Introduction: Acetogenic bacteria are a group of strictly anaerobic bacteria that fix CO₂ and produce acetate as the main product using the Wood-Ljungdahl pathway (WLP). The first step of the WLP is the reduction of CO₂ to formate, catalysed by formate dehydrogenases, but the involved electron carriers differ among acetogens^[1-3]. The model for cytochrome-containing acetogens, Sporomusa ovata has at least three different formate dehydrogenase genes, but the nature of the enzyme remained to be identified.

Goals: We aimed to elucidate the subunit composition, mechanism of electron transport and electron carriers involved in the reduction of CO₂ to formate in S. ovata.

Materials & Methods: Protein purification, biochemical characterization, enzymatic assays and bioinformatic analysis.

Results: The formate dehydrogenase of S. ovata was purified from the cytoplasmic fraction using three different chromatographic steps. After each purification step, the fractions containing formate dehydrogenase also showed hydrogenase activity. The purified protein catalyzed the simultaneous reduction of NAD⁺ and Fd with H₂ as well as with formate as electron donor. The enzyme was able to produce formate from H₂ and CO₂ as well as H₂ from formate.

Summary: The formate dehydrogenase (FdhA) of Sporomusa ovata forms a complex with the electron bifurcating hydrogenase (HydABC) and the complex uses the mechanism of electron bifurcation to reduce CO₂ with electrons derived from molecular hydrogen. Ferredoxin and NAD⁺ are involved as electron carriers.

[1] Li, L.F., Ljungdahl, L., Wood, H.G. (1966) J Bacteriol 92:405-412

[2] Wang, S., Huang, H., Kahnt, J., Müller, A.P., Köpke, M., Thauer, R.K. (2013) J Bacteriol 92:405-412 195:4373-4386

[3] Schuchmann, K., Müller, V. (2013) Science 342:1382-1385