Coenzyme F420-dependent Methylenetetrahydromethanopterin Dehydrogenase (Mtd) from Methanopyrus kandleri: A Methanogenic Enzyme with an Unusual Quarternary Structure

The fourth reaction step of CO2-reduction to methane in methanogenic archaea is catalyzed by coenzyme F420-dependent methylenetetrahydromethanopterin dehydrogenase (Mtd). We have structurally characterized this enzyme in the selenomethionine-labelled form from the hyperthermophilic methanogenic archaeon Methanopyrus kandleri at 1.54 Å resolution using the single wavelength anomalous dispersion method for phase determination. Mtd was found to be a homohexameric protein complex that is organized as a trimer of dimers. The fold of the individual subunits is composed of two domains: a larger α,β domain and a smaller helix bundle domain with a short C-terminal β-sheet segment. In the homohexamer the α,β domains are positioned at the outside of the enzyme, whereas, the helix bundle domains assemble towards the inside to form an unusual quarternary structure with a 12-helix bundle around a 3-fold axis. No structural similarities are detectable to other enzymes with F420 and/or substituted tetrahydropterins as substrates. The substrate binding sites of F420 and methylenetetrahydromethanopterin are most likely embedded into a crevice between the domains of one subunit, their isoalloxazine and tetrahydropterin rings being placed inside a pocket formed by this crevice and a loop segment of the adjacent monomer of the dimer. Mtd revealed the highest stability at low salt concentrations of all structurally characterized enzymes from M. kandleri. This finding might be due to the compact quaternary structure that buries 36% of the monomer surface and to the large number of ion pairs.