Beetles Do It Differently: Two Stereodivergent Cyclisation Modes in Iridoid‐Producing Leaf‐Beetle Larvae

Abstract

Larvae of the Chrysomelina species Phaedon cochleariae, Hydrothassa marginella, Phratora vulgatissima, Gastrophysa viridula, Gastrophysa atrocyanea, Gastrophysa cyanea and Gastrophysa polygoni produce the iridoid chrysomelidial (1) to defend themselves against predators. Feeding experiments with a deuterated precursor ([(2)H(5)]8-hydroxygeraniol 9) and in vitro isotope exchange experiments with defensive secretion in (2)H(2)O revealed differences in the cyclisation of the ultimate precursor 8-oxogeranial (8) to 1, between members of the genus Gastrophysa and all other species. In P. cochleariae, H. marginella and P. vulgatissima 1 is most likely produced by a Rauhut-Currier-type cyclisation via a "transoid dienamine", with loss of a single deuterium atom from C(4) of the precursor. In contrast, members of the genus Gastrophysa cyclise 8 via a "cisoid dienamine" intermediate, with exchange of all three deuterium atoms from the methyl group at C(3). To study whether the different cyclisation modes influence the stereochemistry of 1, the absolute configuration of 1 of the larvae was determined by GC-MS on a chiral column. In accordance with literature (J. Meinwald, T. H. Jones, J. Am. Chem. Soc. 1978, 100, 1883 and N. Shimizu, R. Yakumaru, T. Sakata, S. Shimano, Y. Kuwahara, J. Chem. Ecol. 2012, 38, 29), we found (5S,8S)-chrysomelidial (1) in H. marginella and P. vulgatissima, but P. cochleariae and all investigated members of the genus Gastrophysa synthesise (5R,8R)-chrysomelidial (1).

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

“…After translocation of 8-hydroxygeraniol-O-β-d-glucoside into the defensive glands, its final transformation into iridoids occurs in the secretions. This transformation involves the hydrolysis of the glucoside and oxidation of the two primary hydroxy groups to produce the dialdehyde 8-oxogeranial, which is followed by a cyclization [52,54,39]. In general, hydrolysis of glycosides occurs in the metabolism of all organisms.…”

“…Interestingly, with the same precursor, isotopic tracing studies have shown that there are two mechanistically different cyclization modes in different leaf beetle groups [39]. When deuterium atom labeled [ 2 H 5 ]Ger-8-OH was used for the feeding experiments, the precursor lost a single deuterium atom from C(4) in P. cochleariae, Hydrothassa marginella, and Phratora vulgatissima.…”

“…The detailed biochemical characterization of this protein in vitro, however, is still in progress. As the enzymes from Chrysomelina leaf beetles are known to differ in their stereoselectivity [39], a variety of cyclic products can be produced that could serve as building blocks for new metabolites of pharmacological relevance. Hence, deciphering the reaction mechanism would make the enzyme prospectively a candidate for application in drug development.…”

“…In particular, the juveniles of Chrysomelina beetles evolved specialized pair-wise exocrine glands (composed of a reservoir with adhered glandular cells) on their dorsal segments to release iridoids in droplets as defensive secretions. Considering the evolutionary aspects of Chrysomelina beetles, it has been shown that iridoid de novo synthesis precedes the sequestration of plant-derived secondary metabolites The absolute configuration of chrysomelidial has been measured by gas chromatography-mass spectrometry (GC-MS) on a chiral column (adapted from Kunert et al [39]). de, diastereomeric excess.…”

Die Enzymologie organischer Umwandlungen: Namensreaktionen in biologischen Systemen

“…Iridoidalkaloide sind bicyclische Monoterpene, die in Pflanzen und Insekten vorkommen und zur Verteidigung gegen Räuber dienen. Für die Cyclisierung von 8‐Oxogeranial ( 136 ) zu Iridodial ( 137 ), eine wichtige Zwischenverbindung in der Biosynthese der Iridoide, in Chrysomelina ‐Arten wurde eine Rauhut‐Currier‐Reaktion vorgeschlagen (Abbildung a) . Allerdings könnte die Biosynthese von Iridoiden in Käfern ganz anders ablaufen als in Pflanzen.…”