Methanogenesis Group

These reductases play a key role both in methanogenesis and in the degradation of phenols that carry several nitro groups, which is discussed further in Chapter 9, Part 5. Although these reductases are typically found in methanogens, they have been encountered in a number of other bacteria and archaea ... 

Methane emissions from any ecosystem, particularly a rice agroecosystem (Figure 1), are governed by the magnitude and balance of microbial CH4 production (methanogenesis) and oxidation (methanotrophy), which occur by separate microbial communities. The two groups... 

Fig. 22.8. Energy yields for various anaerobic (top) and aerobic (bottom) metabolisms during mixing of a subsea hydrothermal fluid with seawater, expressed as a function of temperature, per kg of hydrothermal water. Energy yields for acetoclastic methanogenesis and acetotrophic sulfate reduction under oxic conditions are hypothetical, since microbes from these functional groups are strict anaerobes and cannot live in the presence of dioxygen.

The reactions catalyzed by B12 may be grouped into two classes those catalyzed by methylcobalamin and those catalyzed by cofactor B,2. The former reactions include formation of methionine from homocysteine, methanogenesis (formation of methylmercury is an important side reaction), and synthesis of acetate from carbon dioxide (82). The latter reactions include the ribonucleotide reductase reaction and a variety of isomerization reactions (82). Since dehydration and deamination have been studied quite extensively and very possibly proceed via [Pg.257]

Figure 4 Coenzymes of methanogenesis. F-430, coenzyme M (2-thioethanesulfonate), and coenzyme B (7-thioheptanoyl-threonine-O-phosphate [sometimes abbreviated HS-HTP]). Methyl-reducing factor is a structure proposed on the basis of NMR and mass spectrometry, in which the phosphate group of CoB is linked by a carboxylic-phosphoric anhydride to a UDP disaccharide, uridine 5 -(2-acetamido-2-deoxymannopyranuronosyl)-2-acetamido-2-deoxy-glucopyranosyl-diphosphate [171]. The CoB moiety without it appears to be functionally active in the enzyme reaction.

M. barkeri extracts methanogenesis from this labelled protein is at least 100 times faster than the non-enzymatic reaction between CoM and methylcobalamin they postulated that this protein acts as a methyltransferase in methanogenesis from methanol. Finally, Van der Meijden et al. [152-155] resolved the M. barkeri methyl transfer from methanol to CoM into two steps involving methyltransferase 1 and 2, or MTi and MT2. The oxygen-sensitive corrinoid protein MTj in its most reduced state (Co" ) accepts the methyl group from methanol [153,154] ... 

In Methanosarcina and Methanothrix" CH3-C0M, an intermediate of acetoclastic methanogenesis, is formed by transfer of the methyl group of acetyl-CoA to CoM [231,237,239,242]. This conversion. 

Methanogenesis from acetate in extracts of Methanosarcina does not require membrane addition [260]. However, this does not exclude a function for cytochromes in acetoclastic methanogenesis by whole cells. Rather, the role of H2 in cell extracts, the ability of cytochrome b from Methanosarcina species to react with CO, and the observation that membrane-bound cytochromeb of M. barkeri is reduced by H2, and is oxidized by CH3-CoM -I- ATP or CH3-C0M + acetyl-phosphate, all point to the participation of cytochromes in Methanosarcina. A role of cytochromes in transport of electrons generated from carboxyl-group oxidation to the heterodisulfide reductase is a logical hypothesis. 

The final step in methanogenesis is the reductive demethylation of CH3-S-CoM to CH4. This reduction involves two reactions CH3-S-C0M is reduced with jV-7-mercaptoheptanoylthreonine phosphate (H-S-HTP) (Fig. 2B) as electron donor to yield CH4 and a heterodisulfide of H-S-CoM and H-S-HTP (CoM-S-S-HTP) (Reaction 7, Table 2). This reaction is catalyzed by CH3-S-C0M reductase [70-72] which contains a nickel porphinoid, factorF430, as prosthetic group (Fig. 2C) (for a recent review see Friedmann et al. [73]). The subsequent reduction of the heterodisulfide with H2 to yield H-S-HTP and H-S—CoM (Reaction 8, Table 2) is catalyzed by CoM-S-S-HTP-dependent heterodisulfide reductase. The enzyme is an iron-sulfur protein containing FAD as prosthetic group [74]. The physiological electron donor for the heterodisulfide reductase is not known. 

Competition for fermentation products produces a succession of dominant metabolic pathways as distance from the source of electron donors and acceptors increases. Aerobic metabolism dominates the surface of sediments, and methanogenesis the deeper depths, as suggested by their free energy yield (Table 1). There is often very little overlap between each zone, suggesting nearly complete exclusion of one group by another. The same pattern is observed with distance from the surface of a root or burrow, or with distance downstream from an organic pollutant source in rivers and aquifers. 

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