Catalytic role of carbonyl oxygens and water in selinadiene synthase

Terpene synthases (TSs) catalyse the most complex cyclization cascades in nature, with generation and taming of reactive carbocations. Although deprotonation–reprotonation sequences are frequently relevant for TS catalysis, little is known how the enzyme acts in these processes. Here we show, through quantum mechanics (density functional theory)/molecular mechanics molecular dynamics simulations that the main-chain carbonyl oxygen of Gly182 of selina-4(15),7(11)-diene synthase (SdS) has a dual role as a base and an acid and acts in synchrony with one water molecule. The computational model is supported by isotopic labelling experiments confirming the predicted stereochemical course associated with the deprotonation–reprotonation sequence. Gly182 is located within the G1/2 helix break of SdS, with all backbone carbonyl oxygens pointing into the active site having functions in recognizing substrate conformation, stabilizing carbocation intermediates and anchoring their poses. The strict conservation of the G1/2 helix break in type I TSs from bacteria, fungi and plants suggests that its functions as described here may be of general importance in TS catalysis.