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. Iridoids are typically found in plants as

glycoside In chemistry, a glycoside is a molecule in which a sugar is bound to another functional group via a glycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. ...

s, most often bound to

glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...

. The chemical structure is exemplified by

iridomyrmecin Iridomyrmecin is a defensive chemical, classified as an 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-oxoge ...

, a defensive chemical produced by the ant genus ''

Iridomyrmex ''Iridomyrmex'' is a genus of ants called rainbow ants (referring to their blue-green iridescent sheen) first described by Austrian entomologist Gustav Mayr in 1862. He placed the genus in the subfamily Dolichoderinae of the family Formicida ...

'', 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 Oleuropein is a glycosylation, glycosylated seco-iridoid, a type of polyphenol, phenolic bitter compound found in green olive skin, flesh, seeds, and leaves. The term oleuropein is derived from the botanical name of the olive tree, ''Olea europaea ...

and

amarogentin Amarogentin is a chemical compound found in gentian (''Gentiana lutea'') or in ''Swertia chirata''. Gentian root has a long history of use as a herbal bitter in the treatment of digestive disorders and is an ingredient of many proprietary medic ...

Occurrence

The iridoids produced by plants act primarily as a defense against herbivores or against infection by microorganisms. The variable checkerspot butterfly also contains iridoids obtained through its diet which act as a defense against avian predators. 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. Iridoids are prevalent in the plant subclass Asteridae, such as

Ericaceae The Ericaceae () are a Family (biology), family of flowering plants, commonly known as the heath or heather family, found most commonly in acidic and infertile growing conditions. The family is large, with about 4,250 known species spread acros ...

Loganiaceae The Loganiaceae are a family of flowering plants classified in order Gentianales. The family includes up to 13 genera, distributed around the world's tropics. There are not any great morphological characteristics to distinguish these taxa from ot ...

Gentianaceae Gentianaceae is a family of flowering plants of 105 genera and about 1600 species. Etymology The family takes its name from the genus ''Gentiana'', named after the Illyrian king Gentius. Distribution Distribution is cosmopolitan (species), cosm ...

Rubiaceae Rubiaceae () is a family (biology), family of flowering plants, commonly known as the coffee, madder, or bedstraw family. It consists of terrestrial trees, shrubs, lianas, or herbs that are recognizable by simple, opposite leaves with Petiole ( ...

Verbenaceae The Verbenaceae ( ), the verbena family or vervain family, is a family of mainly tropical flowering plants. It contains trees, shrubs, and herbs notable for heads, spikes, or clusters of small flowers, many of which have an aromatic smell. T ...

Lamiaceae The Lamiaceae ( ) or Labiatae are a family (biology), family of flowering plants commonly known as the mint, deadnettle, or sage family. Many of the plants are aromatic in all parts and include widely used culinary herbs like basil (herb), ba ...

Oleaceae Oleaceae, also known as the olive family or sometimes the lilac family, is a taxonomic family of flowering shrubs, trees, and a few lianas in the order Lamiales. It presently comprises 28 genera, one of which is recently extinct.Peter S. Gree ...

Plantaginaceae Plantaginaceae, the plantain family or veronica family, is a large, diverse family (biology), family of flowering plants in the order Lamiales that includes common flowers such as Antirrhinum, snapdragon and Digitalis, foxglove. It is unrelated ...

Scrophulariaceae The Scrophulariaceae are a family of flowering plants, commonly known as the figwort family. The plants are annual and perennial herbs, as well as shrubs. Flowers have bilateral (zygomorphic) or rarely radial (actinomorphic) symmetry. The Scr ...

Valerianaceae The Valerianaceae Batsch, the valerian family, was a family of flowering plants that is now considered part of the Caprifoliaceae The Caprifoliaceae or honeysuckle family is a clade of dicotyledonous flowering plants consisting of about 860 s ...

, and

Menyanthaceae Menyanthaceae is a family of aquatic and wetland plants in the order Asterales. There are approximately 60-70 species in six genera distributed worldwide. The simple or compound leaves arise alternately from a creeping rhizome. In the submers ...

. Iridoids have been the subject of research into their potential biological activities.

Biosynthesis

The iridoid ring scaffold is synthesized, in plants, by the enzyme iridoid synthase. 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 Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses, which require N ...

-dependent reduction step followed by a cyclization step that occurs through either a Diels-Alder reaction or an intramolecular

Michael addition In organic chemistry, the Michael reaction or Michael 1,4 addition is a reaction between a Michael donor (an enolate or other nucleophile) and a Michael acceptor (usually an α,β-unsaturated carbonyl) to produce a Michael adduct by creating a c ...

. The iridoid

secologanin Secologanin is a secoiridoid monoterpene synthesized from geranyl pyrophosphate in the mevalonate pathway. Secologanin then proceeds with dopamine or tryptamine to form ipecac and terpene indole alkaloids, respectively. Biosynthesis Secologa ...

can react with

tryptamine Tryptamine is an indolamine metabolite of the essential amino acid tryptophan. The chemical structure is defined by an indole—a fused benzene and pyrrole ring, and a 2-aminoethyl group at the second carbon (third aromatic atom, with the firs ...

to form the

indole alkaloid Indole alkaloids are a class of alkaloids containing a structural moiety of indole; many indole alkaloids also include isoprene groups and are thus called terpene indole or secologanin tryptamine alkaloids. Containing more than 4100 known differ ...

strictosidine Strictosidine is a natural chemical compound and is classified as a glucoalkaloid and a vinca alkaloid. It is formed by the Pictet–Spengler condensation reaction of tryptamine with secologanin, catalyzed by the enzyme strictosidine synthase. ...

, the precursor of many biologically active compounds such as

strychnine Strychnine (, , American English, US chiefly ) is a highly toxicity, toxic, colorless, bitter, crystalline alkaloid used as a pesticide, particularly for killing small vertebrates such as birds and rodents. Strychnine, when inhaled, swallowed, ...

yohimbine Yohimbine, also known as quebrachine, is an indole alkaloid derived from the bark of the African tree '' Pausinystalia johimbe'' (yohimbe); also from the bark of the unrelated South American tree '' Aspidosperma quebracho-blanco''. Yohimbine is ...

vinca alkaloids ''Vinca'' alkaloids are a set of anti-mitotic and anti-microtubule alkaloid agents originally derived from the periwinkle plant '' Catharanthus roseus'' (basionym ''Vinca rosea'') and other ''vinca'' plants. They block beta-tubulin polymeriza ...

, and

ellipticine Ellipticine is a tetracyclic alkaloid first extracted from the tree species ''Ochrosia elliptica'' and ''Rauvolfia sandwicensis'' which inhibits the enzyme topoisomerase II via intercalation (biochemistry), intercalative binding to DNA. ...

Occurrence

Biosynthesis

See also

References

Further reading