Muraymycins are peptidyl nucleoside antibiotics that contain two Cβ-modified amino acids, (2S,3S)-capreomycidine and (2S,3S)-β-OH-Leu. The former is also a component of chymostatins, which are aldehyde-containing peptidic protease inhibitors thatlike muraymycinare derived from nonribosomal peptide synthetases (NRPSs). Using feeding experiments and in vitro characterization of 12 recombinant proteins, the biosynthetic mechanism for both nonproteinogenic amino acids is now defined. The formation of (2S,3S)-capreomycidine is shown to involve an FAD-dependent dehydrogenase:cyclase that requires an NRPS-bound pathway intermediate as a substrate. This cryptic dehydrogenation strategy is both temporally and mechanistically distinct in comparison to the biosynthesis of other capreomycidine diastereomers, which has previously been shown to proceed by Cβ-hydroxylation of free l-Arg catalyzed by a member of the nonheme Fe2+- and α-ketoglutarate (αKG)-dependent dioxygenase family and (eventually) a dehydration-mediated cyclization process catalyzed by a distinct enzyme(s). Contrary to our initial expectation, the sole nonheme Fe2+- and αKG-dependent dioxygenase candidate Mur15 encoded within the muraymycin gene cluster is instead demonstrated to catalyze specific Cβ hydroxylation of the Leu residue to generate (2S,3S)-β-OH-Leu that is found in most muraymycin congeners. Importantly, and in contrast to known l-Arg-Cβ-hydroxylases, the Mur15-catalyzed reaction occurs after the NRPS-mediated assembly of the peptide scaffold. This late-stage functionalization affords the opportunity to exploit Mur15 as a biocatalyst, proof of concept of which is provided.

Overcoming increasing antibiotic resistance requires the development of novel antibacterial agents that address new targets in bacterial cells. Naturally occurring nucleoside antibiotics (such as muraymycins) inhibit the bacterial membrane protein MraY, a clinically unexploited essential enzyme in peptidoglycan (cell wall) biosynthesis. Even though a range of synthetic muraymycin analogues has already been reported, they generally suffer from limited cellular uptake and a lack of activity against Gram‐negative bacteria. We herein report an approach to overcome these hurdles: a synthetic muraymycin analogue has been conjugated to a siderophore, i. e. the enterobactin derivative EntKL, to increase the cellular uptake into Gram‐negative bacteria. The resultant conjugate showed significantly improved antibacterial activity against an efflux‐deficient E. coli strain, thus providing a proof‐of‐concept of this novel approach and a starting point for the future optimisation of such conjugates towards potent agents against Gram‐negative pathogens.