Structural insights into γ-presodorifen pyrophosphate synthase.
Canonical terpenoids conform to the (C5H8)n formula, whereas an emerging group of terpenoids that break the isoprene rule have been discovered recently. The precursor of C17 homosesquiterpenoid chlororaphens is derived from converting canonical C15 farnesyl pyrophosphate (FPP) by two consecutive methyltransferases (MTs). The first step is catalyzed by a bi-functional S-adenosyl-L-methionine (SAM)-dependent methyltransferase that methylates C10 on FPP to trigger a cyclization cascade and generate the monocyclic C16 γ-presodorifen pyrophosphate (γ-PSPP). Here, we report the crystal structures of γ-PSPP synthase from Variovorax boronicumulans PHE5-4 (VbFPPMT) and Pseudomonas chlororaphis O6 (PcFPPMT), as well as the Mg2+ ion- and geranyl pyrophosphate (GPP) -bound complex of PcFPPMT. These structures reveal the overall structure of γ-PSPP synthases, composition of the catalytic center, and a flexible region responsible for the anchoring of the pyrophosphate (PPi) moiety of the substrate and a Mg2+ ion. Combining structural analysis, molecular docking, and site-directed mutagenesis, we identified key residues that govern the catalytic mechanism of these bi-functional γ-PSPP synthases. These findings advance our understanding into the catalytic mechanism employed by non-canonical methyltransferases, establish a vital framework for further mechanistic understanding and benefit rational engineering of C16/C17 terpenoid biosynthesis.