Content area
Full Text
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 activities1-4. 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 agricultural5-8 and pharmaceutical1-4,9 applications.
(ProQuest: ... denotes formula omitted.)
The crucial step in the biosynthesis of all cyclic terpenes is the cyclization step that gives rise to the individual ring systems. In all kingdoms of life, this step has been shown to be catalysed by terpene cyclases, which are terpene synthases that use polyprenyl diphosphates such as geranyl diphosphate to produce a cationic species that is subsequently cyclized and rearranged to formone of hundreds of possible ring structures10,11. However, the cyclization step that leads to the characteristic 5-6 bicyclic scaffold of iridoids (Fig. 1a) is markedly different. Feeding studies have suggested that the direct precursor of all iridoids in plants is not geranyl diphosphate, but the linear monoterpene 10-oxogeranial12-15. These studies also suggest that the bicyclic compound nepetalactol is the general precursor of the more derivatized iridoids. Assays using crude plant extracts have indicated that 10-oxogeranial can be converted to nepetalactol in aNADH/NADPHdependent manner (Fig. 1b)16. Nevertheless, the molecular identity of the enzyme...