Since its structure contains a secondary amine rather than a primary amine, proline is actually an imino acid rather than an amino acid, but it forms peptide bonds and is incorporated into proteins just like an amino acid. It causes an abrupt and rigid change of direction in the polypeptide chain, and this has a major effect on the final conformation of the protein. The carbon at the 4 position can be hydroxy-lated to form hydroxyproline. Every third residue of the structural protein collagen is a hydroxyproline residue.
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