Pathways terpene indole

Biosynthesis of some classes of terpene indole alkaloids is well understood. In certain cases, many of the enzymes that are responsible for biosynthesis have been cloned and mechanistically studied. In other cases, biosynthesis pathway is only proposed based on the results of feeding studies with isotopically labeled substtates and from the structures of isolated biosynthetic intermediates. All terpene indole alkaloids are derived from tryptophan and the iridoid terpene secologanin (Fig. 14.11). Tryptophan decarboxylase, a pyridoxal-dependent enzyme [29], converts tryptophan to tryptamine [30]. The following strictosidine synthase-catalyzed Mannich reaction connects ttyptamine and secologanin to yield strictosidine [31]. The Apocynaceae, Loganiaceae, Rubiaceae, and Nyssaceae families of plants each produce terpene indole alkaloids with dramatically diverse structures [32-34]. The mechanisms and control of... 

In regard to the experimental evidence available, a substantial number of reports on the chemical constituents of the plant are available, but much less work has been done with the pharmacological properties. Kasture et al., however, made the important observation that a triterpene isolated from R. cordifolia induces anxiety in rodents, an effect accompanied with an increase in serotonin contents in the brain (30). An interesting development from that observation would be to explore further the molecular-pharmacological pathway and the effect of this agent on the serotoninergic system because terpenes, compared with indole alkaloids, are seldom reported for serotoninergic activities. 

Iridoids are a large and stmcturally diverse class of secondary metabolites of monoterpenoid origin [10, 11]. Their sttuc-tural parent system is the iridane skeleton, which is derived from geraniol (6) by a cyclization pathway that is mechanistically different to the cyclization reactions that are usually found in classical terpene chemistry [1, 12], Further enzymatic transformations then give the fundamental iridoid skeleton or the thereof derived secoiridoid motive (with an alternative connectivity as shown in Scheme 6.10), which often serves as a building block for the synthesis of more complex monoterpenoid indole alkaloids [1]. Noteworthy, hereby the isoprene mle is often not fulfilled anymore as the terpenoid parts of such compounds sometimes contain nine carbon atoms only due to a decarboxylation step somewhere in the sequence. Very often iridoids and secoiridoids are present as glycosides in nature. 

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