Geraniol metabolic pathways

From activated isoprene, the metabolic pathway leads via dimerization to activated geraniol (1 = 2) and then to activated farnesol = 3). At this point, the pathway divides into two. Further extension of farnesol leads to chains with increasing numbers of isoprene units—e.g., phytol (1 = 4), dolichol (1 = 14-24), and rubber = 700-5000). The other pathway involves a head-to-head linkage between two farnesol residues, giving rise to squalene (1 = 6), which, in turn, is converted to cholesterol (1 = 6) and the other steroids. 

Metabolic pathways of citronellol (258), citronellal (261), geraniol (271), nerol (272), citral [neral (275) and geranial (276)], and myrcene (302) are summarized in Figure 19.203 (Seubert and Fass, 1964 Hayashi et al., 1968 Rama Devi and Bhattacharyya, 1977a,b). Geraniol (271) is formed from citronellol (258), nerol (272), linalool (20, and geranyl acetate (270) and metabolized through... 

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... 

Considering the activities of the enzymes TDC, SSS, and SG in cultured C. roseus cells, it is clear that the actual production of alkaloids is considerably lower than that which could have been produced by these enzymes. In other words, there seems to be an overcapacity in the activity of these enzymes. Thus, there is a limiting step in the pathway before these enzymes, or the carbon flux from primary metabolism into the pathway is limited, for example, by competition with other pathways or by transport and com-partmentation. A clear limiting factor is the iridoid pathway, in which geraniol-lO-hydroxylase is an obvious point of regulation. Catabolism is a further factor which in cell cultures does limit the alkaloid accumulation. 

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