Methanogens reactions involving

Deppemneier U., Lienard T., and GottschaUc G. (1999) Novel reactions involved in energy conservation by methanogenic archaea. Federat. Euro. Biochem. Soc. Lett. 457, 291—297. 

Heterotrophic methanogens use acetate directly for biosynthesis and as an energy source. During this process, methanogens convert acetate to carbon dioxide and methane by utilizing a different biochemical pathway than the autotrophic methanogens. The sequence of reactions involved are as... 

Methyl coenzyme-M reductase (MCR) catalyzes the final step of methane production in methanogenic archaebacteria, which accounts for all biologically generated methane on the earth (4). The reaction involves methane formation concomitantly with the formation of a disulfide bond between coenzyme M (CoM) and coenzyme B (CoB) ... 

An alternative pathway by which some acetogenic bacteria form acetate is via reversal of the glycine decarboxylase reaction of Fig. 15-20. Methylene-THF is formed by reduction of C02, and together with NH3 and C02 a lipoamide group of the enzyme and PLP forms glycine. The latter reacts with a second methylene-THF to form serine, which can be deaminated to pyruvate and assimilated. Methanogens may use similar pathways but ones that involve methanopterin (Fig. 15-17).191... 

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). 

The cell walls do not contain muramic acid. Ihe characteristic constituent of the pcplidnglycuns that form lxicten.il cell walls. (2l Their mciaholism differs markedly from bacteria. A number nf coenzymes apparently unique 10 methanogens have been identified. Some of these enzymes are involved in methyl transfer reactions, including the formation of methane. One of the coenzymes is possibly the smallesi coen/yme yet... 

Perhaps the best-characterized example of this mechanism involves the synthesis of heme cofactors and their subsequent incorporation into various hemoproteins (see Iron Heme Proteins Electron Transport). Succinctly, enzyme-catalyzed reactions convert either succinyl-CoA or glutamate into 5-ammolevulinic acid. This molecule is further converted through a series of intermediates to form protoporphyrin IX, the metal-ffee cofactor, into which Fe is inserted by ferrochelatase. Analogous reactions are required for the synthesis of other tetrapyrrole macrocycles such as the cobalamins (see Cobalt Bu Enzymes Coenzymes), various types of chlorophylls, and the methanogen coenzyme F430 (containing Co, Mg, or Ni, respectively). Co- and Mg-chelatases have been described for insertion of these metals into the appropriate tetrapyrrolic ring structures. ... 

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