Methanogens metabolic enzymes

The enzyme is present in large amounts in acetogenic bacteria, where it is involved in an unusual pathway for fixation of C02 with the formation of acetate (25,26). In acetogens this reaction is involved both in production of energy, with acetate as a waste product, and in biosynthesis of cell constituents starting from acetate. CO oxidoreductase is also present in methanogenic bacteria, where it is used in biosynthetic metabolism (25, 27). 

Nickel enzymes are particularly prominent in the metabolism of anaerobic bacteria. For example, the methanogenic bacteria, which are classified as Arch-aea, an ancient division of living organisms, can grow on a mixture of H2 and C02 to produce methane [9-11], The metabolism of methanogens involves three... 

The catalysis of the transfer of a methyl group is an important role of enzyme-bound vitamin B12 derivatives in human, animal, and bacterial metabolism. The known enzyme-controlled methyl group-transfer reactions are key steps in the cobamide-dependent methylations of homocysteine to methionine, in the metabolic formation of methane from other Ci-compounds in methanogenic bacteria, and in the fixation of carbon dioxide via the acetyl coenzyme A pathway of some bacterial autotrophs (Figure 10). ... 

With regard to amino acid metabolism, the data are scarce but it is probable that these metabolites give rise to, or are derived from, the oxoacids of the citric acid cycle via transamination or analogous reactions. See the literature [60-62] for examples of such enzymes in the halophilic, thermophilic and methanogenic archaebacteria. 

Figure 4 Metabolic scheme for the degradation of complex organic matter, culminating in methanogen-esis. Polymers are cleaved via extracellular or ceU-surface associated enzymes to monomers that are fermented to organic products, H2 and CO2. Methane is formed primarily from the oxidation of H2 coupled to CO2 reduction or by the fermentation of acetate. Acetate is formed by primary fermentation, acetogenesis from H2/CO2, and from secondary fermentation of primary fermentation products.

Most studies in the microbial metabolism of nitroaromatic compounds used aerobic microorganisms. In most cases no mineralization of nitroaromatics occurs, and only superficial modifications of the structures are reported. However, under anaerobic sulfate-reducing conditions, the nitroaromatic compounds reportedly undergo a series of reductions with the formation of amino compounds. For example, trinitrotoluene under sulfate-reducing conditions is reduced to triaminotoluene by the enzyme nitrite reductase, which is commonly found in many Desulfovibrio spp. The removal of ammonia from triaminotoluene is achieved by reductive deamination catalyzed by the enzyme reductive deaminase, with the production of ammonia and toluene. Some sulfate reducers can metabolize toluene to (X) sub 2. Similar metabolic processes could be applied to other nitroaromatic compounds like nitrobenzene, nitrobenzoic acids, nitrophenols, and aniline. Many methanogenic bacteria can reduce nitroaromatic compounds to amino compounds. 

In comparison to all other heterotrophs, the microorganisms oxidizing methane and other Cj compounds such as methanol, have a unique metabolic pathway which involves oxygenase enzymes and thus requires O. Only aerobic methane-oxidizing bacteria have been isolated and studied in laboratory culture, yet methane oxidation in marine sediments is known to take place mostly anaerobically at the transition to the sulfate zone. Microbial consortia that oxidize methane with sulfate have in particular been studied at methane seeps on the sea floor and the communities can now also be grown in the laboratory (Boetius et al. 2000 Orphan et al. 2001 Nauhaus et al. 2002) Anaerobic methane oxidation is catalyzed by archaea that use a key enzyme related to the coenzyme-M reductase of methanogens, to attack the methane molecule (Kruger et al. 2003 see Sect. 5.1). The best studied of these ANME (ANaerobic MEthane... 

Rbntgen fluorescence spectroscopy and plasma atomic emission allow quantitative measurement of S, P as well as many other elements. The ion mass fraction of Kluyveromyces marxianus and M thermoautotrophicum are listed in Table 7. The large difference in iron and nickel content is related to the metabolism. In fact, these two ions are cofactors of key enzymes in methanogene-sis [35]. The total mass of ions found in biomass is 6.36 g per 100 g biomass for M. thermoautotrophicum and 3.14 for K. marxianus (phosphorus is not taken into account since it is not present as free ion in biomass). The values are in good agreement with [36]. 

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