Strong Implications From Small Deviations in Labeling Patterns: The Mechanism of Burkholderia gladioli...

The sesquiterpene synthase BgPgS from Burkholderia gladioli produces the main product (-)-1-epi-pacifigorgia-6,10-diene, besides a few structurally related compounds. The enzyme mechanism of BgPgS was addressed through isotopic labeling experiments, revealing several intricate mechanistic problems. Unexpectedly, the isotopic labelings for the Me groups C12 and C13 occurred in different positions for the main and the side products. Moreover, as demonstrated in this study, two hydrogen atoms must change from the bottom to the top hemisphere, which is not possible through standard terpene biosynthesis routines with suprafacial hydrogen migrations. Our rational solution involves two mechanistic explanations: First, a key rearrangement may be associated with a conformational change that rotates one hydrogen from bottom to top. Second, a "break-flip-cyclize" sequence explains the change of side by the other hydrogen. DFT calculations show that the proposed terpene cyclization cascades are energetically feasible; only one problematic activation barrier (>25 kcal/mol) remains. However, several mechanistic alternatives either failed to explain the experimental results of the isotopic labeling experiments or were associated with even higher activation barriers. Our biosynthetic proposal for pacifigorgiadiene biosynthesis can be understood as a contribution that awaits further investigation and scientific debate for its ultimate resolution.