Clostridium beijerinckii displays a soluble [FeFe]-hydrogenase/formate dehydrogenase enzyme complex that links H2 and CO2 metabolism.

The production of biofuels by bacterial fermentation receives sustained attention due to the need to develop novel circular and sustainable technologies. Clostridium beijerinckii produces both hydrogen (H2) and carbon-based biofuels acetone, butanol and ethanol (ABE solvents). H2 metabolism in C. beijerinckii is complex and mostly unexplored. Seven hydrogenase genes are contained in the genome, but their exact physiological role is unknown. Here, we report on the characterisation of a novel heterotetrameric soluble enzyme complex composed of an [FeFe]-hydrogenase component stably bound to a formate dehydrogenase subunit, which we name CbFdh/Hyd. We show that the four subunits form a stable complex that can be conveniently overexpressed and purified recombinantly. CbFdh/Hyd is highly sensitive to atmospheric oxygen and displays reversible catalytic features, including H2 evolution, H2 uptake, formate oxidation and the ability to split formate into H2 and CO2 (formate hydrogen lyase activity, FHL) as well as the opposite reaction, H2-driven CO2 reduction (HDCR). CbFdh/Hyd displays functional and spectroscopic features very similar to Fdh/Hyd complexes previously described in acetogens, suggesting that this enzyme is at the basis of the previously reported unconventional ability of C. beijerinckii to fix CO2 into acetate and butyrate. CbFdh/Hyd could also represent a key player in H2 production metabolism by degrading formate produced from the decarboxylation of pyruvate.