FIG 7.

The H₂-dependent electron transport system of Methanosarcina. Electrons enter the transport system as either externally provided H₂ or internally generated Fd_red or F420_red. All pathways involve the oxidation of H₂ by Vht, and the transport of electrons by MP to the heterodisulfide reductase for reduction of the terminal electron acceptor, CoM-CoB. Regeneration of CoM and CoB enables continued methanogenesis. Frh and Ech generate H₂ with electrons from F420_red and Fd_red, respectively. The internally produced H₂ diffuses across the membrane (dashed line) and is oxidized by Vht, thereby transferring 2 H⁺ ions from the inside to the outside of the cell. The exergonic reaction catalyzed by Ech facilitates additional proton translocation. For each pathway, 2 H⁺ ions are consumed from the cytoplasm during MP reduction and released outside of the cell upon MP oxidation. Thus, the transport of 2 electrons results in 4 H⁺ ions translocated for the F420_red:CoM-CoB pathway and 5 H⁺ ions translocated for the Fd_red:CoM-CoB pathway (indicated in red). Brown lines trace the putative pathways of electron transport. Enzyme subunits are identified by letters.