Terpene Synthases: One Fold, Many Products

Terpene synthases/cyclases yield core structures of some of the most valuable bioactive molecules, such as artemisinin and paclitaxel. A detailed understanding of structure–function relationship of terpene synthases/cyclases delineates the evolutionary relationship between different enzymes and molecular basis of their strategies to direct and manipulate carbocation intermdiates throughout the catalysis trajectory. Using Aspergillus terreus aristolochene synthase and Streptomyces citricolor germacradiene-4-ol synthase, both of which are high-fidelity cyclases, as model systems, we have provided structural evidence showing that the pyrophosphate group is responsible for carrying out general base–general acid chemistry throughout the carbocation cascade. We also show that despite the highly reactive nature of carbocation intermediates, terpene cyclases accurately guide active site water molecules that are tethered by surrounding residues to serve as a part of the enzymatic template or are catalytically activated at the right time to hydroxylate the final carbocation intermediate. Phomopsis amygdali fusicoccadiene synthase (PaFS) is the first identified bifunctional diterpene synthase that carries out tandem elongation and cyclization reactions. We have shown that cyclization activity of the bona fide substrate geranylgeranyl diphosphate (GGPP) by the full-length enzyme is subject to allosteric regulation, and hexamerization of PaFS driven by the elongation domain creates proximity effects that accelerate product flux by 2 fold. We also present structural studies that shed light on its catalytic mechanism and reveal its evolutionary relationships between bifunctional diterpene synthases and similar sesterterpene synthases.