Energy conservation in aceticlastic methanogenic archaea and the human gut archaeon <em>Methanomassiliicoccus luminyensis</em>

Abstract

Methanogenic <em>Archaea</em> produce methane from products of anaerobic bacterial metabolism. Therefore, they play a central role in global carbon cycling. The fact that methane is a potent green house gas further stresses the ecological relevance of methane producing <em>Archaea</em>. Furthermore, in recent years several new methanogenic species from the human gastrointestinal tract have been identified, which might be beneficial for the human host. <br /> In this study energy conservation of the aceticlastic methanogenic <em>Archaea</em> <em>Methanosaeta (Mt.) thermoacetophila</em> and <em>Methanosarcina (Ms.) mazei</em> as well as energy conservation in <em>Methanomassiliicoccus (Mm.) luminyensis</em> from the human gut has been investigated.<br /> First, the acetate activation reaction in <em>Mt. thermoacetophila</em> was examined in detail. It was found that the AMP-forming acetyl-CoA synthetase Mthe1194 and a soluble pyrophosphatase (Mthe0236) are involved in the acetate activation reaction. Thus, two ATP equivalents are spent for acetate activation in <em>Mt. thermoacetophila</em> that have to be regained by membrane-bound enzymes of the respiratory chain. Interestingly, in <em>Mt. thermoacetophila</em> the Fpo complex can be found, which is known to oxidize the electron carrier F₄₂₀H₂ and channels electrons into the respiratory chain. However, no F₄₂₀H₂ is built during aceticlastic methanogenesis. Instead, electrons are transferred to ferredoxin. In the course of this study it was shown that in the model organism <em>Ms. mazei</em> the Fpo complex is able to accept electrons not only from F₄₂₀H₂ but also from reduced ferredoxin. The reaction might enable the translocation of three protons across the cytoplasmic membrane instead of two as was previously described. This provides a possible explanation how <em>Mt. thermoacetophila</em> can grow with acetate as substrate.<br /><em>Mm. luminyensis</em> uses H₂ and methylated compounds as substrates. H₂ is used as electron donor and is oxidized via the MvhADG/HdrABC complex. This complex produces reduced ferredoxin that has to be oxidized by membrane-bound enzyme(s). In this study, activity of the Ech hydrogenase from the family of energy converting hydrogenases was demonstrated and thus <em>Mm. luminyensis</em> might conserve energy by translocating protons via the Ech hydrogenase.