Type II Flavin‐Containing Monooxygenases: A New Class of Biocatalysts that Harbors Baeyer–Villiger Monooxygenases with a Relaxed Coenzyme Specificity

Abstract Within a newly identified set of flavin‐containing monooxygenases (FMOs) from Rhodococcus jostii RHA1, we have identified three monooxygenases (FMO‐E, FMO‐F, and FMO‐G) that are effective in catalyzing Baeyer–Villiger oxidations. These type II FMOs display relaxed coenzyme specificity by accepting both NADPH (reduced form of nicotinamide adenine dinucleotide phosphate) and NADH (reduced form of nicotinamide adenine dinucleotide), as a coenzyme, which is a novel and attractive feature among biocatalysts capable of conducting Baeyer–Villiger oxidations. We purified FMO‐E and determined that the Michaelis constants for both coenzymes were in the micromolar range, whereas the activity was highest for NADH. By using the stopped‐flow technique, formation of a peroxyflavin–enzyme intermediate was observed, which indicated that type II FMOs follow a catalytic mechanism similar to that of other class B flavoprotein monooxygenases. A set of cyclobutanones and cyclohexanones were used to probe the regio‐ and enantioselectivity of all three recombinant monooxygenases. The biocatalysts readily accepted small cyclic ketones, which enabled the conversion of previously poorly accepted substrates by other monooxygenases (especially norcamphor), and exhibited excellent and unique regio‐ and enantioselectivities. Sequence analysis revealed that type II FMOs that act as Baeyer–Villiger monooxygenases contain a unique N‐terminal domain. Sequence conservation in this protein domain can be used to identify new NADH‐dependent Baeyer–Villiger monooxygenases, which would facilitate future biocatalyst discovery efforts.