Abstract
Azides are energy-rich compounds with diverse representation in a broad range of scientific disciplines, including material science, synthetic chemistry, pharmaceutical science and chemical biology. Despite ubiquitous usage of the azido group, the underlying biosynthetic pathways for its formation remain largely unknown. Here we report the characterization of an enzymatic route for de novo azide construction. We demonstrate that Tri17, a promiscuous ATP- and nitrite-dependent enzyme, catalyses organic azide synthesis through sequential N-nitrosation and dehydration of aryl hydrazines. Through biochemical, structural and computational analyses, we further propose a plausible molecular mechanism for azide synthesis that sets the stage for future biocatalytic applications and biosynthetic pathway engineering.

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Data availability
All data supporting the findings of this study are available within the paper, the Supplementary Information, Supplementary data file, source data and extended data. The coordinates and structure factor amplitudes for the apo structure of Tri17 and the complex structure with ATP have been deposited to the Protein Data Bank (PDB) under accession codes 8TF7 and 9BQ0, respectively. The AlphaFold2 model is available in ModelArchive with accession code ma-zyg1d. Source data are provided with this paper.
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Acknowledgements
This research was supported financially by grants from the National Institutes of Health (NIH; R01GM136758 and R35GM153289 for W.Z.). A.D.R.F. was supported financially by the Blavatnik Innovation Fellowship and UC Berkeley Chancellor’s Fellowship. X-ray data were collected at Beamline 8.3.1 of the Advanced Light Source, a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231, which is supported in part by the ALS-ENABLE programme funded by the NIH, National Institute of General Medical Sciences (P30 GM124169-01). This work was supported in part by the National Science Foundation (CBET-1704266 and CBET-1846426 for H.J.K. and D.W.K.). H.J.K. holds a Career Award at the Scientific Interface from the Burroughs-Wellcome Fund (H.J.K. and D.W.K.). The funders had no role in study design, data collection and analysis, decision to publish or preparation of the paper. We thank J. Pelton and D. Millar for assistance with NMR and bioinformatic analyses, respectively. We also thank D. Hilvert and C. Khosla for helpful discussions on the proposed azido-forming mechanism.
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A.D.R.F. designed all the experiments, conceptualized and conceived the project, performed biochemical and bioinformatic analyses of Tri17 and variants, designed and conducted chemical synthesis schemes, analysed NMR data, aided with the structural analysis of Tri17, analysed the data and wrote the paper. R.Z. designed the experiments, conducted structural analysis for Tri17, performed docking and MD studies, analysed the data and wrote the paper. D.W.K. designed computational experiments, performed calculations, analysed the data and contributed to the writing about the computational work. K.S. helped conduct in vitro experiments, chemical synthesis, protein purification and kinetic characterization of substrates. W.C. analysed the NMR data and aided with chemical synthesis. S.Y., Y.S., K.D.M. and M.N. aided in protein purification, construction of plasmids and repeating biochemical assays for this study. N.B.D. aided in protein purification, biochemical assays, construction of plasmids and interpretation of LC-MS data. Z.X. aided R.Z. with the structural work of Tri17. D.A.M. helped collect and analyse the NMR data. H.J.K. designed computational experiments, analysed the data and wrote the paper. W.Z. designed the experiments, conceptualized and conceived the project, analysed the data and wrote the paper.
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Extended data
Extended Data Fig. 1 Biochemical analysis of Tri17 assays with 20.
a) EICs demonstrating production of 21 and 22 from assays containing Tri17, ATP, nitrite, and 20. Omission of any of these components led to the abolition of 21 and 22. Utilization of 15N-nitrite resulted in the expected mass spectral shifts for both 21 and 22. No new products were detected when Aha11 was used in place of Tri17. A 10-ppm error mass tolerance was used for each trace. At least three independent replicates were performed for each assay, and representative results are shown. b) Relative amounts of 21 and 22 quantified by LC-HRMS over a 6-hour time course of the Tri17 assay. Error bars correspond to standard deviation of the mean from three replicate experiments. c) Analysis of 22 production from Tri17 assays. A Tri17 biochemical assay with 20 was first incubated at room temperature for 30 minutes and the protein was removed immediately using an Amicon spin filter (2 kDa MWCO). The reaction flowthrough was extracted with ethyl acetate, dried, and served as substrates (containing a mixture of 20, 21, and 22) for new Tri17 reactions and the production of 22 was monitored in a time course. Tri17 wild-type and Tri17_H229F were used in new reactions together with no enzyme control. The data points and error bars represent the average and standard deviations from three independently performed experiments, respectively.
Extended Data Fig. 2 Biochemical analysis of Tri17-mediated dehydration of 18.
A Tri17 biochemical assay with 17 was incubated at room temperature for 30 minutes and the protein was removed immediately using an Amicon spin filter (2 kDa MWCO). The reaction flowthrough was extracted with ethyl acetate, dried, and served as substrates (containing a mixture of 17, 18, and 19) for new Tri17 reactions and the production of 19 was monitored. The EICs demonstrate increased production of 19 in a Tri17-dependent manner. A 10-ppm error mass tolerance was used for each trace. The data points and error bars present in the bar graph represent the average and standard deviations of 19 produced from three independently performed experiments.
Extended Data Fig. 3 Docking of selected nitrosylated species in the hydrophobic tunnel of Tri17 in the ConNuc model.
a–d, Docking poses were screened and clustered, and the stability was checked via MD simulation for 1P (a), 7P (b), 12P (c) and 18 (d). Three bulky aromatic residues (H275, F273, H230) were found to be located at the entrance, in the middle and in the deep end of the tunnel, respectively.
Extended Data Fig. 4 Biochemical analysis of Tri17_H229F with 17 and 20.
a) Relative amounts of 18 and 19 quantified by LC-HRMS over a 6-hour time course from assays containing Tri17_H229F, ATP, nitrite and 17. b) Relative amounts of 21 and 22 quantified by LC-HRMS over a 6-hour time course from assays containing Tri17_H229F, ATP, nitrite and 20. Error bars correspond to standard deviation of the mean from three replicate experiments.
Extended Data Fig. 5 Bioinformatic analysis of Tri17 and its homologs.
a) Sequence similarity network (constructed using the EFI-Enzyme Similarity Tool using default settings130,131) consisting of 1,471 Tri17 homologs represented as nodes. Each node represents proteins that are >45% identical. Tri17, AvaA6, and SpiA7 are highlighted in red, purple, and gray as part of Group 1 (1,159 members), respectively. CreM and Aha11 are highlighted in green and yellow, respectively, as part of Group 2 (191 members). Groups 3 and 4 are composed of 76 and 45 members, respectively. Two bacterial strains other than Streptomyces tsukubaensis have been reported to produce triacsins (Streptomyces aureofaciens29 and Salinispora cortesiana138), which possess Tri17 homologs with 89%/93% and 80%/86% identity/similarity, respectively. Tri17_Aur, reported in this work (Supplemental Fig. 14), is depicted by the same node as Tri17 due to their high sequence similarity. b) Phylogenetic tree of Tri17 suggests that Tri17 is located at a different clade than CreM and Aha11 (see Supplementary Fig. 57 for a larger representation of the phylogenetic tree). The Tri17 clade shown in yellow was putatively annotated to include homologs with >50% sequence identity. The CreM clade is shown in purple consisting of CreM and Aha11. Structural homologs of Tri17 from the Dali server are colored green. Other proteins in black correspond to BLAST results with less than 50% sequence identity with respect to Tri17. c) Sequence alignment between Tri17 and its homologs. The residues highlighted in yellow correspond to putative substrate binding residues.
Extended Data Fig. 6 Proposed reaction mechanism for azidation of 17 by Tri17.
Tri17 utilizes ATP to activate nitrite to generate a nitroso-AMP intermediate that is subjected to nucleophilic attack by 17. After tautomerization, compound 18 undergoes dehydration presumably through acid-base catalysis mediated by His229 to generate 19.
Supplementary information
Supplementary Data 1
Computational data from Supplementary Figs. 19–25.
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Source Data Extended Data Fig. 1/Table 1
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Del Rio Flores, A., Zhai, R., Kastner, D.W. et al. Enzymatic synthesis of azide by a promiscuous N-nitrosylase. Nat. Chem. 16, 2066–2075 (2024). https://doi.org/10.1038/s41557-024-01646-2
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DOI: https://doi.org/10.1038/s41557-024-01646-2