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

The long and winding road to methane The process by which archaea make methane involves a series of reactions and enzymes. First, CO 2 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 CO 2 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