Discovery of Two Native Baeyer-Villiger Monooxygenases for Asymmetric Synthesis of Bulky Chiral Sulfoxides

Abstract

Two Baeyer-Villiger monooxygenases (BVMOs), designated BoBVMO and AmBVMO, were discovered from Bradyrhizobium oligotrophicum and Aeromicrobium marinum, respectively. Both monooxygenases displayed novel features for catalyzing the asymmetric sulfoxidation of bulky and pharmaceutically relevant thioethers. Evolutionary relationship and sequence analysis revealed that the two BVMOs belong to the family of typical type I BVMOs and the subtype ethionamide monooxygenase. Both BVMOs are active toward medium- and long-chain aliphatic ketones as well as various thioether substrates but are ineffective toward cyclohexanone, aromatic ketones, and other typical BVMO substrates. BoBVMO and AmBVMO showed the highest activities (0.117 and 0.025 U/mg protein, respectively) toward thioanisole among the tested substrates. Furthermore, these BVMOs exhibited distinct activity and excellent stereoselectivity toward bulky and prochiral prazole thioethers, which is a unique feature of this family of BVMOs. No native enzyme has been reported for the asymmetric sulfoxidation of bulky prazole thioethers into chiral sulfoxides. The identification of BoBVMO and AmBVMO provides an important scaffold for discovering enzymes capable of asymmetrically oxidizing bulky thioether substrates by genome mining.IMPORTANCE Baeyer-Villiger monooxygenases (BVMOs) are valuable enzyme catalysts that are an alternative to the chemical Baeyer-Villiger oxidation reaction. Although BVMOs display broad substrate ranges, no native enzymes were reported to have activity toward the asymmetric oxidation of bulky prazole-like thioether substrates. Herein, we report the discovery of two type I BVMOs from Bradyrhizobium oligotrophicum (BoBVMO) and Aeromicrobium marinum (AmBVMO) which are able to catalyze the asymmetric sulfoxidation of bulky prazole thioethers (proton pump inhibitors [PPIs], a group of drugs whose main action is a pronounced and long-lasting reduction of gastric acid production). Efficient catalysis of omeprazole oxidation by BoBVMO was developed, indicating that this enzyme is a promising biocatalyst for the synthesis of bulky and pharmaceutically relevant chiral sulfoxide drugs. These results demonstrate that the newly identified enzymes are suitable templates for the discovery of more and better thioether-converting BVMOs.

Keywords: Baeyer-Villiger monooxygenase; asymmetric sulfoxidation; ethionamide monooxygenase; prazole thioether; stereoselectivity.