Light-driven biocatalytic oxidation and reduction reactions: scope and limitations.

The quest for practical regeneration concepts for nicotinamide-dependent oxidoreductases continues. Recently we proposed the use of visible light to promote the direct reductive regeneration of a flavin-dependent monooxygenase. With this enzyme (PAMO-P3) light-driven enantioselective Baeyer-Villiger oxidations were performed. In spite of the significant reduction in the complexity achieved, catalytic performance of the novel approach did not meet the requirements for an efficient biocatalytic oxygenation system. Driven by this ultimate goal, we further investigated the limiting factors of our particular system. We discovered that oxidative uncoupling of the flavin-regeneration reaction from enzymatic O2-activation accounts for the futile consumption of approximately 95% of the reducing equivalents provided by the sacrificial electron donor, EDTA. Furthermore, it was found that the apparent turnover frequency (TOF) for PAMO-P3 in the present setup is approximately two orders of magnitude lower than in conventional setups that use NADPH as reductant. This finding was traced to sluggish electron transfer kinetics that arose from an impeded interaction between PAMO-P3-bound FAD and the reducing catalyst. The limiting factors and potential approaches for their circumvention are discussed. Furthermore, we broadened the light-driven regeneration approach to the class of flavin-dependent reductases. By using the Old Yellow Enzyme homologue YqjM as a model system, a significantly higher catalytic turnover for the enzyme catalyst was achieved, which we assign to a higher accessibility of the prosthetic group as well as to the absence of oxidative uncoupling.