Tabersonine hydroxylation

The kingdom of plants represents a valuable apothecaiy, as it is the origin of many important therapeutic agents. Thus, the expression of plant P450 fusion enzymes in bacteria may permit the high-level production of medically relevant compounds that plants produce naturally at low levels. In Catharanthus roseus, the synthesis of vinblastine and vincristine, two important alkaloids that find apphcation in the treatment of leukemia, starts with tabersonine hydroxylation [201]. Schroder et al. generated a fusion enzyme... 

The first of six steps in the conversion of tabersonine to vindoline is catalyzed by tabersonine 16-hydroxylase (T16H) (Fig.7.3), which was characterized as a P450-dependent monooxygenase.59 A cDNA able to hydroxylate tabersonine at... 

Fig. 8.7 The conversion of tabersonine to vindoline involves an ordered sequence of 6 reactions that include hydroxylation, O-methylation, hydroxylation, A -methylation, hydroxylation and O-acetylation.

Tabersonine is hydroxylated at C-16, followed by methylation and hydration of the 2,3 double bond, N(l)-methylation, hydroxylation at C-4 and 4-0 acetylation. Hydroxlation of tabersonine at C-16 requires a cytochrome P450-mediated monooxygenase (T16H with CYP71D12). This enz)mie was found to be located in the endoplasmic reticulum, was at maximal activity... 

Vandrikidine (27) showed some similarity to tabersonine (28), but from other data an aromatic methoxy group and a secondary hydroxyl needed to be placed. From the chemical shift of the C-ll carbon at 159.9 ppm, this was clearly the site of the aromatic methoxy group. The hydroxyl group was located at C-19 from the chemical shift of C-19 at 67.9 ppm and the smaller downfield shifts of both C-18 and C-21. Vandrikidine therefore was assigned structure 27 (20). The relationship with 11-methoxymino-vincine (9) was not established. 

Biosynthesis The biosynthesis of V., and especially the final stage of a ca. 15-step chain of reactions, has been studied on the enzymatic level The last 6 reactions, beginning at the level of the Catharanthus alkaloid "ta-bersonine are accordingly hydroxylations, methyla-tions, and acetylation. The final reaction in the biosynthesis of V. is catalyzed by a highly specific acetyl-CoA-dependent acetyltransfeiase which converts deacetylvindoline to V. Although catharanthine and ta-bersonine can be produced in cell cultures of C. roseus, the biosynthesis of V. in these cell systems is interrupted at the stage of tabersonine thus the dimeric in-... 

As shown in Scheme 13.63, tabersonine, in the presence of tabersonine 16-hydroxylase (EC 1.14.13.73), is converted to 16-hydroxytabersonine. The reaction requires both NADPH and oxygen (O2), and NADP and water (H2O) are produced. Tabersonine 16-hydroxylase is a heme-thiolate protein (a P45o).The phenolic hydroxyl group on the aromatic ring undergoes methylation (tabersonine 16-O-methyltransferase, EC 2.1.1.94), while 5-adenosylmethionine is converted to... 

While the biosynthesis of catharanthine and tabersonine is poorly characterized (Scheme 4.5), the biosynthetic pathway from tabersonine to vindoline has been extensively studied [2]. Thus, the hydroxylation of tabersonine at position 16 represents the first reaction leading to vindoline (Scheme 4.6), and it is catalyzed by tabersonine 16-hydroxylase (T16H), a cytochrome P450-dependent enzyme... 

---