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The oxygen‐tolerant and NAD+‐dependent formate dehydrogenase from Rhodobacter capsulatus is able to catalyze the reduction of CO2 to formate

The formate dehydrogenase from Rhodobacter capsulatus (RcFDH) is an oxygen‐tolerant protein with an (αβγ)2 subunit composition that is localized in the cytoplasm. It belongs to the group of metal and NAD+‐dependent FDHs with the coordination of a molybdenum cofactor, four [Fe4S4] clusters and one [Fe2S2] cluster associated with the α‐subunit, one [Fe4S4] cluster and one FMN bound to the β‐subunit, and one [Fe2S2] cluster bound to the γ‐subunit. RcFDH was heterologously expressed in Escherichia coli and characterized. Cofactor analysis showed that the bis‐molybdopterin guanine dinucleotide cofactor is bound to the FdsA subunit containing a cysteine ligand at the active site. A turnover rate of 2189 min−1 with formate as substrate was determined. The back reaction for the reduction of CO2 was catalyzed with a kcat of 89 min−1. The preference for formate oxidation shows an energy barrier for CO2 reduction of the enzyme. Furthermore, the FMN‐containing and [Fe4S4]‐containing β‐subunit together with the [Fe2S2]‐containing γ‐subunit forms a diaphorase unit with activities for both NAD+ reduction and NADH oxidation. In addition to the structural genes fdsG, fdsB, and fdsA, the fds operon in R. capsulatus contains the fdsC and fdsD genes. Expression studies showed that RcFDH is only active when both FdsC and FdsD are present. Both proteins are proposed to be involved in bis‐molybdopterin guanine dinucleotide modification and insertion into RcFDH.