Enhancing cofactor recycling in the bioconversion of racemic alcohols to chiral amines with alcohol dehydrogenase and amine dehydrogenase by coupling cells and cell-free system.

Alcohol dehydrogenase (ADH) and amine dehydrogenase (AmDH)-catalyzed one-pot cascade conversion of an alcohol to an amine provides a simple preparation of chiral amines. To enhance the cofactor recycling in this reaction, we report a new concept of coupling whole-cells with the cell-free system to enable separated intracellular and extracellular cofactor regeneration and recycling. This was demonstrated by the respective biotransformation of racemic 4-phenyl-2-butanol 1a and 1-phenyl-2-propanol 1b to (R)-4-phenylbutan-2-amine 3a and (R)-1-phenylpropan-2-amine 3b. Escherichia coli cells expressing S-enantioselective CpsADH, R-enantioselective PfODH, and NADH oxidase (NOX) was developed to oxidize racemic alcohols 1a-b to ketones 2a-b with full conversion via intracellular NAD+ recycling. AmDH and glucose dehydrogenase (GDH) were used to convert ketones 2a-b to amines (R)-3a-b with 89-94% conversion and 891-943 times recycling of NADH. Combining the cells and enzymes for the cascade transformation of racemic alcohols 1a-b gave 70% and 48% conversion to the amines (R)-3a and (R)-3b in 99% ee, with a total turnover number (TTN) of 350 and 240 for NADH recycling, respectively. Improved results were obtained by using the E. coli cells with immobilized AmDH and GDH: (R)-3a was produced in 99% ee with 71-84% conversion and a TTN of 1410-1260 for NADH recycling, the highest value so far for the ADH-AmDH-catalyzed cascade conversion of alcohols to amines. The concept might be generally applicable to this type of reactions.