Light has received increased attention for various chemical reactions but also in combination with biocatalytic reactions. Because currently only a few enzymatic reactions are known, which per se require light, most transformations involving light and a biocatalyst exploit light either for providing the cosubstrate or cofactor in an appropriate redox state for the biotransformation. In selected cases, a promiscuous activity of known enzymes in the presence of light could be induced. In other approaches, light-induced chemical reactions have been combined with a biocatalytic step, or light-induced biocatalytic reactions were combined with chemical reactions in a linear cascade. Finally, enzymes with a light switchable moiety have been investigated to turn off/on or tune the actual reaction. This Review gives an overview of the various approaches for using light in biocatalysis.

Light-driven biocatalysis in recombinant cyanobacteria provides highly atom-efficient cofactor regeneration via photosynthesis, thereby remediating constraints associated with sacrificial cosubstrates. However, despite the remarkable specific activities of photobiocatalysts, self-shading at moderate-high cell densities limits efficient space-time-yields of heterologous enzymatic reactions. Moreover, efficient integration of an artificial electron sink into the tightly regulated network of cyanobacterial electron pathways can be highly challenging. Here, we used C=C bond reduction of 2-methylmaleimide by the NADPH-dependent ene-reductase YqjM as a model reaction for light-dependent biotransformations. Time-resolved NADPH fluorescence spectroscopy allowed direct monitoring of in-cell YqjM activity and revealed differences in NADPH steady-state levels and oxidation kinetics between different genetic constructs. This effect correlates with specific activities of whole-cells, which demonstrated conversions of >99%. Further channelling of electrons toward heterologous YqjM by inactivation of the flavodiiron proteins (Flv1/Flv3) led to a 2-fold improvement in specific activity at moderate cell densities, thereby elucidating the possibility of accelerating light-driven biotransformations by the removal of natural competing electron sinks. In the best case, an initial product formation rate of 18.3 mmol h –1 L –1 was reached, allowing the complete conversion of a 60 mM substrate solution within 4 h.

Light-driven biotransformations in recombinant cyanobacteria allow to employ photosynthetic water-splitting for cofactorregeneration and thus, to save the use of organic electron donors. The genes of three recombinant imine reductases (IREDs) were expressed in the cyanobacterium Synechocystis sp. PCC 6803 and eight cyclic imine substrates were screened in whole-cell biotransformations. While initial reactions showed low to moderate rates, optimization of the reaction conditions in combination with promoter engineering allowed to alleviate toxicity effects and achieve full conversion of prochiral imines with initial rates of up to 6.3 mM h À 1 . The high specific activity of up to 22 U g CDW À 1 demonstrates that recombinant cyanobacteria can provide large amounts of NADPH during whole cell reactions. The excellent optical purity of the products with up to > 99 %ee underlines the usefulness of cyanobacteria for the stereoselective synthesis of amines.Biocatalysis has emerged as important catalytic technology with significant contributions for the development of clean reactions. [1] A large number of biocatalytic processes employ [a] H.