Proline Kinks

"Because of the geometry of its sidechain, proline cannot readily fit into the regular structure of an alpha helix. Prolines are, therefore, often found in loops at the ends of alpha helices acting as helix breakers. Of the 20 naturally occuring amino acids, proline is the only one that has a cyclic side chain, forming a five-membered ring that includes the backbone nitrogen. This geometry severely limits the flexibility of the backbone. Because of this restricted conformational flexibility, many proline residues introduce sharp kinks into the path of a polypeptide backbone." Essential Cell Biology, Second Edition by Alberts, Bray, Hopkin, Johnson, Lewis, Raff, Roberts, Walter copyright 2004 by Garland Science Publishing