Biosynthesis, of methane

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. 

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. 

As indicated in Figure 4.6, the last step in the sedimentary metabolic pathway is methanogenesis. The formation of methane at a depth level, where all sulfate has been consumed, involves a group of strictly anaerobic archaea, collectively called methanogens. They use a small number of different low-molecular-weight substances for the biosynthesis of methane which, together with elemental hydrogen, in turn are formed... 

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. 

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. 

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)...

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) ... 

Coenzyme F430 is a nickel-containing tetrapyrrole that is required by methyl-coenzyme M reductase (Figure 29). It is one of a class of nickel chlorins found in nature.Methyl-coenzyme M reductase catalyzes the last step in methanogenesis, the reduction of methyl-coenzyme M to methane and coenzyme M (step 8, Figure 1). The biosynthesis of F430 follows the common tetrapyrrole biosynthetic pathway from glutamate to... 

C42H5 ClN4NiO,7, Mr 1006.04, uv, (pentamethyl ester, F 430 M in HjO) 431,418,296,274 nm. F. belongs to the structural class of the hydroporphyrins. Occurrence F. (coenzyme F 430), is the prosthetic group of methyl-coenzyme M reductase, which catalyzes the last step of methane formation in methano-genic bacteria. The mechanism of formation of methane from methyl-coenzyme M (H3C-S-CH2-CH2-SOJ) is still unknown. However, the central nickel atom in F. most certainly participates in the reduction. Biosynthesis F. is formed from 5-amino-4-oxova-leric acid via the normal biosynthetic route for porphi-noid compounds. The 2- and 7-methyl groups originate from 5-adenosylmethionine. 

Mechanism 20.7 outlines the steps involved in the haloform cleavage of a methyl ketone. The boxed essay The Haloform Reaction and the Biosynthesis of Trihalo-methanes describes its involvement as an environmental source of chloroform and bromoform. 

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