Biochemistry of methanogens

Most recent findings

In the last two years the group continued to work on the structure and catalytic mechanism of the iron-sulfur-cluster free hydrogenase (renamed [Fe]-hydrogenase) from hydrogenotrophic methanogens and of methyl-coenzyme M reductase from methanogenic and methanotrophic archaea. Finally, after 15 years of struggle, a crystal structure of the [Fe]-hydrogenase was obtained (Shima et al. 2008). Almost of equal importance is our finding that upon reaction of the nickel enzyme methyl-coenzyme M reductase with its coenzymes M and B a nickel hydride complex is formed. In view of the known ability of nickel hydrides to activate methane, and the growing body of evidence that methyl-coenzyme M reductase is involved in the anaerobic oxidation of methane (AOM) the nickel hydride complex could play a key role in helping to understand both the "reverse" and "forward" methanogenesis. For this and other results on methyl-coenzyme M reductase the reader is referred to the publications by Hinderberger et al. (2006), Kern et al. (2007), Kahnt et al. (2007), and Harmer et al. (2008). Recents Reviews on the subject are by Jaun and Thauer (2007) and Thauer and Shima (2008). In our work on [Fe]-hydrogenase we have continued our intensive collaboration with Ulrich Ermler from the Max Planck Institute for Biophysics in Frankfurt and in our work on methyl-coenzyme M reductase with Bernhard Jaun and Arthur Schweiger from the ETH Zürich.

The discovery of a novel pathway of heme biosynthesis in archaea is described in the paper by Buchenau et al. (2006) and the identification of the enzyme catalyzing the synthesis of Re-citrate in bacteria by Li et al. (2007)

Several new questions were taken up, the most important being how methanogens without cytochromes (all but the members of the Methanosarcinales) conserve energy during methyl-coenzyme M reduction with H2 despite the fact that all the components involved are apparently cytoplasmic and not associated with the cytoplasmic membrane (Thauer et al. 2008). A similar situation prevails in Clostridium kluyveri in which the reduction of crotonyl-CoA with NADH is coupled with energy conservation. The cytoplasmic butyryl-CoA dehydrogenase/electron transfer flavoprotein complex was shown by us to couple the exergonic reduction of crotonyl-CoA (Eº´ = -10 mV) with NADH (-320 mV) with the endergonic reduction of ferredoxn (-420 mV) with NADH (-320 mV) (Li et al. 2008). The novel coupling mechanism is referred to as "flavin-based electron bifurcation" in analogy to "quinone-based electron bifurcation" involved in energy conservation in the bc1 complex of the respiratory chain (Herrmann, G., Jayamani, E., Mai, G., and Buckel, W. (2008) Energy conservation via electron-transferring flavoprotein in anaerobic bacteria. J. Bacteriol. 190, 784-791).