Enzymatic synthesis of chiral amino-alcohols by coupling transketolase and transaminase-catalyzed reactions in a cascading continuous-flow microreactor system

Rapid biocatalytic process development and intensification continues to be challenging with currently available methods. Chiral amino-alcohols are of particular interest as they represent key industrial synthons for the production of complex molecules and optically pure pharmaceuticals. (2S,3R)-2-amino-1,3,4-butanetriol (ABT), a building block for the synthesis of protease inhibitors and detoxifying agents, can be synthesized from simple, non-chiral starting materials, by coupling a transketolase- and a transaminase-catalyzed reaction. However, until today, full conversion has not been shown and, typically, long reaction times are reported, making process modifications and improvement challenging. In this contribution, we present a novel microreactor-based approach based on free enzymes, and we report for the first time full conversion of ABT in a coupled enzyme cascade for both batch and continuous-flow systems. Using the compartmentalization of the reactions afforded by the microreactor cascade, we overcame inhibitory effects, increased the activity per unit volume, and optimized individual reaction conditions. The transketolase-catalyzed reaction was completed in under 10 min with a volumetric activity of 3.25 U ml^-1 . Following optimization of the transaminase-catalyzed reaction, a volumetric activity of 10.8 U ml^-1 was attained which led to full conversion of the coupled reaction in 2 hr. The presented approach illustrates how continuous-flow microreactors can be applied for the design and optimization of biocatalytic processes.