Hydrogen is a preferred intermediate in the energy-conserving electron transport chain of Methanosarcina barkeri

Abstract

Methanogens use an unusual energy-conserving electron transport chain that involves reduction of a limited number of electron acceptors to methane gas. Previous biochemical studies suggested that the proton-pumping F 420H2 dehydrogenase (Fpo) plays a crucial role in this process during growth on methanol. However, Methanosarcina barkeri ⌬fpo mutants constructed in this study display no measurable phenotype on this substrate, indicating that Fpo plays a minor role, if any. In contrast, ⌬frh mutants lacking the cytoplasmic F 420-reducing hydrogenase (Frh) are severely affected in their ability to grow and make methane from methanol, and double ⌬fpo/⌬frh mutants are completely unable to use this substrate. These data suggest that the preferred electron transport chain involves production of hydrogen gas in the cytoplasm, which then diffuses out of the cell, where it is reoxidized with transfer of electrons into the energy-conserving electron transport chain. This hydrogen-cycling metabolism leads directly to production of a proton motive force that can be used by the cell for ATP synthesis. Nevertheless, M. barkeri does have the flexibility to use the Fpo-dependent electron transport chain when needed, as shown by the poor growth of the ⌬frh mutant. Our data suggest that the rapid enzymatic turnover of hydrogenases may allow a competitive advantage via faster growth rates in this freshwater organism. The mutant analysis also confirms the proposed role of Frh in growth on hydrogen/carbon dioxide and suggests that either Frh or Fpo is needed for aceticlastic growth of M. barkeri.hydrogen electron transport ͉ F420 ͉ H2 cycling ͉ methanogenesis

Physiology, Biochemistry, and Applications of F420- and Fo-Dependent Redox Reactions

“…While Frh primarily sustains H 2 -mediated F 420 reduction during hydrogenotrophic growth, it may mediate F 420 H 2 -mediated H 2 production during methylotrophic methanogenesis and formate-dependent growth (224,225). This is consistent with the observations of severe defects of Ms. barkeri ⌬frh mutants during growth on methanol and on H 2 production during formate-dependent growth of Mc.…”

“…Consistently, trimethylamine-cultured ⌬fpo mutants of Ms. mazei are severely compromised in growth and methane formation compared to the wild-type strain (193). Surprisingly, these findings do not extend to Ms. barkeri; in this organism, Fpo appears to be dispensable for methylotrophic growth, whereas Frh is essential (224). On this basis, Kulkarni et al (224) …”

“…7) (221)(222)(223). This hydrogenase is essential for growth on H 2 /CO 2 in Methanosarcina barkeri (Ms. barkeri) (224), but it appears to be dispensable in methanogens with genes that encode an alternative pathway for F 420 reduction such as Methanococcus maripaludis (Mc. maripaludis) (225).…”

“…It has been proposed that Frh is physiologically active in both the forward and reverse directions (224,225). While Frh primarily sustains H 2 -mediated F 420 reduction during hydrogenotrophic growth, it may mediate F 420 H 2 -mediated H 2 production during methylotrophic methanogenesis and formate-dependent growth (224,225).…”

Genomic composition and dynamics amongMethanomicrobialespredict adaptation to contrasting environments

“…The importance of this point is highlighted by the fact that seemingly subtle differences in energy conservation and electron trafficking pathways can be central to different lifestyles in contrasting environments, as seen, for example, in freshwater versus marine Methanosarcina spp. (Ferry and Lessner, 2008;Kulkarni et al, 2009).…”