Hedycaryol synthase in complex with nerolidol reveals terpene cyclase mechanism

The biosynthesis of terpenes is catalysed by class I and II terpene cyclases. Here we present structural data from a class I hedycaryol synthase in complex with nerolidol, serving as a surrogate for the reaction intermediate nerolidyl diphosphate. This prefolded ligand allows mapping of the active site and hence the identification of a key carbonyl oxygen of Val179, a highly conserved helix break (G1/2) and its corresponding helix dipole. Stabilising the carbocation at the substrate's C1 position, these elements act in concert to catalyse the 1,10 ring closure, thereby exclusively generating the anti-Markovnikov product. The delineation of a general mechanistic scaffold was confirmed by site-specific mutations. This work serves as a basis for understanding carbocation chemistry in enzymatic reactions and should contribute to future application of these enzymes in organic synthesis. High-precision structure: The structure of the bacterial sesquiterpene synthase hedycaryol synthase from Kitasatospora setae in complex with the reaction intermediate analogue nerolidol was obtained at high resolution (1.5 Å). Detailed mechanistic principles for the late steps of terpene cyclisations were delineated and extensively tested by analysis of twelve site-specific mutants.