Hydroxyproline is derived from the amino acid proline and is used almost exclusively in structural proteins including collagen, connective tissue in mammals, and in plant cell walls. An unusual feature of this amino acid is that it is not incorporated into collagen during biosynthesis at the ribosome, but is formed from proline by a posttranslational modification by an enzymatic hydroxylation reaction. Non-hydroxylated collagen is commonly termed pro-collagen.