Characterization of a new Baeyer-Villiger monooxygenase and conversion to a solely N-or S-oxidizing enzyme by a single R292 mutation.

BACKGROUND: Ar-BVMO is a recently discovered Baeyer-Villiger monooxygenase from the genome of Acinetobacter radioresistens S13 closely related to medically relevant ethionamide monooxygenase EtaA (prodrug activator) and capable of inactivating the imipenem antibiotic.

METHODS: The co-substrate preference as well as steady-state and rapid kinetics studies of the recombinant purified protein were carried out using stopped-flow spectroscopy under anaerobic and aerobic conditions. Kd values were measured by isothermal calorimetry. Enzymatic activity was determined by measuring the amount of product formed using high pressure liquid chromatography or gas chromatography. Site-directed mutagenesis experiments were performed to decipher the role of the active site arginine-292.

RESULTS: Ar-BVMO was found to oxidize ethionamide as well as linear ketones. Mechanistic studies on the wild type enzyme using stopped-flow spectroscopy allowed for the detection of the characteristic oxygenating C4a-(hydro)peroxyflavin intermediate, which decayed rapidly in the presence of the substrate. Replacement of arginine 292 in Ar-BVMO by glycine or alanine resulted in greatly reduced or no Baeyer-Villiger activity, respectively, demonstrating the crucial role of this residue in catalysis of ketone substrates. However, both the R292A and R292G mutants are capable of carrying out N- and S-oxidation reactions.

CONCLUSIONS: Substrate profiling of Ar-BVMO confirms its close relationship to EtaA; ethionamide is one of its substrates. The active site Arginine 292 is required for its Baeyer-Villiger activity but not for heteroatom oxidation.

GENERAL SIGNIFICANCE: A single mutation converts Ar-BVMO to a unique S- or N-monooxygenase, a useful biocatalyst for the production of oxidized metabolites of human drug metabolizing enzymes.