Semi-rational design based on the interaction between SmFMO and FAD isoalloxazine ring to enhance the enzyme activity

Flavin monooxygenases (FMOs) have been widely used in the biosynthesis of natural compounds due to their excellent stereoselectivity, regioselectivity and chemoselectivity. Stenotrophomonas maltophilia flavin monooxygenase (SmFMO) has been reported to catalyze the oxidation of various thiols to corresponding sulfoxides, but its activity is relatively low. Herein, we obtained a mutant SmFMO^F52G which showed 4.35-fold increase in k_cat/K_m (4.96 mM^-1s^-1) and 6.84-fold increase in enzyme activity (81.76 U/g) compared to the SmFMO^WT (1.14 mM^-1s^-1 and 11.95 U/g) through semi-rational design guided by structural analysis and catalytic mechanism combined with high-throughput screening. By forming hydrogen bond with O4 atom of FAD isoalloxazine ring and reducing steric hindrance, the conformation of FAD isoalloxazine ring in SmFMO^F52G is more stable, and NADPH and substrate are closer to FAD isoalloxazine ring, shortening the distances of hydrogen transfer and substrate oxygenation, thereby increasing the rate of reduction and oxidation reactions and enhancing enzyme activity. Additionally, the overall structural stability and substrate binding capacity of the SmFMO^F52G have significant improved than that of SmFMO^WT. The strategy used in this study to improve the enzyme activity of FMOs may have generality, providing important references for the rational and semi-rational engineering of FMOs.