Impact of Expanded Small Alkyl-Binding Pocket by Triple Point Mutations on Substrate Specificity of Thermoanaerobacter ethanolicus Secondary Alcohol Dehydrogenase

Site-directed mutagenesis was employed to generate five different triple point mutations in the double mutant (C295A/I86A) of Thermoanaerobacter ethanolicus alcohol dehydrogenase (TeSADH) by computer-aided modeling with the aim of widening the small alkyl-binding pocket. TeSADH engineering enables the enzyme to accept sterically hindered substrates that could not be accepted by the wild-type enzyme. The underline in the mutations highlights the additional point mutation on the double mutant TeSADH introduced in this work. The catalytic efficiency ( k ca /K M ) of the M151A/C295A/I86A triple TeSADH mutant for acetophenone increased about 4.8-fold higher than that of the double mutant. A 2.4-fold increase in conversion of 3'-methylacetophenone to ( R )-1-(3-methylphenyl)-ethanol with a yield of 87% was obtained by using V115A/C295A/I86A mutant in asymmetric reduction. The A85G/C295A/I86A mutant also produced ( R )-1-(3-methylphenyl)-ethanol (1.7-fold) from 3'-methylacetophenone and ( R )-1-(3-methoxyphenyl)-ethanol (1.2-fold) from 3'- methoxyacetophenone, with improved yield. In terms of thermal stability, the M151A/ C295A/I86A and V115A/C295A/I86A mutants significantly increased ΔT1/2 by +6.8°C and +2.4°C, respectively, with thermal deactivation constant ( k d ) close to the wild-type enzyme. The M151A/C295A/I86A mutant reacts optimally at 70 °C with almost 4 times more residual activity than the wild type. Considering broad substrate tolerance and thermal stability together, it would be promising to produce ( R )-1-(3-methylphenyl)-ethanol from 3'- methylacetophenone by V115A/C295A/I86A, and ( R )-1-phenylethanol from acetophenone by M151A/C295A/I86A mutant, in large-scale bioreduction processes.