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The methanogenic CO2 reducing-and-fixing enzyme is bifunctional and contains 46 [4Fe-4S] clusters

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Science  07 Oct 2016:
Vol. 354, Issue 6308, pp. 114-117
DOI: 10.1126/science.aaf9284

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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

Abstract

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.

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