Functionally redundant formate dehydrogenases enable formate-dependent growth inMethanococcus maripaludis

Abstract

AbstractMethanogens are essential for the complete remineralization of organic matter in anoxic environments. Most cultured methanogens are hydrogenotrophic, using H2as an electron donor to reduce CO2to CH4, but in the absence of H2many can also use formate. Formate dehydrogenase (Fdh) is essential for formate oxidation, where it transfers electrons for reduction of coenzyme F420or 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. UsingMethanococcus 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 F420at similar rates. F420reduction preceded flavin-based electron bifurcation activity for both enzymes. In a Δfdh1mutant 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 activitiesin vivomay 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.