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

摘要

F420-dependent methylenetetrahydromethanopterin (methylene-H4MPT) dehydrogenase (Mtd) ofnMethanopyrus kandleri is an enzyme of themethanogenic energymetabolism that catalyzes the reversible hydridentransfer between methenyl-H4MPTnþnand methylene-H4MPT using coenzyme F420 as hydride carrier. Wendetermined the structures of the Mtd-methylene-H4MPT, Mtd-methenyl-H4MPTnþn, and the Mtd-methenyl-nH4MPTnþn-F420H2 complexes at 2.1, 2.0, and 1.8 A ° resolution, respectively. The pterin-imidazolidine-phenylnring system is present in a new extended but not planar conformation which is virtually identical in methenyl-nH4MPTnþnandmethylene-H4MPT at the current resolution. Both substratesmethenyl-H4MPTnþnand F420H2 bindnin a face to face arrangement to an active site cleft, thereby ensuring a direct hydride transfer between their C14anandC5 atoms, respectively. The polypeptide scaffold does not reveal any significant conformational change uponnbinding of the bulky substrates but in turn changes the conformations of the substrate rings either to avoid clashesnbetween certain ring atoms or to adjust the rings involved in hydride transfer for providing an optimal catalyticnefficiency.