Tropane Alkaloid Biosynthesis

The biosynthetic building blocks leading to the tropane alkaloids are theamino acids L-arginine and L-phenylalanine (Fig. 10.11). On the pathway leading from L-arginine to the N-methylpyrrolinium ion, the enzyme putrescine N-methyltransferase (pmt) has been well characterized at the enzymatic and molecular genetic levels (Hibi et al., 1992). Several pmt genes have been isolated from Nicotiana tabacum, Atropa belladonna, Hyoscyamus niger, N. sylvestris, and N. attenuate (Hibi et al., 1994; Shoji et al., 2000; Suzuki et al., 1999a; Winz and Baldwin, 2001). Further along the biosynthetic pathway to tropane alkaloids, tropinone is reduced by tropinone reductase I (tr-I) to tropine. tr-I specifically reduces the tropinone 3-keto moiety to the 3α-hydroxyl group of tropine, the biosynthetic precursor of hyoscyamine and scopolamine (Hashimoto et al., 1992; Koelen and Gross, 1982). Tropinone reductase II (tr-II) reduces the 3-keto group of tropinone to the 3β-hydroxy moiety of pseudotropine, which serves as precursor to the calistegins (Dräger and Schaal, 1994). The gene tr-I has been characterized from datura stramonium and H. niger (Nakajima et al., 1993, 1999). The gene tr-II is known from D. stramonium, H. niger, and Solanum tuberosum (Keiner et al., 2002; Nakajima et al., 1993, 1999). The final gene fromthe scopolamine biosynthetic pathway that has been

FIGURE 10.11 Schematic representation of the biosynthetic pathway leading from L-arginine to nicotine, scopolamine, calistegins, and cocaine. pmt, putrescineN-methyltransferase; tr-I, tropinone reductase I; tr-II, tropinone reductase II; h6h, hyoscyamine 6β-hydroxylase.

identified is h6h encoding hyoscyamine 6β-hydroxylase (Hashimoto and Yamada, 1986). This 2-oxoglutarate-dependent dioxygenase is bifunctional, catalyzing both the monooxygenation of hyoscyamine to 6b-hydroxyhyoscyamine and the subsequent epoxidation to scopolamine. This gene has been characterized from H. niger and A. belladonna (Matsuda et al., 1991; Suzuki et al., 1999b).

With the cloning of the genes pmt, tr-I, tr-II, and h6h, our understanding of the cellular localization of solanaceous alkaloid biosynthesis has also advanced. The current view is discussed in the following sections.