Formate methanogenesis from

Fig. 1. The pathways of methanogenesis. Intermediates are abbreviated as in the text. The thick lines indicate the pathway for H2-CO2 methanogenesis, which is also in common to some extent with methanogenesis from one or more other substrates. The thin lines indicate specialized portions of pathways of methanogenesis from methanol, formate, and acetate, (a) Two different dehydrogenases have been reported, one dependent on H2F420, and one dependent on H2. (b) The source of these electrons may be H2F420 in some cases, but in other cases it is unknown see the text, (c) This is a possible alternative for methanol oxidation to the methylene-RjMPT level see the text for details.

The first evidence for cobalamin involvement in the conversion of methanol to methane was provided by Blaylock and Stadtman [196,216-218] with extracts of methanol-grown M. barkeri they demonstrated enzymatic formation of methylcobalamin from methanol, and subsequent reduction of methylcobalamin to methane. Later Blaylock [196] showed that conversion of methanol to methylcobalamin requires a heat-stable cofactor and at least three proteins, a 100-200 kDa Bi2-enzyme (methyltransferase), a ferredoxin, and an unidentified protein. Blaylock speculated that the role of hydrogen and ferredoxin in the conversion of methanol to methylcobalamin was in the reduction of the Bi2-protein. This work led to the proposal that methylcobalamin was the direct precursor of methane in methanogenesis from various substrates [196,218]. 

In order to explain the Na stimulation of ATP synthesis driven by a diffusion potential the presence of a Na /H antiporter was proposed [175]. In this artificial system the acidification of the cytoplasm, which occurs in response to electrogenic potassium efflux, could be prevented by the antiporter. Subsequently, Na /H antiporter activity has been demonstrated in both Methanobacterium thermoautotrophicum [176] and in Methanosarcina harden [108]. An important result of these studies was that the Na /H antiporter could be inhibited by amiloride and harmaline, which have been described as inhibitors of eucaryotic Na" /H" antiporters [177]. Using these inhibitors it has been shown that an active antiporter is essential for methanogenesis from H2/CO2 [176,178]. The antiporter also accepts Li instead of Na, since Li stimulates CH4 formation from H2/CO2 in the absence of Na [176]. In subsequent studies the use of amiloride and the more potent derivative ethyl-isopropylamiloride permitted the discrimination of primary and secondary Na potentials generated in partial reactions of the CO2 reduction pathway. 

According to this scheme the antiporter has a stoichiometric function in CO2 reduction to CH4 and ATP synthesis. Besides this function a role of the methanogenic Na"/FI" antiporter in pFl regulation (for reviews see refs. [177,180]) has also been proposed [ 176] to explain Na"-dependent ApFl formation (inside alkaline at external pH 5, inside acidic at external pH 9) in cell suspensions of Methanobacterium thermoautotrophicum. However, the data are difficult to interpret because of interference of Na"-dependent methanogenesis from H2/CO2 (for a discussion of pH regulation in methanogens see ref [16]). 

Fig. 12. Proposed role of methyl-tetrahydromethanopterin (CH3-H4MPT) coenzymeM (H-S-CoM) methyltransferase in methanogenesis from CO2, acetate and methanol methyltransferase as a reversible Na -translocating membrane protein. During CH4 formation from CO2 and from acetate, methyltransferase is involved in primary Na" extrusion generating A/INa during methanol disproportionation to CH4 and CO2 the enzyme is involved in A/iNa -driven methanol oxidation.

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]

In contrast to Sec-tRNA and fMet-tRNA formation, Pyl-tRNA y formation can occur by direct acylation of pyrrolysine onto tRNA i Pyrrolysine (Pyl), the twenty-second amino acid, was discovered incorporated in methylamine methyltrans-ferases from Methanosarcinaceae, a branch of methanogenic archaea that has the ability to reduce a wide variety of compounds to methane including carbon dioxide, acetate, methanol, methylated thiols, and methylated amines (72). Methanogenesis... 

Methane formation from CH3-C0M in acetate methanogenesis follows the same course described for H2-CO2 methanogenesis methyl-coenzyme M reductases have been purified from acetate-grown Methanosarcina and Methanothrix [242,26 i]. 

It was shown later that ATP synthesis can be coupled to methanogenesis at very low A/iH values (—90 to —lOOmV) [103] Proton potentials of defined magnitude, adjusted with K gradient in the presence of valinomycin, were applied to cells of Methanosarcina barkeri and Methanobacterium thermoautotrophicum, and the ATP pool and the rates of methane formation from Hz/methanol and from Hz/COz were followed as a function of... 

Acetate formation from pyruvate in the absence of methanogenesis... 

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