Computational studies on the sterol-like cyclization of a monodomain class II terpene cyclase

Recently the first example of a class II terpene cyclase comprised of only a single domain was reported. Class II synthases are a diverse group of enzymes that catalyze exceptionally complex reactions, including the remarkable cyclization of steroids. This discovery of a single-domain enzyme being able to catalyze a steroid-like product contradicted the long-held tenet that complex class II cyclizations required double-domain enzymes. The proposed mechanism for the sterol-like cyclization of a monodomain class II terpene cyclase was studied computationally by using density functional theory (DFT). The complete pathway for the conversion of 5-geranyl-3,4-dihydroxybenzoate to the steroid-like pentacyclic product merosterolic acid A was elucidated. The formation of a tricyclic carbocation intermediate with three cyclohexane rings was found to be a concerted, but asynchronous, cyclization. The formation of the fourth ring proceeds with a low energy activation Friedel-Crafts reaction. Subsequent deprotonation of this pentacyclic system gave as the final product merosterolic acid. The overall conversion was found to be highly exothermic due to the conversion of three C-C double bonds to C-C single bonds. Delineation of the mechanism with DFT calculations.