Structure of methylene-tetrahydromethanopterin dehydrogenase from Methylobacterium extorquens AM1

NADP-dependent methylene-H4MPT dehydrogenase, MtdA, from Methylobacterium extorquens AM1 catalyzes the dehydrogenation of methylene-tetrahydro-methanopterin and methylene-tetrahydrofolate with NADP(+) as cosubstrate. The X-ray structure of MtdA with and without NADP bound was established at 1.9 Angstrom resolution. The enzyme is present as a homotrimer. The alpha,beta fold of the monomer is related to that of methylene-H4F dehydrogenases, suggesting a common evolutionary origin. The position of the active site is located within a large crevice built up by the two domains of one subunit and one domain of a second subunit. Methylene-H4MPT could be modeled into the cleft, and crucial active site residues such as Phe18, Lys256, His260, and Thr102 were identified. The molecular basis of the different substrate specificities and different catalytic demands of MtdA compared to methylene-H4F dehydrogenases are discussed.