The 4-hydroxyacetophenone monooxygenase (HAPMO) from Pseudomonas fluorescens ACB catalyzes NADPH-and oxygen-dependent Baeyer-Villiger oxidation of 4-hydroxyacetophenone to the corresponding acetate ester. Using the purified enzyme from recombinant Escherichia coli, we found that a broad range of carbonylic compounds that are structurally more or less similar to 4-hydroxyacetophenone are also substrates for this flavin-containing monooxygenase. On the other hand, several carbonyl compounds that are substrates for other Baeyer-Villiger monooxygenases (BVMOs) are not converted by HAPMO. In addition to performing Baeyer-Villiger reactions with aromatic ketones and aldehydes, the enzyme was also able to catalyze sulfoxidation reactions by using aromatic sulfides. Furthermore, several heterocyclic and aliphatic carbonyl compounds were also readily converted by this BVMO. To probe the enantioselectivity of HAPMO, the conversion of bicyclohept-2-en-6-one and two aryl alkyl sulfides was studied. The monooxygenase preferably converted (1R,5S)-bicyclohept-2-en-6-one, with an enantiomeric ratio (E) of 20, thus enabling kinetic resolution to obtain the (1S,5R) enantiomer. Complete conversion of both enantiomers resulted in the accumulation of two regioisomeric lactones with moderate enantiomeric excess (ee) for the two lactones obtained [77% ee for (1S,5R)-2 and 34% ee for (1R,5S)-3]. Using methyl 4-tolyl sulfide and methylphenyl sulfide, we found that HAPMO is efficient and highly selective in the asymmetric formation of the corresponding (S)-sulfoxides (ee > 99%). The biocatalytic properties of HAPMO described here show the potential of this enzyme for biotechnological applications.In nature, many oxygenation reactions are carried out by flavin-dependent monooxygenases (24). The diversity of conversions that can be catalyzed is large; the reactions include Baeyer-Villiger reactions, aromatic hydroxylations, sulfoxidations, amine oxidations, and epoxidations. Many of these conversions occur with high enantio-and/or regioselectivity. The variety of reactivity is also reflected in the diverse physiological processes in which these enzymes play a prominent role, including xenobiotic compound metabolism in humans (49), biosynthesis of toxins (21), and pollutant degradation by bacteria (46).Enantio-and regioselective oxygenations are often difficult to achieve by chemical means, while these types of reactions can lead to valuable optically active compounds. Due to their exquisite regio-and/or enantioselectivity and catalytic efficiency, flavin-dependent monooxygenases are useful biocatalysts for the synthesis of a variety of fine chemicals (12,35,41,47). However, so far, only a limited number of flavin-dependent monooxygenase genes have been cloned and overexpressed, which has limited the application of these biocatalysts for synthetic processes.The Baeyer-Villiger reaction, i.e., the oxidation of ketones or aldehydes by peroxides resulting in oxygen insertion adjacent to the carbonyl group (2), has many applications in organic ...