Mining coral-derived terpene synthases and mechanistic studies of the coral biflorane synthase

Biflorane diterpenoids are unique natural products often seen in marine animals. Recent studies have reported a small number of biflorane synthases. However, the catalytic mechanism and structural basis for biflorane formation remain unclear. To address these issues, we conducted genome mining of terpene synthases from the sea whip coral Paramuricea clavata, resulting in the discovery of a biflorane synthase PcTS1. We performed a series of isotope labeling, crystallography, quantum mechanics/molecular mechanics calculations, and mutagenesis studies toward PcTS1 to investigate the mechanism. Isotopic labeling studies, together with calculations, elucidate a cascade of 1,10-cyclization, 1,3-hydride shift, 1,6-cyclization, 1,2-hydride shift, 2,6-cyclization, cyclopropane ring opening, and deprotonation by the generated pyrophosphate, forming the biflorane scaffold. Crystallography, quantum mechanics/molecular mechanics, and mutagenesis studies confirmed the cascade and produced different terpene scaffolds. Our work demonstrated the mechanism of marine biflorane formation, elucidated the second crystal structure of a coral terpene synthase, and realized the terpene skeleton expansion.