Cofactors assist enzymes to catalyze reactions and are indispensable and ubiquitous in nature, playing a central role in metabolic pathways. In biocatalysis, common redox cofactors such as nicotinamide, flavin and heme can be activated by light or synthetized to vary redox potentials, leading to different types of reactions for the formation of interesting chiral products, unattainable through classical chemical methods. This chapter will focus on light-driven cell-free biocatalytic reactions activated via their redox cofactors.

Direct, NAD(P)H-independent regeneration of Old Yellow Enzymes represents an interesting approach for simplified reaction schemes for the stereoselective reduction of conjugated C=C-double bonds. Simply by illuminating the reaction mixtures with blue light in the presence of sacrificial electron donors enables to circumvent the costly and unstable nicotinamide cofactors and a corresponding regeneration system. In the present study, we characterise the parameters determining the efficiency of this approach and outline the current limitations. Particularly, the photolability of the flavin photocatalyst and the (flavin-containing) biocatalyst represent the major limitation en route to preparative application.

An increasing number of biocatalytic oxidation reactions rely on H ₂ O ₂ as a clean oxidant. The poor robustness of most enzymes towards H ₂ O ₂ , however, necessitates more efficient systems for in situ H ₂ O ₂ generation. In analogy to the well-known formate dehydrogenase to promote NADH-dependent reactions, we here propose employing formate oxidase (FOx) to promote H ₂ O ₂ -dependent enzymatic oxidation reactions. Even under non-optimised conditions, high turnover numbers for coupled FOx/peroxygenase catalysis were achieved.

Elemental metals are shown to be suitable sacrificial electron donors to drive the stereoselective reduction of conjugated C=C double bonds using Old Yellow Enzymes as catalysts. Both direct electron transfer from the metal to the enzyme as well as mediated electron transfer is feasible, although the latter excels by higher reaction rates. The general applicability of this new chemoenzymatic reduction method is demonstrated, and current limitations are outlined.

We report visible light-driven, asymmetric hydrogenation of C=C bonds using an ene-reductase from Thermus scotoductus SA-01 (TsOYE) and a light-harvesting dye (rose bengal, RB) co-immobilized in an alginate hydrogel. Highly efficient encapsulation of RB in alginate hydrogel was achieved using the intrinsic affinity between TsOYE and RB, which allowed for the construction of robust RB-TsOYE-loaded alginate capsules. In the absence of NADH, the photobiocatalytic system facilitated asymmetric reduction of 2-methylcyclohexenone to an enantiopure (R)-2-methylcyclohexanone (ee > 99%; max. conversion, 70.4%; turnover frequency, 1.54 min ^-1 turnover number, 300.2) under illumination. A series of stability tests revealed a significant enhancement of TsOYE's robustness in alginate hydrogel against heat and chemical denaturants. This study provides insight into a greener and sustainable approach of cofactor-free OYE catalysis for producing value-added chemicals using light energy.

Natural deep eutectic solvents (NADES) are proposed as alternative solvents for peroxygenase-catalysed oxyfunctionalization reactions. Choline chloride-based NADES are of particular interest as they can serve as solvent, enzyme-stabiliser and sacrificial electron donor for the in situ H₂O₂ generation. This report provides the first proof-of-concept and basic characterisation of this new reaction system. Highly promising turnover numbers for the biocatalysts of up to 200,000 have been achieved.

Biocatalytic oxyfunctionalisation reactions are traditionally conducted in aqueous media limiting their production yield. Here we report the application of a peroxygenase in neat reaction conditions reaching product concentrations of up to 360 mM.

Deazaflavins are potentially useful redox mediators for the direct, nicotinamide-independent regeneration of oxidoreductases. Especially the O₂-stability of their reduced forms have attracted significant interest for the regeneration of monooxygenases. In this contribution we further investigate the photochemical properties of deazaflavins and investigate the scope and limitations of deazaflavin-based photoenzymatic reaction systems.