β-Terrecyclene synthase constructs the quadrane backbone in terrecyclic acid biosynthesis.

Quadrane sesquiterpenes featuring a distinctive tricyclic skeleton exhibit potent antimicrobial and anticancer activities. Although extensive studies have attempted to reveal the multistep carbocation rearrangement involved in the formation of the tricyclic quadrane scaffold, the exact biosynthetic pathway and chemical logic to generate the quadrane structure remains mysterious. Here we identified a novel sesquiterpene synthase that is capable of generating β-terrecyclene possessing the quadrane scaffold and characterized the biosynthetic pathway of a representative fungal quadrane terrecyclic acid. Further mutagenesis coupled with isotopically sensitive branching studies of this β-terrecyclene synthase provided insight into the mechanism involved in the formation of the quadrane scaffold.