Characterization of the oxygen-tolerant formate dehydrogenase from Clostridium carboxidivorans

Fixation of CO 2 into the organic compound formate by formate dehydrogenases (FDHs) is regarded as the oldest autotrophic process on Earth. It has been proposed that an FDH-dependent CO 2 fixation module could support CO 2 assimilation even in photoautotrophic organisms. In the present study, we characterized FDH from Clostridium carboxidivorans ( cc FDH) due to its ability to reduce CO 2 under aerobic conditions. During the production of recombinant cc FDH, in which the selenocysteine codon was replaced by Cys, we were able to replace the W with Mo as the transition metal in the cc FDH metal cofactor, resulting in a two-fold increase of 6 μmol formate min −1 in enzyme activity. Then, we generated cc FDH variants in which the strict NADH preference of the enzyme was changed to NADPH, as this reducing agent is produced in high amounts during the photosynthetic light process. Finally, we showed that the native cc FDH can also directly use ferredoxin as a reducing agent, which is produced by the photosynthetic light reactions at photosystem I. These data collectively suggest that cc FDH and, particularly, its optimized variants can be regarded as suitable enzymes to couple formate production to photosynthesis in photoautotroph organisms, which could potentially support CO 2 assimilation via the Calvin–Benson–Bassham (CBB) cycle and minimize CO 2 losses due to photorespiration.