Protein Building Blocks and the Expansion of the Genetic Code

The proteins of all known organisms are built of a set of 20 canonical amino acids prescribed by the genetic code. Many more amino acids occur in nature but they are excluded from ribosomal translation. Nevertheless, nature exploits their vast chemical diversity for the production of highly bioactive peptides by non-ribosomal biosynthesis routes. The extraordinarily rich structural and functional repertoire of the noncanonical amino acids holds great promise for the future of protein engineering, yet we have only just begun to tap the cornucopia of noncanonical building blocks for the biosynthesis of synthetic proteins.