Diversity of electron-bifurcating CO₂-fixing supercomplexes in methanogens

In the hydrogenotrophic methanogenic pathway, formylmethanofuran dehydrogenase (Fmd) reduces and fixes CO₂, driven by low-potential electrons provided by electron-bifurcating heterodisulfide reductase (Hdr) complexed with electron-donating proteins such as Mvh hydrogenase. Here, we report the structure of a C2-symmetric (Mvh-Hdr)₂-Fmd₄ supercomplex from a Class I methanogen, Methanothermobacter marburgensis, which is architecturally different from the previously reported ring-shaped D3-symmetric supercomplex of a methanogen belonging to phylogenetically distinct Class II methanogens. In this C2-symmetric form, the redox active sites of Hdr and Fmd are connected by two MvhB polyferredoxins, whose branching electron paths appear to be available for electron transfer to/from other partners. The ancestral form was likely C2 symmetric, whereas D3-symmetric supercomplexes were acquired by horizontal gene transfer, a transition probably helpful for growth in substrate-poor environments.