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The long and winding road to methane
The process by which archaea make methane involves a series of reactions and enzymes. First, CO2 and methanofuran (MFR) are reduced to formyl-MFR by an as yet unresolved mechanism. Wagner et al. solved the x-ray crystal structure of a tungsten-containing formyl-MFR dehydrogenase complex. Two active sites in the complex are separated by a 43-Å tunnel, which is responsible for transferring the formate made after CO2 reduction. The complex also contains a chain of 46 iron-sulfur clusters. Although the exact function of this chain is unclear, it may electronically couple the four tungsten redox centers.
Science, this issue p. 114
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.