Proposed mechanism for diterpene synthases in the formation of phomactatriene and taxadiene

To obtain insight into how the cyclization pathway is controlled, the mechanism of diterpene synthase reactions (the putative phomactatriene synthase and taxadiene synthases) involving the same intermediate was investigated in detail. The mechanism of the initial transformation of GGDP to verticillen-12-yl cation (A+) was proposed based on the labelling pattern of phomactatriene (9a) obtained in the feeding experiments with 13C-labelled acetates. To obtain information on the reaction pathway of A+ to 9a and taxadiene (10), reactions of verticillol (5) with various acids were conducted. Structural determination of products 9b, 9c, 13a, 13b and 14 allowed us to propose a reaction pathway via cations A+, D+, E+, F+ and G+. Identification of hydrocarbons 6, 9b, 13a and 13b in mycelial extracts of phomactin-producing fungus supported the proposed reaction mechanism. Based on the results of ab initio calculations for highly flexible cation intermediates, a mechanism is proposed.