The methanogenic CO ₂ reducing-and-fixing enzyme is bifunctional and contains 46 [4Fe-4S] clusters

Biological methane formation starts with a challenging adenosine triphosphate (ATP)-independent carbon dioxide (CO₂) fixation process. We explored this enzymatic process by solving the x-ray crystal structure of formyl-methanofuran dehydrogenase, determined here as Fwd(ABCDFG)₂ and Fwd(ABCDFG)₄ complexes, from Methanothermobacter wolfeii The latter 800-kilodalton apparatus consists of four peripheral catalytic sections and an electron-supplying core with 46 electronically coupled [4Fe-4S] clusters. Catalysis is separately performed by subunits FwdBD (FwdB and FwdD), which are related to tungsten-containing formate dehydrogenase, and subunit FwdA, a binuclear metal center carrying amidohydrolase. CO₂ is first reduced to formate in FwdBD, which then diffuses through a 43-angstrom-long tunnel to FwdA, where it condenses with methanofuran to formyl-methanofuran. The arrangement of [4Fe-4S] clusters functions as an electron relay but potentially also couples the four tungstopterin active sites over 206 angstroms.