The Substrate-Bound Crystal Structure of a Baeyer–Villiger Monooxygenase Exhibits a Criegee-like Conformation

The Baeyer–Villiger monooxygenases (BVMOs) are a family of bacterial flavoproteins that catalyze the synthetically useful Baeyer–Villiger oxidation reaction. This involves the conversion of ketones into esters or cyclic ketones into lactones by introducing an oxygen atom adjacent to the carbonyl group. The BVMOs offer exquisite regio- and enantiospecificity while acting on a wide range of substrates. They use only NADPH and oxygen as cosubstrates, and produce only NADP<sup>+</sup> and water as byproducts, making them environmentally attractive for industrial purposes. Here, we report the first crystal structure of a BVMO, cyclohexanone monooxygenase (CHMO) from <i>Rhodococcus</i> sp. HI-31 in complex with its substrate, cyclohexanone, as well as NADP<sup>+</sup> and FAD, to 2.4 Å resolution. This structure shows a drastic rotation of the NADP<sup>+</sup> cofactor in comparison to previously reported NADP<sup>+</sup>-bound structures, as the nicotinamide moiety is no longer positioned above the flavin ring. Instead, the substrate, cyclohexanone, is found at this location, in an appropriate position for the formation of the Criegee intermediate. The rotation of NADP<sup>+</sup> permits the substrate to gain access to the reactive flavin peroxyanion intermediate while preventing it from diffusing out of the active site. The structure thus reveals the conformation of the enzyme during the key catalytic step. CHMO is proposed to undergo a series of conformational changes to gradually move the substrate from the solvent, via binding in a solvent excluded pocket that dictates the enzyme’s chemospecificity, to a location above the flavin–peroxide adduct where catalysis occurs.