Crystal structure of the methane formation enzyme elucidated
The crystal structure of the enzyme that catalyzes the formation of methane has recently been solved at unusually high resolution, as reported in Science (November 1997). This was achieved in close cooperation with scientists from the two Max Planck Institutes for Biophysics in Frankfurt and for Terrestrial Microbiology in Marburg. Methane is always formed where biomass is broken down by microorganisms and oxygen is used up. Methane-forming microorganisms settle here, e.g. in sediments from water bodies, in swamps, in the soil of paddy fields, in the digestion tower of Kadra plants, in landfills, in the rumen of ruminants and in the intestinal tract of termites. In total, in these biotopes approx.1 billion tons of methane formed per year.
For us humans, methane is important as a possible fuel, but also as a greenhouse gas. For the methanogens, it is the end product of their energy metabolism. The now elucidated crystal structure of the methane-forming enzyme, which is called methyl-coenzyme-M-reductase, allows conclusions to be drawn about the mechanism of methane formation. Nickel and most likely radical intermediates are involved. The structure shows the binding of the two involved coenzymes B and M and the nickel-containing cofactor F 430 within a hydrophobic anhydrous chamber that is only accessible through a 6 engstrom narrow channel from the enzyme surface. The binding of the products can be seen in a second enzyme form.
The high resolution of the crystal structure up to 1.45 E is also amazing. It is far better than that of most structurally characterized enzymes. And this despite the fact that the methanogenic enzyme has a molecular mass of around 300.000 days Has. Until recently, such an exact structure determination seemed impossible for such large proteins.