Iridoid

{{Short description|Class of chemical compounds}} thumb|Chemical structure of iridomyrmecin '''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.<ref>[http://dynamo.ku.dk/calendar/events/fgf/ Gene discovery in the biosynthesis of medicinal alkaloids in Catharanthus roseus]</ref> 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== {{multiple image|caption_align=center|header_align=center | width = 200 | image1=Aucubin skeletal.svg | alt1=Aucubin | image2=Catalpol skeletal.svg | alt2=Catalpol | footer = Aucubin and catalpol are two of the most common iridoids in the plant kingdom. }} The iridoids produced by plants act primarily as a defense against herbivores or against infection by microorganisms.{{citation needed|date=April 2016}} The variable checkerspot butterfly also contains iridoids obtained through its diet which act as a defense against avian predators.<ref>{{cite journal|last1=Bowers|first1=M. Deane|title=Unpalatability as a Defense Strategy of Western Checkerspot Butterflies (Euphydryas scudder, Nymphalidae)|journal=Evolution|date=March 1981|volume=35|issue=2|pages=367–375|doi=10.2307/2407845|pmid=28563381|jstor=2407845}}</ref> 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|date=April 2016}} Iridoids are prevalent in the plant subclass Asteridae, such as Ericaceae, Loganiaceae, Gentianaceae, Rubiaceae, Verbenaceae, Lamiaceae, Oleaceae, Plantaginaceae, Scrophulariaceae, Valerianaceae, and Menyanthaceae.<ref name=Tundis>{{cite journal | doi = 10.2174/138955708783955926| title = Biological and Pharmacological Activities of Iridoids: Recent Developments| journal = Mini-Reviews in Medicinal Chemistry| volume = 8| issue = 4| pages = 399–420| year = 2008| last1 = Tundis| first1 = Rosa| last2 = Loizzo| first2 = Monica| last3 = Menichini| first3 = Federica| last4 = Statti| first4 = Giancarlo| last5 = Menichini| first5 = Francesco}}</ref>

Iridoids have been the subject of research into their potential biological activities.<ref name=Tundis/><ref>{{cite journal | doi = 10.1248/cpb.55.159| pmid = 17268091| title = Naturally Occurring Iridoids. A Review, Part 1| journal = Chemical & Pharmaceutical Bulletin| volume = 55| issue = 2| pages = 159–222| year = 2007| last1 = Dinda| first1 = Biswanath| last2 = Debnath| first2 = Sudhan| last3 = Harigaya| first3 = Yoshihiro| doi-access = free}}</ref>

==Biosynthesis== The iridoid ring scaffold is synthesized, in plants, by the enzyme iridoid synthase.<ref name="Geu-Flores2012">{{Cite journal | last1 = Geu-Flores | first1 = F. | last2 = Sherden | first2 = N. H. | last3 = Courdavault | first3 = V. | last4 = Burlat | first4 = V. | last5 = Glenn | first5 = W. S. | last6 = Wu | first6 = C. | last7 = Nims | first7 = E. | last8 = Cui | first8 = Y. | last9 = O'Connor | first9 = S. E. | doi = 10.1038/nature11692 | title = An alternative route to cyclic terpenes by reductive cyclization in iridoid biosynthesis | journal = Nature | volume = 492 | issue = 7427 | pages = 138–142 | year = 2012 | pmid = 23172143| bibcode = 2012Natur.492..138G }}</ref> 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.<ref name="Geu-Flores2012"/>

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== *Deacetylasperulosidic acid, an iridoid compound found in a few medicinal plants, such as ''Morinda citrifolia''

==References== {{reflist}}

==Further reading== {{cite journal|last1=Moreno-Escobar|first1=Jorge A.|last2=Alvarez|first2=Laura|last3=Rodrıguez-Lopez|first3=Veronica|last4=Marquina Bahena|first4=Silvia|title=Cytotoxic glucosydic iridoids from Veronica Americana|journal=Phytochemistry Letters|date=2 March 2013|volume=6|issue=4|pages=610–613|doi=10.1016/j.phytol.2013.07.017}}

Category:Iridoids