Iridoid
Iridoids are a type of monoterpenoids in the general form of cyclopentanopyran, found in a wide variety of plants and some animals. They are biosynthetically derived from 8-oxogeranial.[1] Iridoids are typically found in plants as glycosides, most often bound to glucose.
The chemical structure is exemplified by iridomyrmecin, a defensive chemical produced by the ant genus Iridomyrmex, for which iridoids are named. Structurally, they are bicyclic cis-fused cyclopentane-pyrans. Cleavage of a bond in the cyclopentane ring gives rise to a subclass known as secoiridoids, such as oleuropein and amarogentin.
Occurrence
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The iridoids produced by plants act primarily as a defense against herbivores or against infection by microorganisms.[citation needed] The variable checkerspot butterfly also contains iridoids obtained through its diet which act as a defense against avian predators.[2] To humans and other mammals, iridoids are often characterized by a deterrent bitter taste.
Aucubin and catalpol are two of the most common iridoids in the plant kingdom.[citation needed] Iridoids are prevalent in the plant subclass Asteridae, such as Ericaceae, Loganiaceae, Gentianaceae, Rubiaceae, Verbenaceae, Lamiaceae, Oleaceae, Plantaginaceae, Scrophulariaceae, Valerianaceae, and Menyanthaceae.[3]
Iridoids have been the subject of research into their potential biological activities.[3][4]
Biosynthesis
[edit | edit source]The iridoid ring scaffold is synthesized, in plants, by the enzyme iridoid synthase.[5] In contrast with other monoterpene cyclases, iridoid synthase uses 8-oxogeranial as a substrate. The enzyme uses a two-step mechanism, with an initial NADPH-dependent reduction step followed by a cyclization step that occurs through either a Diels-Alder reaction or an intramolecular Michael addition.[5]
The iridoid secologanin can react with tryptamine to form the indole alkaloid strictosidine, the precursor of many biologically active compounds such as strychnine, yohimbine, vinca alkaloids, and ellipticine.
See also
[edit | edit source]- Deacetylasperulosidic acid, an iridoid compound found in a few medicinal plants, such as Morinda citrifolia
References
[edit | edit source]- ^ Gene discovery in the biosynthesis of medicinal alkaloids in Catharanthus roseus
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Further reading
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