Terpene synthases typically form complex molecular scaffolds by concerted activation and cyclization of linear starting materials in a single enzyme active site. Here we show that iridoid synthase, an atypical reductive terpene synthase, catalyzes the activation of its substrate 8-oxogeranial into a reactive enol intermediate, but does not catalyze the subsequent cyclization into nepetalactol. This discovery led us to identify a class of nepetalactol-related short-chain dehydrogenase enzymes (NEPS) from catmint (Nepeta mussinii) that capture this reactive intermediate and catalyze the stereoselective cyclisation into distinct nepetalactol stereoisomers. Subsequent oxidation of nepetalactols by NEPS1 provides nepetalactones, metabolites that are well known for both insect-repellent activity and euphoric effects in cats. Structural characterization of the NEPS3 cyclase reveals that it binds to NAD+ yet does not utilize it chemically for a non-oxidoreductive formal [4 + 2] cyclization. These discoveries will complement metabolic reconstructions of iridoid and monoterpene indole alkaloid biosynthesis.
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The sequences of N. mussinii NEPS enzyme have been deposited in GenBank/EMBL/DDBJ with the accession codes MG677124 (NmNEPS1), MG677125 (NmNEPS2) and MG677126 (NmNEPS3). The NAD+ bound NmNEPS3 (7S-cis-cis-nepetalactol cyclase) X-ray structure has been deposited in the PDB with the accession code 6F9Q. The mass spectrometry proteomics data have been deposited to the ProteomeXchange Consortium via the PRIDE partner repository with the data set identifier PXD008704. Detailed experimental procedures and can be found in the Supplementary Information. The authors declare that all other data supporting the findings of this study are available within this article and its Supplementary Information or from the authors upon reasonable request.
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We acknowledge funding from UK Biotechnological and Biological Sciences Research Council (BBSRC) and Engineering and Physical Sciences Research Council (EPSRC) joint-funded OpenPlant Synthetic Biology Research Centre (BB/L014130/1) and from the National Science Foundation Plant Genome Research Program (IOS- 1444499). For the X-ray data collection, we acknowledge Diamond Light Source for access to beamline I03 under proposal MX13467, with support from the European Community’s Seventh Framework Program (FP7/2007–2013) under grant agreement 283570 (BioStruct-X). We are grateful to: P. Brett (John Innes Centre) for assistance with GC–MS analysis and M. Vigoroux (John Innes Centre) for assistance with proteome annotations. We also thank K. Houk, J. Fell, H. Kries and D. Whitaker for discussions concerning the iridoid synthase and cyclization mechanisms.
A UK patent application has been submitted based on the work reported here (GB1808663.7).
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Lichman, B.R., Kamileen, M.O., Titchiner, G.R. et al. Uncoupled activation and cyclization in catmint reductive terpenoid biosynthesis. Nat Chem Biol 15, 71–79 (2019). https://doi.org/10.1038/s41589-018-0185-2
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