in connective tissue - bone, tendons, and skin. The
objective of this exercise is to develop an understanding of the fibrous portion of collagen and to show how the different levels of protein structure come
together and form a highly ordered and stable fiber. Three helicial peptides combine to form tropocollagen, and five
tropocollagens combine to form a fiber segment. It is important to realize that in an actual fiber segment the ends of the
tropocollagen would be staggered rather then flush-cut as shown in the model presented herein. The staggered ends would
permit overlap in the association of the tropocollagens in adjacent segments, and thus aid in forming a strong fiber.
Collagen's properties of rigidity and inextensibility are due to this highly ordered structure. The non-structurally order part of collagen is not illustrated in this model. This part of the protein complex having a different amino acid composition, lysine and hydroxylysine are particularly important residues, is globular in nature and not as structurally organized. Lysine and hydroxylysine form covalent crosslinks in the protein complex, thereby adding strength and some flexibility to the fiber. This covalent crosslinking continues throughout life and produces rigid collagen and brittle bones in older adults. More general information or movies of assembly of triple helix of type I and IV collagen.