Studies on the biosynthesis of naturally occurring antitumor agents

The naturally occurring antitumor agents, taxol and the ansamitocins were investigated for their biosynthetic pathways. The investigation into taxol biosynthesis focused upon the first committed step in the overall biosynthetic pathway which is the cyclization of geranylgeranyl diphosphate (GGDP) into taxa-4(5), 11(12)-diene by the enzyme taxadiene synthase (TS). The conformation of GGDP during its enzyme-catalyzed conversion into taxa-4(5), 11(12)-diene was inferred using two complimentary approaches. The first approach relied upon specific deuterium labeling of GGDP and incubation of the labeled substrate with taxadiene synthase; the resulting taxa-4(5), 11(12)-diene was analyzed for the presence and location of deuterium. The second approach involved the use of computer modeling to analyze the putative intermediates generated within the TS-GGDP Michaelis complex. These two complementary approaches describe an overall cyclization pathway for the conversion of GGDP to taxa-4(5), 11(12)-diene.; The studies on the biosynthesis of the ansamitocin family of antitumor compounds were directed at discovering the origin and mode of formation of the unusual chain extension unit found at the C9 and C10 positions of the ansamitocins. The so-called ‘post-polyketide’ (post-PKS) modifications taking place during ansamitocin biosynthesis were also investigated. Although a complete picture of how the unusual extender unit is formed and incorporated into the process of PKS assembly remained elusive, our studies provided insight into the order of post-PKS modifications and the enzymes responsible for these tailoring reactions.