| Triple-helical structure of collagens
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The structural hallmark of collagens is their triple-helical structure, formed by folding of three peptide chains. These chains vary in size from 600-3000 amino acids . X-ray diffraction analysis indicates that three left-handed helical chains are wrapped around one another in a rope-like fashion, to form a right-handed superhelix structure (Fig. 27.1). The left-handed helix is more extended than the α-helix of globular proteins (Fig. 2.10), having nearly twice the rise per turn and only three, rather than 3.6, amino acids per turn. Every third amino acid is glycine, because only this amino acid, with the smallest side chain, fits into the crowded central core. The characteristic, repeating sequence of collagen is Gly-X-Y, where X and Y can be any amino acid, but most often X is proline and Y is hydroxyproline. Because of their restricted rotation and bulk, proline and hydroxyproline confer rigidity to the helix. The intra- and inter-chain helices are stabilized by hydrogen bonds, largely between peptide NH and C=O groups. The side chains of the X and Y amino acids point outward from the helix, and thus are on the surface of the protein, where they form lateral interactions with other triple helices or proteins.
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