Terpenoid indole alkaloids geraniol, pathway

Fig. (3). Compartmentalization of the biosynthetic pathway of terpenoid indole alkaloids in plant cells. G10H geraniol 16-hydroxylase SLS secologanin synthase TDC tryptophan decarboxylase STR strictosidine synthase SGD strictosidine P-D-glucosidade T16H tabersonine 16-hydroxylase OMT S-adenosyl - L-methionine 16-hydroxytabereonine - 16-O-methyltransferase NMT S-adenosyl - /.-methionine 16-methoxy - 2,3-dihydro-3-hydroxytabersonine - A -methyltransferase D4H desacetoxy vindoline 4-hydroxylase DAT acetylcoenzyme A 4-O-deacetylvindoline 4-O-aeetyltransferase PRX peroxidase.

In the present review we divide the pathway leading to the Catharanthus alkaloids into five parts (Fig. 2). The first two concern the biosynthesis of tryptophan and geraniol diphosphate they are similar to (or even part of) primary metabolism and occur in all plant species. Whether these pathways in C. roseus are differently regulated, or whether even an additional pathway exists parallel to the normal primary metabolism, is a question not yet answered. The third and the fourth part coneern the steps from tryptophan to tryptamine and from geraniol to secologanin, respectively. Both pathways occur also in other plants, including plants that do not produce terpenoid indole alkaloids. The fifth part is the condensation of secologanin and tryptamine to strictosidine and the subsequent conversion into a plethora... 

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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