Electron flow in hydrogenotrophic methanogens under nickel limitation
Shunsuke Nomura, Pablo San Segundo‐Acosta, E. A. Protasov, Masanori Kaneko, Jörg Kahnt, Bonnie J. Murphy, Seigo Shima
Abstract
Abstract Methanogenic archaea are the main producers of the potent greenhouse gas methane 1,2 . In the methanogenic pathway from CO 2 and H 2 studied under laboratory conditions, low-potential electrons for CO 2 reduction are generated by a flavin-based electron-bifurcation reaction catalysed by heterodisulfide reductase (Hdr) complexed with the associated [NiFe]-hydrogenase (Mvh) 3–5 . F 420 -reducing [NiFe]-hydrogenase (Frh) provides electrons to the methanogenic pathway through the electron carrier F 420 (ref. 6 ). Here we report that under strictly nickel-limited conditions, in which the nickel concentration is similar to those often observed in natural habitats 7–11 , the production of both [NiFe]-hydrogenases in Methanothermobacter marburgensis is strongly downregulated. The Frh reaction is substituted by a coupled reaction with [Fe]-hydrogenase (Hmd), and the role of Mvh is taken over by F 420 -dependent electron-donating proteins (Elp). Thus, Hmd provides all electrons for the reducing metabolism under these nickel-limited conditions. Biochemical and structural characterization of Elp–Hdr complexes confirms the electronic interaction between Elp and Hdr. The conservation of the genes encoding Elp and Hmd in CO 2 -reducing hydrogenotrophic methanogens suggests that the Hmd system is an alternative pathway for electron flow in CO 2 -reducing hydrogenotrophic methanogens under nickel-limited conditions.