Structural Basis of the Hydride Transfer Mechanism in F420-Dependent Methylenetetrahydromethanopterin Dehydrogenase

F-420-dependent methylenetetrahydromethanopterin (methylene-H4MPT) dehydrogenase (Mtd) of Methanopyrus kandleri is an enzyme of the methanolgenic energy metabolism that catalyzes the reversible hydride transfer between methenyl-H4MPT+ and methylene-H4MPT using coenzyme F-420 as hydride carrier. We determined the structures of the Mtd-methylene-H4MPT, Mtd-methenyl-H4MPT+, and the Mtd-methenyl-H4MPT+-F420H2 complexes at 2.1, 2.0, and 1.8 angstrom resolution, respectively. The pterin-imidazolidine-phenyl ring system is present in a new extended but not planar conformation which is virtually identical in methenyl-H4MPT+ and methylene-H4MPT at the current resolution. Both substrates methenyl-H4MPT+ and F420H2 bind in a face to face arrangement to an active site cleft, thereby ensuring a direct hydride transfer between their C14a and C5 atoms, respectively. The polypeptide scaffold does not reveal any significant conformational change upon binding of the bulky substrates but in turn changes the conformations of the substrate rings either to avoid clashes between certain ring atoms or to adjust the rings involved in hydride transfer for providing an optimal catalytic efficiency.