Functionally redundant formate dehydrogenases enable formate-dependent growth in Methanococcus maripaludis

Methanogens are essential for the complete remineralization of organic matter in anoxic environments. Most cultured methanogens are hydrogenotrophic, using H2 as an electron donor to reduce CO2 to CH4, but in the absence of H2 many can also use formate. Formate dehydrogenase (Fdh) is essential for formate oxidation, where it transfers electrons for reduction of coenzyme F420 or to a flavin-based electron bifurcating reaction catalyzed by heterodisulfide reductase (Hdr), the terminal reaction of methanogenesis. How these competing reactions are coordinated is unknown. Furthermore, methanogens that use formate encode at least two isoforms of Fdh in their genomes, but how these different isoforms participate in methanogenesis is also unknown. Using Methanococcus maripaludis, we undertook a biochemical characterization of both Fdh isoforms involved in methanogenesis. Both Fdh1 and Fdh2 interacted with Hdr to catalyze the flavin-based electron bifurcating reaction, and both reduced F420 at similar rates. F420 reduction preceded flavin-based electron bifurcation activity for both enzymes. In a {Delta}fdh1 mutant background, a suppressor mutation was required for Fdh2 activity. Genome sequencing revealed that this mutation resulted in loss of a specific molybdopterin transferase (moeA), allowing for Fdh2-dependent growth. This suggests that both isoforms of Fdh are functionally redundant, but their activities in vivo may be limited by gene regulation under different growth conditions. Together these results expand our understanding of formate oxidation and the role of Fdh in methanogenesis.