Ribosomal Synthesis of Peptides with C-terminal Lactams, Thiolactones, and Alkylamides

All change at the C terminus: We have established a novel methodology for the ribosomal synthesis of peptides featuring C-terminal cyclization and various modifications, including macrocyclization, by making use of genetic code reprogramming. The C-terminal amide modification of linear and cyclic peptides should enhance their physiological stabilities, and open up the possibility of developing new drug-like peptides. The C terminus of a peptide expressed by the translation apparatus generally ends in a carboxylate group. On the other hand, the C termini of some naturally occurring peptides have amide moieties instead of carboxylates, which are believed to give better biostability. Here, we describe a new strategy for the ribosomal synthesis of peptides featuring C-terminal lactam, thiolactone, and alkylamide units. The method was based on the concept of genetic code reprogramming involving the flexizymes (flexible tRNA acylation ribozymes) and the PURE (peptide synthesis using recombinant elements) system, in which vacant codons are reassigned to nonproteinogenic amino acids; this enabled us to convert the C termini of peptides into the above functionalities. We have also applied this method to the synthesis of a macrocyclic peptide closed by an amide bond formed between a lysine side chain and the peptide C terminus. This method thus offers us new opportunities to express various peptides with C-terminal modifications as well as macrocyclic peptides using the translation apparatus, and potentially to accelerate the discovery of peptidic drugs designed for various therapeutic targets.