Engineering of Thermostable β‐Hydroxyacid Dehydrogenase for the Asymmetric Reduction of Imines

The β‐hydroxyacid dehydrogenase from Thermocrinus albus (Ta‐βHAD), which catalyzes the NADP(+)‐dependent oxidation of β‐hydroxyacids, was engineered to accept imines as substrates. The catalytic activity of the proton‐donor variant K189D was further increased by the introduction of two nonpolar flanking residues (N192 L, N193 L). Engineering the putative alternative proton donor (D258S) and the gate‐keeping residue (F250 A) led to a switched substrate specificity as compared to the single and triple variants. The two most active Ta‐βHAD variants were applied to biocatalytic asymmetric reductions of imines at elevated temperatures and enabled enhanced product formation at a reaction temperature of 50 °C.