Geosmin biosynthesis. Mechanism of the fragmentation-rearrangement in the conversion of germacradienol to geosmin.

Abstract

Geosmin (1) is responsible for the characteristic odor of moist soil. Incubation of farnesyl diphosphate (2, FPP) with recombinant Streptomyces coelicolor germacradienol/geosmin synthase generated geosmin (1), germacradienol (3), germacrene D (4), octalin 5, and acetone, which was trapped as 2,2-dimethylthiazolidine (9) by reaction with cysteamine. The acetone was shown to be derived by fragmentation of the 2-hydroxypropyl moiety of germacradienol by incubation with [13,13,13-2H3]FPP (2a), giving rising to the corresponding derivative of d3-acetone, 9a. GC−MS analysis of the labeling pattern of [6-2H]geosmin (1b) resulting from the incubation of germacradienol/geosmin synthase with [2-2H]FPP (2b) revealed that the H-2 proton on FPP undergoes the predicted 1,2-hydride shift to ring B of geosmin. Both sets of experimental results are consistent with a proposed mechanism of geosmin formation involving protonation-cyclization of germacradienol with loss of the 2-hydroxypropyl moiety as acetone by a retro-Prin...