An alternative route to cyclic terpenes by reductive cyclization in iridoid biosynthesis

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 activities. Additionally, certain iridoids are used as sex pheromones in agriculturally important species of aphids, a fact that has underpinned innovative and integrated pest management strategies. 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 agricultural and pharmaceutical applications.

Deciphering the route to cyclic monoterpenes in Chrysomelina leaf beetles: source of new biocatalysts for industrial application?

“…The iridoid synthase is a member of the Rossmann-fold NAD(P) + -binding protein superfamily. It cyclizes 8-oxogeranial to cis-trans-iridodials and cis-trans-nepetalactol under the consumption of NAD(P)H [32]. In a proteomic analysis, however, no protein similar to the iridoid synthase from C. roseus could be detected in the secretions of iridoid-producing Chrysomelina larvae.…”

“…The iridoid pathway starts with its key intermediate, geraniol diphosphate, and comprises a number of oxidation, reduction, glycosylation, and methylation reactions [5,29]. The biosynthesis of secologanin through the intermediates iridodial and iridotrial in the Madagascar periwinkle, Catharanthus roseus, has been best understood [30][31][32][33][34][35]. The pathway in C. roseus is organized in a complex manner, with the enzymes localized in different cell types and subcellular compartments [36,37].…”

The occurrence of progesterone 5β-reductase is not limited to the angiosperms: a functional gene was identified in Picea sitchensis and expressed in Escherichia coli

“…Minor peaks do not represent pregnane-like compounds on the basis of a MS data comparison P5ßR activity. Geu-Flores et al (2012) reported that P5ßR activity is expressed in C. roseus during the biosynthesis of a bicyclic monoterpene, namely the iridoid compound nepetalactol (Geu-Flores et al 2012. Subsequently, it has been demonstrated that the precursor substance 8-oxogeranial is converted in a NADPH-dependent manner by a number of recombinant P5ßR enzymes.…”

Dual Catalytic Activity of a Cytochrome P450 Controls Bifurcation at a Metabolic Branch Point of Alkaloid Biosynthesis in Rauwolfia serpentina

“…Comparative transcriptomics and metabolomics have been widely and successfully used to elucidate the genes of plant alkaloid metabolism 15, 16, 17, 18. Here we report the use of an available Rauwolfia transcriptome (Medicinal Plant Genomics Resource (MPGR), http://medicinalplantgenomics.msu.edu/index.shtml) to identify and characterize a cytochrome P450 enzyme (CYP82S18) that hydroxylates vinorine to generate vomilenine, which we show exists as rapidly interconverting epimers in solution.…”