Cytochrome P450- and Dehydrogenase-Mediated Regiospecific and Stereoselective Formation of β- and γ-Lactones in Drimane-Type Sesquiterpenoid Biosynthesis.
Lactone-containing natural products are important candidates for drug discovery. Drimane-type sesquiterpenes (DTSs), characterized by a bicyclic trans-decalin scaffold, can bear both β- and γ-lactone moieties. While γ-lactone-containing DTSs have frequently been reported, β-lactone-containing derivatives are rare, and their biosynthesis remains unexplored. Here, we identified a biosynthetic gene cluster (dri) in Aspergillus ustus and confirmed ustidrimane A (1), a β- and γ-lactone-containing DTS, as its product. Heterologous gene expression, precursor feeding, and enzymatic investigation provided evidence for the formation of both lactone rings. In both cases, the reaction cascade is initiated by regiospecific (and stereoselective) methyl hydroxylation, followed by regiospecific and stereoselective oxidation of one hydroxymethyl group to an aldehyde. The resulting hemiacetal was proven to be subsequently oxidized to a lactone. The β-lactone formation is catalyzed by two cytochrome P450 enzymes (DriE and DriF), followed by two oxidation steps catalyzed by two dehydrogenases (DriG and DriH). These findings differ entirely from the known β-lactone formation in fatty acid-, PKS-, and NRPS-derived metabolites. The subsequent γ-lactone formation is catalyzed by a P450 (DriJ) and a dehydrogenase (DriD). DriJ has been shown to be involved in both methyl hydroxylation and hemiacetal formation, while DriD is responsible for the hemiacetal oxidation and also contributes moderately to its formation. Collectively, these findings establish a sequential P450/dehydrogenase-mediated oxidative cascade for the construction of two distinct lactone motifs within a single DTS scaffold. Moreover, they provide the first insight into the β-lactone formation in terpenes, thus unveiling a new strategy for the construction of this structural motif.