Methane biosynthesis

This unique redox catalyst links the oxidation of H2 or of formate to the reduction of NADP+229 and also serves as the reductant in the final step of methane biosynthesis (see Section E) 228 It resembles NAD+ in having a redox potential of about -0.345 volts and the tendency to be only a two-electron donor. More recently free 8-hydroxy-7,8-didemethyl-5-deazaribo-flavin has been identified as an essential light-absorbing chromophore in DNA photolyase of Methanobacterium, other bacteria, and eukaryotic algae.230 Roseoflavin is not a coenzyme but an antibiotic from Streptomyces davawensisP1 Many synthetic flavins have been used in studies of mechanisms and for NMR232 and other forms of spectroscopy. 

Vitamin B12 is virtually nontoxic, even at high oral or injected doses excessive amounts are rapidly excreted. However, occasionally allergic responses to injected vitamin B12 occur (Fisher 1973), and adverse reactions to the combined administration of large injected doses of vitamin B12 and of oral vitamin C have been reported (Schrauzer 1979). Vitamin B12 is required for methionine biosynthesis and functions in conjunction with folic acid as the intermediate carrier of the methyl group. In its coenzyme form (5 -deoxyadenosylcobala-min), it is required for the conversion of methylmalonyl-CoA to succinyl-CoA. (Friedrich 1987). Bacteria utilize vitamin Bjj or its coenzyme in certain dehydrases, deaminases, and in methane biosynthesis. 

The unprotonated forms react by a bimolecular step with rate constant 1.7 x 10 M s at 25 °C in 40% MeOH-HgO. Reaction of the acid form is imimolecular with a rate constant 1.6 x 10 s involving two-electron oxidation of the metal from cobalt(i) to cobalt(m). This requires a rapid reaction between cobalt(i) and cobalt(iii) to give the cobalt(ii) product. Methylcobalamin is also involved in methane biosynthesis. ... 

The lower than expected yields can be explained by the nature of methane oxidation to methanol in these bacteria. This reaction, catalysed by methane mono-oxygenase, is a net consumer of reducing equivalents (NADH), which would otherwise be directed to ATP generation and biosynthesis. In simple terms the oxidation of methane to methanol consumes energy, lowering the yield. 

Experiments have been carried out to mimic the reactions of model systems for coenzyme F430 that is involved in the terminal step in the biosynthesis of methane, and that is able to dechlorinate CCI4 successively to CHCI3 and CH2CI2 (Krone et al. 1989). Nickel(I) isobacteriochlorin anion was generated electrolytically and used to examine the reactions with alkyl halides in dimethylformamide (Helvenston and Castro 1992). The three classes of reaction were the same as those observed with Fe(II) deuteroporphyrin IX that have already been noted. 

In tracing the evolutionary development of iron ligands it is of interest to examine the machinery employed by organisms which carry out reactions on those substances believed to have been present on the primitive Earth. Specific substrates acted on by this group include, besides ferrous iron itself, hydrogen sulfide, hydrogen gas, methane and reduced nitrogen compounds. Species which perform photosynthesis may be presumed to have the capacity to synthesize protoporphyrin IX since this substance is an intermediate in chlorophyll biosynthesis (43). 

DiMarco, A. A., T. A. Bobik, and R. S. Wolfe, Unusual coenzymes of methanogenesis. Ann. Rev. Biochem. 59 355 (1990). A review of the recently characterized coenzymes required for the biosynthesis of methane in methanogenic bacteria. 

Urhahn T, Ballschmiter K (1998) Chemistry of the Biosynthesis of Halogenated Methanes Cl-Organohalogens as Pre-Industrial Chemical Stressors in the Environment Chemosphere 37 1017... 

Harper DB, McRoberts WC, Kennedy JT (1996) Comparison of the Efficacies of Chloro-methane, Methionine, and S-Adenosylmethionine as Methyl Precursors in the Biosynthesis of Veratryl Alcohol and Related Compounds in Phanerochaete chrysosporium. Appl Environ Microbiol 62 3366... 

Figure 10 Methylcorrinoids as the source of a methyl group in the biosynthesis of methionine from homocysteine (top), of methane (middle), and of the acetyl group of acetyl coenzyme A (bottom)...

Almost all methanogen species can grow on H2+CO2 carrying out autotrophic cell carbon biosynthesis [183]. Due to simplicity of the catabolites, C02-reduction to methane has been extensively used as a model system to study the biochemistry of methanogenesis. Most of this work has used M. thermoautotrophicum, and to a lesser extent, M. barkeri. 

Although they are known to be synthesized by a wide variety of cultured aerobic bacteria there does not appear to be any obligate requirement for oxygen in their biosynthesis. The biosynthesis and cyclization of squalene to a pentacyclic triterpenoid with a hopane skeleton does not seem to require oxygen and, therefore, hopanoid synthesis might also be possible in anaerobes. For instance, analysis of microbial mats at methane seeps under anoxic Black Sea water revealed the presence of C-depleted (8 C = -lS%c) hopanoids with an unusual stereochemistry. This isotopic depletion indicates in situ production and, therefore, suggests that anaerobes are responsible (Thiel et ai, 2003). 

The present studies confirm the earlier studies indicating the relatively great biosynthetic abilities of the methane bacteria and suggest that much of the cellular carbon compounds are probably synthesized from acetate and carbon dioxide. In view of the carbon dioxide and acetate requirements and the reductive carboxylation reactions shown to be involved in isoleucine synthesis in M. ruminantium (26) and the probability of similar carboxylation reactions in biosynthesis of isoleucine, alanine, and other amino acids in MOH, suggested by the studies on M. omelianskii (34), the operation of the pyruvate synthase reaction and some other reactions of the reductive carboxylic acid cycle (35, 36) as major pathways of biosynthesis of cellular materials in these bacteria is an attractive hypothesis. 

The formation of the bromomethanes CHBr and CH2Br2 by marine organisms is believed to be an enzymatic bromination of ketone metabolites (38, 39). A possible biosynthesis of bromoform and dibromomethane, postulated by Moore (38), is presented in Figure 7.5. Dibromochloromethane and dichlorobromo-methane are possibly directly produced by marine organisms or can also be formed by nucleophilic substitution of bromoform with chloride of sea water according to equations (3) and (4) (3, 23) ... 

Roy R, Knowles R (1995) Differential inhibition by allylsulfide of nitrification and methane oxidation in freshwater sediment. Appl Environ Microbiol 61 4278 1283 Saiki K, Mogi T, Anraku Y (1992) Heme O biosynthesis in Escherichia coli the cyoE gene in the cytochrome bo operon encodes a protoheme IX famesyl transferase. Biochem Biophys Res Commun 189 1491-1497... 

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