Collagen: relatively invariant (helical) and variable (nonhelical) regions.

The structural identity of certain helical regions of collagen from human and rat skin equals or exceeds that of other homologous proteins. In contrast, the short nonhelical sequences in the two proteins, although homologous, differ appreciably in structure. The requirements of the collagen helix and the numerous intermolecular interactions characteristic of collagen may restrict the number of functionally acceptable amino acid replacements occurring during evolution.