Collagen is a multifunctional family of proteins of unique structural characteristics. It is the most abundant and ubiquitous protein in the body, its functions ranging from serving crucial biomechanical functions in bone, skin, tendon, and ligament to controlling cellular gene expressions in development [Nimni and Harkness, 1988]. Collagen molecules like all proteins are formed
by enzymaticregulated step-wise polymerization reaction between amino and carboxyl groups of amino acids, where R is a side group of an amino acid residue:
The simplest amino acid is
(Gly) (R = H), where a hypothetical flat sheet organization of polyglycine molecules can form and be stabilized by intermolecular hydrogen bonds (Fig. 6.1a). However, when R is a large group as in most other amino acids, the stereochemical constraints frequently force the
chain to adopt a less constraining conformation by rotating the bulky R groups away from the crowded interactions, forming a helix, where the large R groups are directed toward the surface of the helix (Fig. 6.1b). The hydrogen bonds are allowed to form within a helix between the hydrogen
attached to nitrogen in one amino acid residue and the oxygen attached to a second amino acid residue. Thus, the final conformation of a protein, which is directly related to its function, is governed primarily by the amino acid sequence of the particular protein.