Engineering silica nanocoated whole-cell asymmetric biocatalyst for efficient preparation of a key chiral intermediate of (S)-Rivastigmine.

In our previous study, the whole cells containing an aldo-keto reductase (yhdN) and glucose dehydrogenase (GDH) were constructed and applied in a stereoselective carbonyl reduction reaction to prepare (S)-NEMCA-HEPE, being a key chiral intermediate of (S)-Rivastigmine which is widely prescribed for the treatment of Alzheimer's disease. Although the conversion and enantiomeric excess (e.e.) could reach to 78.2% and 99%, respectively, ionic liquid as an additive was required to improve the permeability of cell membrane. To further simplify the reaction, the molecular docking and saturation mutagenesis technology were used here to obtain an activity-improved yhdN variant such as G19A. And then, both excellent conversion and e.e. of 99% for (S)-NEMCA-HEPE could be achieved within 40min by using only G19A-GDH whole cell as a catalyst without any additive. However, the use of the whole cells still faces the issues of poor operation stability and adverse application prospect. Subsequently, a hydrophobic "cell-in-shell" complex of G19A-GDH@O-Silica was constructed by using a silica nanocoated technology. The obtained G19A-GDH@O-Silica exhibited an excellent conversion towards the asymmetric carbonyl reduction, and a good tolerance in changing thermal, pH, and storage environmental. Giving 76.3% of reaction conversion even after the 11th cycle of reuse, indicated that G19A-GDH@O-Silica also possessed ideal recyclability. The aim of this study is to provide a rapid, and cost-effective nanocoated whole-cell biocatalyst for efficient preparation of (S)-NEMCA-HEPE. The simplicity and robustness of the immobilization approach may become a powerful tool to utilize whole-cell catalysts towards organic catalysis.