Iridoid Monoterpenes
Iridoid monoterpenes
About 600 iridoid monoterpenes are known. They are mostly glycosides; about 100 non-glycosides have been described
This is a specialized pathway and differs from the usual monoterpene pathway.
Although these compounds are mostly found in plants, the name comes from iridomyrmecin, a compound isolated from ants of the genus Iridomyrmex.
Iridoid monoterpenes also are known from other insects, such as aphids.
Biosynthetic pathways
Iridoid monoterpenes are not associated with oil glands as is the case for more usual monoterpenes; sometimes as much as 20-25% of the plant weight is iridoids.
It is not established whether they are derived from the DOXP or MVA pathway, but the fact that MVA is incorporated at higher levels than for other monoterpenes and that the compounds are not associated with special glands suggests that MVA origin may be possible.
However, they are derived from geranyl pyrophosphate or diphosphate (GPP) (via IPP and DMAPP) as is true for other monoterpenes.
There are three distinct pathways leading to these compounds from the original precursors:
1) most important one from 10-hydroxygeraniol to iridodial to iridotrial and ending in deoxyloganic acid
2) a second from 10-hydroxygeraniol, via epi-iridodial and epi-iridotrial and ending in epi-deoxyloganic acid
3) a third route begins with 10-hydroxycitronellol, but also involves iridodial.
Route one is involved in the formation of deoxyloganic acid and secologanin. This series give rise to monoterpene derived alkaloids, or indole alkaloids and is very important for that reason.
the mechanism of ring closure is not known with certainty
scrambling of radioactive label in 3 and 11 occurs in some groups of compounds and not in others --- this usually does happen in the synthesis of deoxyloganin by this route.
Loganin to seco-loganin
loganin is the key intermediate to a large number of iridoid monoterpenes as well as indole alkaloids
how the ring cleavage occurs to yield seco-loganin is not well understood
Restricted distribution
Because this is a relatively long pathway and involves some steps not commonly seen in other pathways ... it would not be expected to have arisen often in the course of evolution.
Iridoid monoterpenes found in about 50 plant families.
the families that possess these compounds belong to two major groups ... now in the course of being reevaluated.
Biological activity
defensive compounds by ants Iridomyrmex
exciting activity on cats
aphid pheromones
feeding stimulants for some butterflies, e.g., checkerspot butterflies on iridoid containing plants
sequestered by larvae of some insects that feed on Penstemon species, for example.
medical .. Valeriana officinalis used as a sedative
bitters .. mostly based on Gentiana species
Cyclization of 10-oxogeranial and formation of iridodial, iridotrial, and loganin
Deoxyloganic acid biosynthesis
Biosynthesis involving 10-hydroxycitranellalol; nepetalactone biosynthesis
Biosynthesis of the epi-iridodial series
Lecture Slides
Plants with Iridoid Monoterpenes
© David S. Seigler, Integrative Biology 425, Plant Secondary Metabolism, Department of Plant
Biology, 265 Morrill Hall, 505 S. Goodwin Ave., University of Illinois, Urbana, Illinois 61801, USA.
217-333-7577. seigler@life.uiuc.edu.
