Hydroclassified Combinatorial Saturation Mutagenesis: Reshaping Substrate Binding Pockets of KpADH for Enantioselective Reduction of Bulky–Bulky Ketones
A hydroclassified combinatorial saturation mutagenesis (HCSM) strategy was proposed for reshaping the substrate binding pocket by dividing 20 amino acids into four groups based on their hydrophobicity and size. These smart HCSM libraries could significantly reduce screening effort especially for the simultaneous mutagenesis of three or more residues and lacking high throughput screening methods. Employing HCSM strategy, the stereoselectivity of KpADH, an alcohol dehydrogenase from Kluyveromyces polysporus, was efficiently improved to 99.4% ee. (4-Chlorophenyl)(pyridin-2-yl)methanone (CPMK), generally regarded as a “hard-to-reduce” ketone, was used as a model substrate, and its corresponding chiral alcohol products could be utilized as antihistamine precursors. The best variant 50C10 displayed higher binding affinity and catalytic efficiency toward CPMK with KM/kcat of 59.3 s–1·mM–1, 3.51-fold that of KpADH. Based on MD simulations, increased difference between two binding pockets, enhanced hydrophobicity, and π–π and halogen–alkyl interactions were proposed to favor the enantioselective recognition and substrate binding in 50C10. Substrate spectrum analysis revealed that 50C10 exhibited improved enantioselectivity toward diaryl ketones especially with halo- or other electron-withdrawing groups. As much as 500 mM CPMK could be asymmetrically reduced into chiral diaryl alcohols with ee of 99.4% and a space–time yield of 194 g·L–1·d–1 without addition of external NADP+. This study provides an effective mutagenesis strategy for the protein engineering of substrate specificity and enantioselectivity.
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