An engineered methionyl-tRNA synthetase enables azidonorleucine incorporation in methionine prototrophic bacteria

An Engineered Escherichia coli Strain with Synthetic Metabolism for in-Cell Production of Translationally Active Methionine Derivatives

In the last decades, it has become clear that the canonical amino acid repertoire codified by the universal genetic code is not up to the needs of emerging biotechnologies. For this reason, extensive genetic code re-engineering is essential to expand the scope of ribosomal protein translation, leading to reprogrammed microbial cells equipped with an alternative biochemical alphabet to be exploited as potential factories for biotechnological purposes. The prerequisite for this to happen is a continuous intracellular supply of noncanonical amino acids through synthetic metabolism from simple and cheap precursors. We have engineered an Escherichia coli bacterial system that fulfills these requirements through reconfiguration of the methionine biosynthetic pathway and the introduction of an exogenous direct trans-sulfuration pathway. Our metabolic scheme operates in vivo, rescuing intermediates from core cell metabolism and combining them with small bio-orthogonal compounds. Our reprogrammed E. coli strain is capable of the in-cell production of l-azidohomoalanine, which is directly incorporated into proteins in response to methionine codons. We thereby constructed a prototype suitable for economic, versatile, green sustainable chemistry, pushing towards enzyme chemistry and biotechnology-based production.

Synthesis and Characterization of Heterodimers and Fluorescent Nisin Species by Incorporation of Methionine Analogues and Subsequent Click Chemistry

Noncanonical amino acids form a highly diverse pool of building blocks that can render unique physicochemical properties to peptides and proteins. Here, four methionine analogues with unsaturated and varying side chain lengths were successfully incorporated at four different positions in nisin in Lactococcus lactis through force feeding. This approach allows for residue-specific incorporation of methionine analogues into nisin to expand their structural diversity and alter their activity profiles. Moreover, the insertion of methionine analogues with biorthogonal chemical reactivity, e.g., azidohomoalanine and homopropargylglycine, provides the opportunity for chemical coupling to functional moieties and fluorescent probes as well as for intermolecular coupling of nisin variants. All resulting nisin conjugates retained antimicrobial activity, which substantiates the potential of this method as a tool to further study its localization and mode of action.

Initiating ribosomal peptide synthesis with exotic building blocks

Ribosomal peptide synthesis begins almost exclusively with the amino acid methionine, across all domains of life. The ubiquity of methionine initiation raises the question; to what extent could polypeptide synthesis be realized with other amino acids, proteinogenic or otherwise? This highlight describes the breadth of building blocks now known to be accepted by the ribosome initiation machinery, from subtle methionine analogues to large exotic non-proteinogenic structures. We outline the key methodological developments that have enabled these discoveries, including the exploitation of methionyl-tRNA synthetase promiscuity, synthetase and tRNA engineering, and the utilization of artificial tRNA-loading ribozymes, flexizymes. Using these methods, the number and diversity of validated initiation building blocks is rapidly expanding permitting the use of the ribosome to synthesize ever more artificial polymers in search of new functional molecules.

Synthesis and evaluation of clickable block copolymers for targeted nanoparticle drug delivery

Polymeric nanoparticles with multifunctional capabilities, including surface functionalization, hold great promise to address challenges in targeted drug delivery. Here, we describe a concise, robust synthesis of a heterofunctional polyethylene glycol (PEG), HO-PEG-azide. This macromer was used to synthesize polylactide (PLA)-PEG-azide, a functional diblock copolymer. Rapid precipitation of this copolymer with a hydrophobic cargo resulted in the generation of monodisperse nanoparticles with azides in the surface corona. To demonstrate conjugation to these nanoparticles, a regioselectively modified alkyne-folate was employed as a model small molecule ligand, and the artificial protein A1 with an alkyne moiety introduced by unnatural amino acid substitution was selected as a model macromolecular ligand. Using the copper-catalyzed azide-alkyne ligation reaction, both ligands exhibited good conjugation efficiency even when low concentrations of ligands were used.