The energy sector currently faces two major challenges: the depleting fossil fuel reserves and the excess emission of greenhouse gases, especially carbon dioxide. Different chemical and biological methods have been explored for conversion of carbon dioxide to liquid fuels and other useful chemicals. Some of these methods include heterogeneous catalysis, electrocatalysis, photosynthesis and metabolic engineering, etc. However, these methods have shortcomings, including harsh reaction conditions; low selectivity and yield; or multiple enzymes involvement. Therefore, direct conversion of CO2 to a liquid fuel, such as formic acid, through one enzyme, formate dehydrogenase, is a more attractive option. Formate dehydrogenases (FDHs) can be broadly classified into three categories: metal-independent/NAD(P)+-dependent FDHs, metal-containing/NAD(P)+-independent FDHs, and metal-containing/NAD(P)+-dependent FDHs. FDHs originally catalyze the oxidation reaction of formic acid to carbon dioxide by generating two protons and two electrons. Recently, several FDHs have been reported to catalyze the reverse reaction, i.e. reduction of carbon dioxide to formic acid under appropriate conditions. The main challenges with these enzymes are a very low CO2-reducing activity and/or high oxygen sensitivity. In this study, we employed an oxygen tolerant and molybdenum-containing FdsABG formate dehydrogenase from Cupriavidus ncecator (formerly known as Ralstonia eutropha) for conversion CO2 to formic acid. The reversibility of the reaction catalyzed by the FdsABG has been investigated. We find that the enzyme is kinetically competent to catalyze the reverse reaction, reduction of CO2 to formate by NADH, with a kcat of 10 s-1, yielding a kcatforward/kcatreverse of ~20. The substrate in the reverse direction is CO2, consistent previous work indicating that CO2 is the product of reaction in the forward reaction. Accumulation of formate in the reverse reaction is quantified and found to quantitatively account for consumption of NADH. It appears that all molybdenum- and tungsten-containing formate dehydrogenases, operate via a simple hydride transfer mechanism and are effective in catalyzing the reversible interconversion of CO2 and formate. Those enzymes such as the C. necator FdsABG formate dehydrogenase, that possess cysteine rather than selenocysteine coordinated to the active site metal, are air-stable and have great potential for industrial applications.

Reversibility of the reaction catalyzed by the molybdenum-containing formate dehydrogenase from Cupriavidus necator (Ralstonia eutropha)

253rd National Meeting (2017)

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