Integrating biocatalysis with continuous flow: current status, challenges, and future perspectives

Continuous-flow biocatalysis is gaining attention for the synthesis of highly valuable pharmaceuticals and fine chemicals. The integrated approach combines the high activity and selectivity of biocatalysts with the superior efficiency in mass and heat transfer of continuous-flow systems. Considerable progress has been made in the continuous-flow biocatalysis, but there are only a few reviews on the specific challenges and strategies encountered in continuous-flow reactions involving different biocatalysts. This review aims to present a general overview of the implementation of various biocatalysts in continuous-flow systems for synthetic applications, and their specific issues and strategies associated with each category were discussed in detail. Future directions for biocatalysis in continuous flow, including current problems and potential strategies were proposed to offer insights and support for the advancement of continuous-flow biocatalysis. Recent advances in continuous-flow biocatalysis were summarized according to the types of biocatalysts involved, encompassing single enzymes, multienzyme catalysts, whole-cell catalysts, metal-enzymatic catalysts, and organo-enzymatic catalysts, along with their combination with photocatalysis and electrocatalysis. The main challenges faced by diverse biocatalysts in continuous-flow processes have been systematically discussed, including poor substrate solubility with lipases, low oxygen availability for oxidoreductases, cofactor regeneration, catalyst leaching, as well as incompatibility problems in multi-enzyme cascade catalysis, and biocatalysis coupling with chemo-catalysis or other catalysis. Various strategies have been developed for addressing these challenges, such as construction of continuous-flow biphasic systems, design of self-sufficient biocatalysts, easy preparation of bioreactors mediated by fusion-tag. Future researches should focus on machine learning-assisted protein engineering and materials design, as well as computer-aided optimization of continuous-flow cascade systems, to reduce enzyme activity loss due to immobilization, and address mismatches between multiple enzymes and chemo-/photocatalysts in cascade processes, thereby developing sustainable, efficient, and industrially viable processes.