So I know that hydroxyproline is created from proline via hydroxylation as a post-translational modification. I also know that proline is considered an amino acid. However, once you hydroxylize it, you have made a covalent modification to the original amino acid. Can hydroxyproline be correctly considered an amino acid? Or is it simply an amino acid derivative and NOT an amino acid?
Or, a more general question - how much can you change an amino acid and still call it an amino acid?