Comparative genomics reveals electron transfer and syntrophic mechanisms differentiating methanotrophic and methanogenic archaea
Fig 6
Cytoplasmic electron carrier oxidation.
Some energy conservation systems discovered in methanogenic archaea are conserved in ANME archaea (colored fill), while others appear absent (transparent gray with diagonal line fill). (A) F420H2 oxidation is coupled to proton translocation in methylotrophic methanogens via the Fpo/Fqo complex or by the production of H2 by Frh and subsequent oxidation by Vht. In either case, electrons are ultimately deposited on MpH2. Neither Frh or Vht complexes have been observed in any ANME genomes analyzed here. (B) Fd2− oxidation can be coupled either to sodium motive force or proton motive force in methylotrophic methanogens. The Rnf complex catalyzes Fd2−:Mp oxidoreductase reaction coupled to sodium translocation and is found in a number of methanogens and ANME. The ANME-2c contain most of the complex but lack the cytochrome c, cytochrome b, and MA0665 subunits, so their activity is difficult to predict. Ech and Vht can combine to produce net proton translocation via H2 diffusion in methylotrophic methanogens, but neither complex is found in ANME. FpoF can catalyze a Fd2−:F420 oxidoreductase reaction, and F420H2 could then pass through the Fpo/Fqo complex. Various HdrABC complexes are present in all ANME genomes and could in principle oxidize Fd2− and CoM-SH/CoB-SH through a reversal of electron bifurcation reaction. The electron acceptor in this process is likely to not be H2 in most ANME groups due to the absence of MvhG and MvhA. (C) Besides, the HdrABC complexes mentioned above and second possible CoM-SH/CoB-SH oxidation strategy would be a reversal of the HdrDE reaction found in methylotrophic methanogens. In ANME, the reaction would have to proceed in the direction illustrated and therefore would dissipate proton motive force by consuming a proton on the positive side of the membrane. For presence/absence of these systems in ANME genomes analyzed here, see S6 Fig. ANME, anaerobic methanotrophic; Ech, energy-conserving hydrogenase; Fpo, F420H2:methanophenazine oxidoreductase; Fqo, F420H2:quinone oxidoreductase complex; Frh, F420-reducing hydrogenase.