Structure of Collagen

AN arrangement of three non-coaxial helical chains linked to one another by hydrogen bonds approximately perpendicular to the length of the chains was suggested as the basis of the structure of collagen two years ago1. This structure explained the occurrence of a fraction of more than one-third of glycine residues and could readily accommodate proline and hydroxyproline residues, besides explaining the infra-red dichroism. The exact nature of the helices (namely, three residues per turn) was later found not to be quite correct for collagen, the X-ray pattern of stretched collagen2 indicating the occurrence of 3 residues per turn3. The presence of such a non-integral number of residues per turn required that the three chains must all be further coiled around. The coiled-coil structure4 retained the essential features of the earlier one as regards the location of amino-acid residues and the orientation of the NH- and CO-bonds. However, it is interesting to note that the simpler non-coiled-coil structure has been found to be the basis of the arrangement of polypeptide chains in polyproline5 and in polyglycine II 6. Thus, it appears that the triple chain structure, with minor modifications, is a configuration which might be found also in other proteins and polypeptides. There is, in fact, good evidence to show that elastin belongs to this type7.