Roles of Coenzyme F 420 -Reducing Hydrogenases and Hydrogen- and F 420 -Dependent Methylenetetrahydromethanopterin Dehydrogenases in Reduction of F 420 and Production of Hydrogen during Methanogenesis

Abstract

ABSTRACT Reduced coenzyme F 420 (F 420 H 2 ) is an essential intermediate in methanogenesis from CO 2 . During methanogenesis from H 2 and CO 2 , F 420 H 2 is provided by the action of F 420 -reducing hydrogenases. However, an alternative pathway has been proposed, where H 2 -dependent methylenetetrahydromethanopterin dehydrogenase (Hmd) and F 420 H 2 -dependent methylenetetrahydromethanopterin dehydrogenase (Mtd) together reduce F 420 with H 2 . 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 H 2 and CO 2 . Furthermore, we show that during growth on formate instead of H 2 , where F 420 H 2 is a direct product of formate oxidation, H 2 production occurs. H 2 presumably arises from the oxidation of F 420 H 2 , 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 H 2 and F 420 H 2 the Hmd-Mtd cycle.