In this review focusing on

In this review, focusing on biocatalyst “formate dehydrogenase FDH” catalyzing both of the formic 4-Aminobutyric acid oxidation to CO2 and the CO2 reduction to formic acid, representative examples of properties, types, structure of active-site of FDH and, reaction mechanism of formic acid oxidation and CO2 reduction with FDH are outlined. The methods for improving CO2 reduction catalytic activity of FDH using a genetic engineering modification and FDH immobilized various support also are introduced. Moreover, chemical and electrochemical approach of CO2 reduction to formic acid with FDH, homogenous system for visible-light driven CO2 reduction to formic acid with FDH and device for visible-light driven CO2 reduction to formic acid with FDH are also introduced as an application of FDH.
Conclusion In this review, focusing on biocatalyst “FDH” catalyzing CO2 reduction to formic acid, the following points are outlined. Particularly in second part, the visible-light driven CO2 reduction to formic acid with FDH via the NAD+ photoredcution and BP photoreduction are described in detail. There is no doubt that it is effective to use the NAD+ photoreduction for the visible-light driven CO2 reduction to formic acid with FDH. The affinity between NAD+ or NADH and FDH does not change, however, FDH catalytic activity cannot be controlled by using NAD+/ NADH redox coupling. As the produced NADH acts as a sacrificial reagent and is consumed, moreover, the NAD+/ NADH redox coupling is not suitable to use for the visible-light driven redox system. Even if the effective NAD+ reduction to NADH with visible-light driven redox system could be achieved, NAD+ is a very expensive biological reagent. Thus, it is necessary to design and synthesize a simple molecule that is easily photoreduced and acts as a co-enzyme for FDH. Various 4,4\'- or 2,2\'- BPs, that have been widely used as an electron carrier molecule in the visible-light driven redox system, has attracted attention, because BPs can be easily chemically modified. By using chemically modified BPs, the catalytic activity of FDH for CO2 reduction to formic acid can be controlled. There are reports that it is better to use natural co-enzyme NAD+ because BP is toxic material [165]. However, this is an irrelevant idea. Controlling the catalytic activity of FDH with cheap molecules is an important point for the practical application of visible-light driven redox system with biocatalyst. In the future, it is expected that new technologies with FDH will be developed for visible-light driven CO2 reduction to formic acid with FDH and device for visible-light driven CO2 reduction to formic acid with FDH.
Acknowledgments Our work introduced in this review was partially supported by Precursory Research for Embryonic Science and Technology (PRESTO, Japan Science and Technology Agency JST), Grant-in-Aid for Challenging Exploratory Research (Japan Society for the Promotion of Science) (15K14239), and Grant-in-Aid for Scientific Research on Innovative Areas “Artificial Photosynthesis (2406)” and “Innovations for Light-Energy Conversion (4906)”.
Introduction The mitochondria of eukaryotes constitute a platform for a variety of processes that provide cell energy homeostasis. They control energy production and dissipation to maintain the proper conditions for the cell to function. In a canonical respiratory chain, there are four basic protein complexes enabling electron transport and proton pumping that result in electrochemical gradient generation and ATP synthesis. In contrast to the classical constitution of the mammalian respiratory chain, plants, fungi, and protists contain additional components that branch from the primary canonical chain (Moller, 2001, Rasmusson et al., 2008). These extra components include rotenone-insensitive type II NAD(P)H dehydrogenases or alternative NAD(P)H dehydrogenases (NDH2) and a cyanide-insensitive alternative oxidase (AOX), which all belong to mitochondrial energy-dissipating systems (Fig. 1). They dissipate energy indirectly, since they do not pump protons and provide a bypass of electrons from the classical respiratory complexes. In this manner, type II NAD(P)H dehydrogenases and the AOX modulate the efficiency of energy conservation by the mitochondrial respiratory chain and ATP synthesis through oxidative phosphorylation (Moller, 2001, Rasmusson et al., 2008). Interestingly, recent studies have identified several Arabidopsis thaliana type II NAD(P)H dehydrogenases as dual targeted proteins (Xu et al. 2013). They target either mitochondria and peroxisomes or mitochondria and chloroplasts. The dual targeting ability of NDH2 probably arose early in the evolution of land plants. However, the NDH2 are usually described in relation to the role they play in the mitochondria.