Monoterpenoid Indole Alkaloid Biosynthesis

FIGURE 10.1 Schematic representation of the biosynthetic pathway leading from L-tryptophan and geraniol to the hypothetical intermediate 3α(S)-strictosidine-aglycone. tdc, tryptophan decarboxylase; str1, strictosidine synthase; G10H (CYP76B6), geraniol 10-hydroxylase; cyp72a1, secologanin synthase; SGD, strictosidine glucosidase.

On the pathway leading from L-tryptophan and geraniol to the central monoterpenoid indole alkaloid intermediate 3α(S)-strictosidine aglycone, the cDNAs encoding five biosynthetic enzymes have been described (Fig. 10.1). These are tydc (tryptophan decarboxylase) (De Luca et al., 1989), g10h or cyp76b6 (geraniol 10-hydroxylase) (Collu et al., 2001), cyp72a1 (secologanin synthase) (Irmler et al., 2000), str1 (strictosidine synthase) (Kutchan et al., 1988, 1989; Mcnight et al., 1990), and sgd (strictosidine glucosidase) (Geerlings et al., 2000; Gerasimenko et al., 2002). 3α(S)-Strictosidine aglycone is an unstable intermediate that can be transformed into a number of chemical structures, which then lead into specific monoterpenoid indole alkaloid pathways. Along the biosynthetic pathway that leads from tabersonine to vindoline, three cDNAs encoding biosynthetic enzymes have been identified (Fig. 10.2). The cytochrome P450-dependent monooxygenase gene cyp71d12 encodes tabersonine 16-hydroxylase (Schröder et al., 1999; St-Pierre and De Luca, 1995). Desacetoxyvindoline is hydroxylated at the 4- position by the oxoglutarate-dependent dioxygenase desacetoxyvindoline–4-hydroxylase (encoded by d4h) (De Carolis and De Luca, 1993; Vazquez-Flota et al., 1997). Finally, deacetylvindoline is acetyla

FIGURE 10.2 Schematic representation of the biosynthetic pathway leading from tabersonine to vindoline. CYP71D12, tabersonine 16-hydroxylase; d4h, desacetoxyvindoline 4-hydroxlyase; dat, deacetylvindoline 4-O-acetyltransferase.

ted to vindoline by acetylcoenzyme A: deacetylvindoline 4-O-acetyltransferase, the gene product of dat (Fahn et al., 1985; Power et al., 1990; St-Pierre et al., 1998).

Although not specific to monoterpenoid indole alkaloid biosynthesis, the nonmevalonate pathway has been shown to be the biosynthetic route to loganin and secologanin (Contin et al., 1998; Eichinger et al., 1999), and three cDNAs encoding 1-deoxy-D-xylulose 5-phosphate synthase, 1-deoxy-D-xylulose 5-phosphate reductoisomerase, and 2C-methyl-D-erythritol 2,4-cyclodiphosphate synthase of this pathway have been isolated from C. roseus (Chahed et al., 2000; Veau et al., 2000).

FIGURE 10.3 Schematic representation of the biosynthetic pathway leading from polyneuridine aldehyde to ajmaline. PNAE, polyneuridine aldehyde esterase; VS, vinorine synthase.

Progress has also been made in identifying cDNAs encoding enzymes of monoterpenoid indole alkaloid biosynthesis in R. serpentina (Fig. 10.3). Two cDNAs involved in the transformation of the sarpagan alkaloid polyneuridine aldehyde into the ajmalan-type alkaloid ajmaline have been characterized: pnae encoding polyneuridine aldehyde esterase (Dogru et al., 2000; Pfitzner and Stöckigt, 1983) that converts polyneuridine aldehyde into epivellosimine which is then rearranged and acetylated to vinorine by vinorine synthase encoded by the vs gene (Bayer et al., 2004).

With the current collection of cDNAs encoding enzymes of monoterpenoid indole alkaloid biosynthesis, progress has also been made with respect to our understanding of the regulation of this biosynthesis. The first topic to be covered here is the cellular localization of monoterpenoid indole alkaloid biosynthesis.