Biosynthesis of the sesquiterpene patchoulol from farnesyl pyrophosphate in leaf extracts of Pogostemon cablin (patchouli): mechanistic considerations

Several mechanistic alternatives have been proposed for the enzyme-catalyzed, electrophilic cyclization of farnesyl pyrophosphate to the tricyclic sesquiterpene alcohol patchoulol, which is the characteristic component of the essential oil of Pogostemon cablin (patchouli). These alternatives include schemes involving deprotonation-reprotonation steps and the intermediacy of the monocyclic and bicyclic olefins germacrene and bulnesene, respectively, and involving a 1,3-hydride shift with only tertiary cationic intermediates and without any deprotonation-reprotonation steps. Analytical studies, based on analyses of P. cablin leaf oil at different stages of plant development, and in vivo time-course investigations, using 14CO2 and [14C]sucrose, gave no indication that germacrene and bulnesene were intermediates in patchoulol biosynthesis. A soluble enzyme system from P. cablin leaves was prepared, which was capable of converting farnesyl pyrophosphate to patchoulol, and isotopic dilution experiments with both labeled and unlabeled olefins were carried out with this system to confirm that sesquiterpene olefins did not participate as fre intermediates in the transformation of the acyclic precursor to patchoulol. Patchoulol derived biosynthetically from [12,13-14C;1-3H]farnesyl pyrophosphate was chemically degraded to establish the overall construction pattern of the product. Similar studies with [12,13-14C;6-3H]farnesyl pyrophosphate as a precursor eliminated deprotonation steps to form bound olefinic intermediates in the biosynthesis of patchoulol, while providing supporting evidence for the hydride shift mechanism