Semisynthetic production of unnatural amino acids and their direct incorporation into peptides and proteins

The main source of chemical diversity in the majority of mature proteins and peptides is due to posttranslational modifications (PTMs). Mimicking these PTM machineries is not trivial. Probably, the most straightforward way to tackle this problem is to specifically insert natural or synthetic non-canonical amino acids (ncAA) directly during translation, expanding the scope of ribosomal protein synthesis beyond the 20 canonical amino acids – in other words, to re-engineer the genetic code. In this context, it is highly desirable that synthetic amino acids of interest are directly produced by the host, by means of engineering metabolic pathways so as to provide the cells with target synthetic amino acids formed intracellularly from simple carbon sources or precursors. In this respect, enzymatic pathways mediated by pyridoxal-5'-phosphate (PLP)-dependent enzymes are most promising targets.