Identification two key residues at the intersection of domains of a thioether monooxygenase for improving its sulfoxidation performance

AcCHMO, a cyclohexanone monooxygenase from Acinetobacter calcoaceticus, is a typical Type I Baeyer-Villiger monooxygenase (BVMO). We previously obtained the AcCHMO_M6 mutant, which oxidizes omeprazole sulfide (OPS) to the chiral sulfoxide drug esomeprazole. To further improve the catalytic efficiency of the AcCHMO_M6 mutant, a focused mutagenesis strategy was adopted at the intersections of the FAD-binding domain, NADPH-binding domain, and α-helical domain based on structural characteristics of AcCHMO. By using focused mutagenesis and subsequent global evolution two key residues (L55 and P497) at the intersections of the domains were identified. Mutant of L55Y improved catalytic efficiency significantly, whereas the P497S mutant alleviated substrate inhibition remarkably. AcCHMO_M7 (L55Y/P497S) was obtained by combining the two mutations, which increased the specific activity from 18.5 (M6) to 108 U/g, and an increase in the K_i of the substrate OPS from 34 to 265 μM. The results indicate that catalytic performance can be elevated by modification of the sensitive sites at the intersection of the domains of AcCHMO. The results also provided some insights for the engineering of other Type I BVMOs or other multidomain proteins.