Extreme Synergistic Mutational Effects in the Directed Evolution of a Baeyer–Villiger Monooxygenase as Catalyst for Asymmetric Sulfoxidation

Structure-based directed evolution utilizing iterative saturation mutagenesis (ISM) has been applied to phenyl acetone monooxygenase (PAMO), a thermally robust Baeyer-Villiger monooxygenase, in the quest to access a mutant which displays reversed enantioselectivity in the asymmetric sulfoxidation of prochiral thioethers. Whereas WT PAMO leads to 90% ee in the sulfoxidation of p-methylbenzyl methyl thioether with preference for the (S)-sulfoxide, the evolved mutant I67Q/P440F/A442N/L443I is 95% (R)-selective in the reaction of this and other thioethers. Partial deconvolution of the (R)-selective mutant with generation of the respective four single mutants shows that all of them are (S)-selective, which points to pronounced synergism (cooperative nonadditivity) when they interact in concert. Complete deconvolution with formation of all combinatorial forms of the respective double and triple mutants allows the designed construction of a fitness landscape featuring all 24 upward pathways leading from WT to the (R)-selective quadruple mutant. In all 24 trajectories strong cooperative mutational effects were found as well, which indicates that such mutational changes in enzymes constitute nonlinear systems. A theoretical analysis based on induced fit docking explains many of the observed effects on a molecular level.