On the Biosynthesis of Triacsins
Natural products are a source of engineering innovation and design for small molecules due to their relevance to wide swaths of the chemical sector including medical, agricultural, food and fragrance, and commodity chemical fields. Their structural complexity comprising of numerous chiral centers and an abundance of heteroatoms makes organic synthesis challenging, expensive, and generally infeasible. While combinatorial chemistry hoped to address these obstacles by enabling rapid diversification and screening methods, it is still limited by access to an initial scaffold on which to act upon. As such there is great potential for leveraging biosynthesis in combination with synthetic strategies to facilitate sustainable production of new bioactive compounds. In this manuscript we present our findings on the biosynthesis of NN bonds in the context of the triacsin natural product family. By elucidating the biosynthesis of a compound with multiple NN chemical bonds, we have discovered multiple enzymatic strategies employed by nature to create a linkage which is synthetically challenging due to the inherent nucleophilicity of nitrogen atoms. As such this research provides insight into the biogenesis of NN bonds and addresses the aforesaid synthetic challenges in chemical access to new bioactive compounds.
Triacsins are notable for the conserved N-hydroxytriazene moiety that all members of the family bear. In addition to two sequential NN bonds, the terminal nitrogen itself is a member of an additional heteroatom-heteroatom linkage in the form of a nitrogen-oxygen bond. As with many stories in natural product biosynthesis, this manuscript begins with the genomic sequencing of the originally reported native producer of triacsins. Mutagenesis and isotopically labeled precursor feeding led to the identification of the essential genes required for triacsin biosynthesis and led to the discovery of another native triacsin-producing organism. Cultivation of mutant strains and analysis of organic extracts from said strains led to the structural characterization of a key late-stage chemical intermediate that informed the biochemical timing of N-hydroxytriazene biosynthesis. Subsequent in vitro reconstitution of several encoded enzymes has advanced our knowledge of biochemical strategies for NN bond biogenesis. Finally, nascent work on the detailed characterization of an NN bond-forming enzyme will provide a full mechanistic understanding of this catalytic transformation. Collectively, this work contributes to the biocatalytic formation of NN bonds.