Protein complexing in a methanogen suggests electron bifurcation and electron delivery from formate to heterodisulfide reductase

Abstract

In methanogenic Archaea, the final step of methanogenesis generates methane and a heterodisulfide of coenzyme M and coenzyme B (CoM-S-S-CoB). Reduction of this heterodisulfide by heterodisulfide reductase to regenerate HS-CoM and HS-CoB is an exergonic process. Thauer et al. [Thauer, et al. 2008 Nat Rev Microbiol 6:579-591] recently suggested that in hydrogenotrophic methanogens the energy of heterodisulfide reduction powers the most endergonic reaction in the pathway, catalyzed by the formylmethanofuran dehydrogenase, via flavin-based electron bifurcation. Here we present evidence that these two steps in methanogenesis are physically linked. We identify a protein complex from the hydrogenotrophic methanogen, Methanococcus maripaludis, that contains heterodisulfide reductase, formylmethanofuran dehydrogenase, F 420 -nonreducing hydrogenase, and formate dehydrogenase. In addition to establishing a physical basis for the electronbifurcation model of energy conservation, the composition of the complex also suggests that either H 2 or formate (two alternative electron donors for methanogenesis) can donate electrons to the heterodisulfide-H 2 via F 420 -nonreducing hydrogenase or formate via formate dehydrogenase. Electron flow from formate to the heterodisulfide rather than the use of H 2 as an intermediate represents a previously unknown path of electron flow in methanogenesis. We further tested whether this path occurs by constructing a mutant lacking F 420 -nonreducing hydrogenase. The mutant displayed growth equal to wild-type with formate but markedly slower growth with hydrogen. The results support the model of electron bifurcation and suggest that formate, like H 2 , is closely integrated into the methanogenic pathway.energy conservation | Archaea | formate dehydrogenase | formylmethanofuran dehydrogenase | F 420 -nonreducing hydrogenase T he biochemical steps in methanogenesis from CO 2 are well known, but the interactions that lead to net energy conservation are not well understood. The steps in the pathway are diagrammed in Fig. 1 (1). The first step involves the reduction of CO 2 and covalent attachment to a unique cofactor, methanofuran (MFR), via the action of formylmethanofuran dehydrogenase (Fwd) to generate formyl-MFR. This represents an energy-consuming step in the pathway and is dependent on reduced ferredoxin, thought to be produced at the expense of a chemiosmotic membrane potential via the energy-conserving hydrogenase, Eha. Next, the formyl group is transferred to another carrier, tetrahydromethanopterin (H 4 MPT), and is then reduced to generate methyl-H 4 MPT. The methyl group is then transferred to yet another carrier, coenzyme M (HS-CoM), by methyl-H 4 MPT-CoM methyltransferase (Mtr) to generate methyl-S-CoM. At this point, Na + ions are translocated across the cell membrane. The final step involves reduction of the methyl group to CH 4 and capture of HS-CoM by coenzyme B (HS-CoB) to form a CoM-S-S-CoB heterodisulfide. To regenerate HS-CoM and HS-CoB, another enzyme is used, heterodisulfid...

Genomic composition and dynamics amongMethanomicrobialespredict adaptation to contrasting environments

“…The proximity of MvhD to HdrABC ( Figure 5) suggests that this subunit could interface with HdrABC during electron bifurcation as previously demonstrated in the Methanococcales order (Costa et al, 2013). However, the organization of Mvh subunits ( Figure 6) and lack of homologs to MvhB differs to what has been observed in M. maripaludis (Costa et al, 2010) where experimental evidence showed that proximally encoded MvhABGD subunits (canonical enzyme system) mediated electron bifurcation. Overall, this indicates that there are profound differences in the mechanisms of electron bifurcation (particularly in the generation of low potential electrons) within the Methanomicrobiales and between the Methanomicrobiales and other orders of cytochrome-deficient methanogens.…”

“…All strains had a cytoplasmic F 420 -reducing hydrogenase, but they showed differential gene content at other steps of methanogenesis. In distantly related orders of cytochrome-deficient methanogens, the Methanobacteriales and Methanococcales, it was shown, during hydrogenotrophic methanogenesis, that two pairs of electrons are bifurcated in a process whereby the exergonic reduction of the coenzyme M-coenzyme B heterodisulfide is energetically coupled to the endergonic production of formyl-methanofuran, in a protein complex that incorporates cytoplasmic heterodisulfide reductase (HdrABC), formyl-methanofuran dehydrogenase, subunit D of Mvh and one of either formate dehydrogenase (Fdh), MvhABG, or an F 420 -reducing hydrogenase (Costa et al, 2010) (Figure 4). The proximity of MvhD to HdrABC ( Figure 5) suggests that this subunit could interface with HdrABC during electron bifurcation as previously demonstrated in the Methanococcales order (Costa et al, 2013).…”

Coupling of ferredoxin and heterodisulfide reduction via electron bifurcation in hydrogenotrophic methanogenic archaea

“…Fig. 5 does not take the recent evidence into account that in Methanococcus maripaludis the MvhADG/HdrABC complex forms a supercomplex with the formylmethanofuran dehydrogenase complex (38) and that therefore the first and the last step of methanogenesis from H 2 and CO 2 proceed in close proximity which was already proposed 20 y ago in a review by Rouvier and Wolfe (39).…”

Essential anaplerotic role for the energy-converting hydrogenase Eha in hydrogenotrophic methanogenesis

“…The Hdr-associated hydrogenases (Vhu and Vhc) (Fig. 1), which provide electrons to the last reductive step of methanogenesis and potentially the first step via electron bifurcation (1,6,7), can be substituted by formate dehydrogenase (Fdh) during growth on formate (7). The F 420 -reducing hydrogenases (Fru and Frc) generate F 420 H 2 for the second and third reductive steps of methanogenesis, but the Fdh is also F 420 reducing (12,13).…”