Abstract
The iridoids comprise a large family of distinctive bicyclic monoterpenes that possess a wide range of pharmacological activities, including anticancer, anti-inflammatory, antifungal and antibacterial activities1,2,3,4. Additionally, certain iridoids are used as sex pheromones in agriculturally important species of aphids, a fact that has underpinned innovative and integrated pest management strategies5. To harness the biotechnological potential of this natural product class, the enzymes involved in the biosynthetic pathway must be elucidated. Here we report the discovery of iridoid synthase, a plant-derived enzyme that generates the iridoid ring scaffold, as evidenced by biochemical assays, gene silencing, co-expression analysis and localization studies. In contrast to all known monoterpene cyclases, which use geranyl diphosphate as substrate and invoke a cationic intermediate, iridoid synthase uses the linear monoterpene 10-oxogeranial as substrate and probably couples an initial NAD(P)H-dependent reduction step with a subsequent cyclization step via a Diels–Alder cycloaddition or a Michael addition. Our results illustrate how a short-chain reductase was recruited as cyclase for the production of iridoids in medicinal plants. Furthermore, we highlight the prospects of using unrelated reductases to generate artificial cyclic scaffolds. Beyond the recognition of an alternative biochemical mechanism for the biosynthesis of cyclic terpenes, we anticipate that our work will enable the large-scale heterologous production of iridoids in plants and microorganisms for agricultural5,6,7,8 and pharmaceutical1,2,3,4,9 applications.
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Change history
05 December 2012
The two stereoisomers in Fig. 3a were corrected.
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Acknowledgements
The present study was funded by the Danish Council for Independent Research (Natural Sciences) through Postdoctoral Fellowship 10-082858 granted to F.G.-F. Additional funding was received from the NIH (GM074820), the j004561SRC (BB/J004561/1 and BB/J009091/1) and the John Innes Foundation. This work was conducted under Defra Plant Health Licence PHSI 449/6612(07/2011). W.S.G. gratefully acknowledges a National Science Foundation Predoctoral Fellowship. Y.C. would like to thank the NSF for grant DMS-1209112. K. R. Chauhan is acknowledged for providing initial nepetalactol samples. We thank S. Fairhurst, L. Hill and A. Jones for NMR, LC–MS and GC–MS assistance, respectively. J. Ward and N. Hawkins are acknowledged for the GC–MS-based high-resolution mass spectra (HRMS) of synthesized compounds. R. Buell and E. Góngora-Castillo provided many helpful discussions on the transcriptome data. The purified EasA protein was provided J. Z. Cheng, to whom we are deeply indebted. S.E.O. receives salary support through a synergy initiative between the John Innes Centre and the University of East Anglia.
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F.G.-F. and S.E.O. conceived the project; C.W., Y.C. and F.G.-F. constructed the G10H mutual ranking list; E.N. cloned the iridoid synthase from cDNA; F.G.-F. and W.S.G. expressed the recombinant protein; N.H.S. performed all chemical synthesis; F.G.-F. carried out all enzyme assays, enzyme-based synthesis, VIGS experiments, and feeding experiments; V.C. performed the sub-cellular localization studies; V.B. carried out the cellular localization studies; F.G.-F. and S.E.O. drafted the manuscript; and S.E.O. acted as overall study director.
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This file contains Supplementary Figures 1-8, Supplementary Tables 1-2, Supplementary Text, which explains the methods used to synthesize the chemicals used in the main paper and additional references. (PDF 2146 kb)
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Geu-Flores, F., Sherden, N., Courdavault, V. et al. An alternative route to cyclic terpenes by reductive cyclization in iridoid biosynthesis. Nature 492, 138–142 (2012). https://doi.org/10.1038/nature11692
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DOI: https://doi.org/10.1038/nature11692
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