Mechanistic Characterisation of the Bacterial Sesterviridene Synthase from Kitasatospora viridis

A gene coding for a terpene synthase homolog from Kitasatospora viridis was cloned and expressed in Escherichia coli. The purified recombinant protein possessed sesterterpene synthase activity and efficiently converted geranylfarnesyl diphosphate (GFPP) with 19 % yield into the sesterterpene hydrocarbon sesterviridene A. Large scale enzymatic conversions also allowed for the isolation of two side products that are generated with very low yields of ca. 0.1 %. Several derivatives of sesterviridene A were obtained by chemical transformations, securing the NMR-based structural assignments. The absolute configuration of sesterviridene A was determined by chemical correlation using stereoselectively deuterated precursors and by anomalous dispersion X-ray crystallography. The cyclisation mechanism from GFPP to sesterviridene A was extensively studied through isotopic labelling experiments and DFT calculations.