Roles of coenzyme F420-reducing hydrogenases and hydrogen- and F420-dependent methylenetetrahydromethanopterin dehydrogenases in reduction of F420 and production of hydrogen during methanogenesis

Abstract

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.

FIG. 1.

The hydrogenotrophic methanogenic pathway. See reference for a full description of methanogenesis. The Hmd-Mtd cycle is boxed. Abbreviations: CoB, coenzyme B; CoM, coenzyme M; F₄₂₀, coenzyme F₄₂₀; Fd, ferredoxin; Frc, cysteine-containing F₄₂₀-reducing hydrogenase; Fru, selenocysteine-containing F₄₂₀-reducing hydrogenase; Mer, methylenetetrahydromethanopterin reductase; MFR, methanofuran.

FIG. 2.

Growth and H₂ production by wild-type and mutant strains of M. maripaludis. (A) Growth on H₂; (B) growth and H₂ production on formate. For the Δmtd mutant on formate, data from three separate growth experiments are plotted and are represented by a single line. OD₆₆₀, optical density at 660 nm.