Unnatural amino acids: production and biotechnological potential.

Unnatural amino acids (UAAs) are valuable building blocks in the manufacture of a wide range of pharmaceuticals. UAAs exhibit biological activity as free acids and they can be incorporated into linear or cyclic peptides with biological activity. However, the scope of biotechnological application of UAAs goes beyond this, as they can be used to investigate the structure and dynamics of proteins, to study protein interactions, or to modulate the activity of proteins in living cells. The means to expand nature's repertoire of amino acids include chemical and biological routes. An UAA can be made through chemical modifications of natural amino acids, or related compounds. These modifications typically rely on utilisation of ligands and palladium catalysts. Employing biocatalysts in the synthesis of UAAs can also afford novel molecules with different physical and chemical properties. A number of transaminases for example have been identified and employed in the production of UAAs. This review will compare the chemical and biological routes for the synthesis of UAAs and provide an overview of their applications.