Enzymatic catalysis favours eight-membered over five-membered ring closure in bicyclomycin biosynthesis

The construction of O-heterocycles is an important step in organic synthesis and biosynthesis for producing valuable ring compounds. Although enzyme-catalysed five- or six-membered ring closures in O-heterocycle biosynthesis have been studied extensively, the enzymatic formation of eight-membered O-heterocycles has been proposed only recently. Here we demonstrate a group of enzymes that catalyse an intramolecular attack of alcohol on epoxide for the construction of an eight-membered ring rather than an intrinsically more favourable five-membered tetrahydrofuran. The detailed mechanism is revealed through biochemical experiments, chemical syntheses, crystallographic structural analyses, computational simulations of potential energies and molecular dynamics, and site-directed mutagenesis. This study provides a vivid example of an enzyme that non-covalently protects an intrinsically more active hydroxyl group through a hydrogen-bond network, and reverses the inherent size selectivity in ring-closure reactions, despite the presence of multiple nucleophiles in the substrate.