Photoredox systems with biocatalysts for CO2 utilization

Visible-light driven redox systems have been attracting significant research attention due to their ability to produce hydrogen and reduce and utilize CO2 for the production of solar fuels and chemicals. Generally, visible-light driven redox systems consist of an electron donor, a photocatalytic dye, an electron mediator and a catalyst. One of the important components in visible-light driven redox systems is an effective catalyst for hydrogen production and reduction and utilization of CO2. The catalysts used in visible-light driven redox systems are classified into metal nanoparticles, molecular catalysts and biocatalysts. Among them, biocatalysts are promising due to their excellent reaction and substrate selectivity, especially for the reduction and utilization of CO2, which is remarkably higher than that of other catalysts. Among the biocatalysts for CO2 reduction and utilization, NAD(P)(+)-dependent dehydrogenases, which are commercially available, are widely used in visible-light driven redox systems. Formate dehydrogenase (FDH) from Candida boidinii is a typical NAD(P)(+)-dependent dehydrogenase for the visible-light driven redox reduction of CO2 to formate. Furthermore, the addition of commercially available aldehyde (aldDH), formaldehyde (FldDH) and alcohol (ADH) dehydrogenase to this system results in the reduction of CO2 to methanol via formate and formaldehyde as intermediates in visible-light driven redox systems. Furthermore, NAD(P)(+)-dependent dehydrogenases for decarboxylation are also commercially available and widely used for the visible-light driven formation of carbon-carbon bonds from CO2 and organic molecules. Malic enzyme (ME) from chicken liver is a typical NAD(P)(+)-dependent dehydrogenase with decarboxylating ability for the visible-light driven production of malate, which is based on the formation of carbon-carbon bonds from CO2 and pyruvate. In this review, visible-light driven CO2 reduction and utilization systems involving the photoreduction of NAD(P)(+) and biocatalysts are introduced. Furthermore, visible-light driven CO2 reduction and utilization systems involving the photoreduction of bipyridinium salt (viologen)-based electron mediators and biocatalysts also are introduced. In particular, the simplification of visible-light driven CO2 reduction and utilization systems by utilizing viologen-based electron mediators and the improvement in their efficiency without changing the structure of the biocatalyst are also discussed.