Iridoid

Image:Iridomyrmecin.svg

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.[http://dynamo.ku.dk/calendar/events/fgf/ Gene discovery in the biosynthesis of medicinal alkaloids in Catharanthus roseus] 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|date=April 2016}} The variable checkerspot butterfly also contains iridoids obtained through its diet which act as a defense against avian predators.{{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}} 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.{{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}}

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

Biosynthesis

The iridoid ring scaffold is synthesized, in plants, by the enzyme iridoid synthase.{{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 }} 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.

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.

Iridoid

Occurrence

Biosynthesis

See also

References

Further reading