Iridoids are a type of

monoterpenoids Monoterpenes are a class of terpenes that consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear (acyclic) or contain rings (monocyclic and bicyclic). Modified terpenes, such as those containing oxygen funct ...

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 glycoside ...

s, most often bound to

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...

. The chemical structure is exemplified by

iridomyrmecin Iridomyrmecin is a defensive chemical, classified as an iridoid, isolated from ants of the genus ''Iridomyrmex''. It has also evolved into a sex pheromone in wasps such as ''Leptopilina'', with host species using the smell of iridomyrmecin as a wa ...

, 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 Formici ...

'', 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 glycosylated seco-iridoid, a type of 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''. Because of its bitter ...

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 med ...

Occurrence

The iridoids produced by plants act primarily as a defense against herbivores or against infection by microorganisms. The

variable checkerspot The variable checkerspot or Chalcedon checkerspot (''Euphydryas chalcedona'') is a butterfly in the family Nymphalidae. It is found in western North America, where its range stretches from Alaska in the north to Baja California in the south and e ...

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 Aucubin is an iridoid glycoside. Iridoids are commonly found in plants and function as defensive compounds. Iridoids decrease the growth rates of many generalist herbivores. Natural occurrences Aucubin, as other iridoids, is found in asterids ...

and

catalpol Catalpol is an iridoid glucoside. This natural product falls in the class of iridoid glycosides, which are simply monoterpenes with a glucose molecule attached. Natural occurrence First isolated in 1962, catalpol was named for plants in the ge ...

are two of the most common iridoids in the plant kingdom. Iridoids are prevalent in the plant subclass

Asteridae ''Asteridae'' is an obsolete botanical name at the rank of subclass. Composition of the subclass has also varied; however, by definition it always includes the family Asteraceae (Compositae). In the modern APG IV system of classification, asterid ...

, such as

Ericaceae The Ericaceae are a 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 c.4250 known species spread across 124 genera, making it th ...

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 103 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. Characteri ...

Rubiaceae The Rubiaceae are a 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 interpetiolar stipules a ...

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. The ...

, Lamiaceae,

Oleaceae Oleaceae, also known as the olive 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. Green. 2004. "Oleaceae". pages 2 ...

Plantaginaceae Plantaginaceae, the plantain family, is a large, diverse family of flowering plants in the order Lamiales that includes common flowers such as snapdragon and foxglove. It is unrelated to the banana-like fruit also called "plantain." In older cl ...

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. Plants are generally herbaceous, and their foliage often has a strong, disagreeable odor. They are found native in ...

, 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 cyclase Monoterpenes are a class of terpenes that consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear (acyclic) or contain rings (monocyclic and bicyclic). Modified terpenes, such as those containing oxygen funct ...

s, 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 In organic chemistry, the Michael reaction or Michael 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 carbon ...

Loganic acid Loganic acid is 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-oxogeranial. Iridoids are typically foun ...

is an iridoid substrate converted to

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. ...

which reacts 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 f ...

, eventually leading to the

indole alkaloids 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 differe ...

which include many biologically active compounds such as strychnine,

yohimbine Yohimbine (), also known as quebrachine, is an indole alkaloid derived from the bark of the African tree '' Pausinystalia johimbe''; also from the bark of the unrelated South American tree ''Aspidosperma quebracho-blanco''. Yohimbine is an α2 ...

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 polymerizatio ...

, and

ellipticine Ellipticine is an alkaloid first extracted from trees of the species '' Ochrosia elliptica'' and ''Rauvolfia sandwicensis'', which inhibits the enzyme topoisomerase II via intercalative binding to DNA. Natural occurrence and synthe ...

Occurrence

Biosynthesis

See also

References

Further reading