Methanogens cytochromes

The sulfite reductase from the hyperthermophilic methanogen Methanocaldococcus jannashii is able to reduce the otherwise toxic sulfite to sulfide that is required for growth. In contrast to most organisms that use nicotinamides and cytochromes as electron carriers, this organism uses a coenzyme p42o-dependent reductase (Johnson and Mukhopadhyay 2005). 

The metal complexes found in geological media are based on four classes of ligand the porphyrins, the simplest being porphin (19), dihydroporphyrins (chlorins) (20), tetrahydropor-phyrins (21) and the corrins (22). These in turn are generally thought to arise from the breakdown products of the chlorophylls, haem-type centres, especially the cytochromes, and related complexes such as vitamin B12 and nirrins, associated perhaps with metal enzymes in methanogenic and allied bacteria. 

The MR from M. thermophila does not require A proteins in vitro [268]. The enzyme is reductively activated by Ti(IlI) (a process stimulated by, but not requiring, ATP), to give a specific activity of 0.22 pmol/minmg. The minimal components needed for activity are MR, Ti(III) and HSHTP, but physiologically it is likely that ferredoxin, cytochrome b, and heterodisulfide reductase are involved. The MR from M voltae, M. jannaschii, and M. barkeri 227 have also been purified [386], and it was shown with these enzymes that crude A components were active when used with MR from other methanogens. 

The formation of methane by the methanogens is called carbon dioxide respiration, as described previously (p. 112). In general, cytochromes occur as the electron carrier in the respiration. Thus, some methanogens have cytochromes b and c. However, not all methanogens have cytochromes, as already mentioned. This may mean that carbon dioxide respiration is an early evolving stage of the respiration from the noncytochrome-type to the cytochrome-type. The electron transfer systems of the sulfur respirers contain cytochrome b or cytochromes b and c (Hedderich et al., 1999). In the iron respiration system of Pyrobaculum islandicum, cytochrome b seems to participate in the Fe(III) reducing system (Childers and Lovley, 2001). These results will show that the fundamental features of the respiration may have already been established in the hyperthermophilic chemolithoautotrophic archaea (Fig. 7.2). 

Kuhn W, Fiebig K, Hippe H, Mah RA, Huser BA, Gottschalk G (1983) Distribution of cytochromes in methanogenic bacteria. FEMS Microbiol Lett 20 407-410 Kuhn W, Gottschalk G (1983) Characterization of the cytochromes occurring in Methanosarcina species. Eur J Biochem 135 89-94... 

Kumazawa Y, Fujiwara T, Fukumori Y, Koga Y, Yamanaka T (1994) Cytochrome be purified from the methanogen Methanosarcina barkeri. Curr Microbiol 29 53-56 Kurihara Y (1999) Exploitation of anti-bacterial concrete which guards concrete from its corrosion (in Japanese). Cement Concrete 633, 44 19... 

With the possible exception of the redox enzymes cytochrome P-450 (iron) and the methanogenic bacterial cofactor F-430 (nickel) [5-7], the cobalamines are the only... 

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