Synthesis of Formate from CO Gas Catalyzed by an O-Tolerant NAD-Dependent Formate Dehydrogenase and Glucose Dehydrogenase.

Direct biocatalytic conversion of CO to formic acid is an attractive means of reversibly storing energy in chemical bonds. Formate dehydrogenases (FDHs) are a heterogeneous group of enzymes that catalyze the oxidation of formic acid to carbon dioxide, generating two protons and two electrons. Several FDHs have recently been reported to catalyze the reverse reaction, i.e., the reduction of carbon dioxide to formic acid, under appropriate conditions. The main challenges with these enzymes are relatively low rates of CO reduction and high oxygen sensitivity. Our earlier studies (Yu et al. (2017) J. Biol. Chem. 292, 16872-16879) have shown that the FdsABG formate dehydrogenase from Cupriavidus necator is able to effectively catalyze the reduction of CO, using NADH as a source of reducing equivalents, with a good oxygen tolerance. On the basis of this result, we have developed a highly thermodynamically efficient and cost-effective biocatalytic process for the transformation of CO to formic acid using FdsABG. We have  cloned the full-length soluble formate dehydrogenase (FdsABG) from C. necator and expressed it in Escherichia coli with a His-tag fused to the N terminus of the FdsG subunit; this overexpression system has greatly simplified the FdsABG purification process. Importantly, we have also combined this recombinant C. necator FdsABG with another enzyme, glucose dehydrogenase, for continuous regeneration of NADH for CO reduction and demonstrated that the combined system is highly effective in reducing CO to formate. The results indicate that this system shows significant promise for the future development of an enzyme-based system for the industrial reduction of CO.