Roles of Coenzyme F420-Reducing Hydrogenases and Hydrogen- and F420-Dependent Methylenetetrahydromethanopterin Dehydrogenases in Reduction of F420 and Production of Hydrogen during Methanogenesis

摘要

Reduced coenzyme F420 (F420H2) is an essential intermediate in methanogenesis from CO2. During methanogenesis from H2 and CO2, F420H2 is provided by the action of F420-reducing hydrogenases. However, an alternative pathway has been proposed, where H2-dependent methylenetetrahydromethanopterin dehydrogenase (Hmd) and F420H2-dependent methylenetetrahydromethanopterin dehydrogenase (Mtd) together reduce F420 with H2. Here we report the construction of mutants of Methanococcus maripaludis that are defective in each putative pathway. Their analysis demonstrates that either pathway supports growth on H2 and CO2. Furthermore, we show that during growth on formate instead of H₂, where F₄₂₀H₂ is a direct product of formate oxidation, H₂ production occurs. H₂ presumably arises from the oxidation of F₄₂₀H₂, and the analysis of the mutants during growth on formate suggests that this too can occur by either pathway. We designate the alternative pathway for the interconversion of H₂ and F₄₂₀H₂ the Hmd-Mtd cycle.