From Monocyclization to Pentacyclization: A Versatile Plant Cyclase Produces Diverse Sesterterpenes with Anti-Liver Fibrosis Potential.

A prolific multi-product sesterterpene synthase CbTPS1 is characterized from the medicinal Brassicaceae plant Capsella bursa-pastoris. Twenty different sesterterpenes including 16 undescribed compounds, possessing 10 different mono-/di-/tri-/tetra-/penta-carbocyclic skeletons, including the unique 15-membered macrocyclic and 24(15→14)-abeo-capbuane scaffolds, are isolated and structurally elucidated from engineered Escherichia coli strains expressing CbTPS1. Site-directed mutagenesis assisted by molecular dynamics simulations resulted in the variant L354M with up to 13.2-fold increased sesterterpene production. These structurally diverse products suggest a comprehensive cyclization mechanism for plant sesterterpenes and provide compelling evidence for the initial cyclization of geranylfarnesyl diphosphate via a crucial 15-membered monocyclic carbocation. The activities of these sesterterpenes against liver fibrosis is inferred from the inhibition of the transforming growth factor-β/Smad signaling pathway and collagen synthesis. These findings greatly expand the chemical space and biological functions of sesterterpenes and provide new insights into the catalytic mechanism of terpene synthases.