Acetogens pathway

The autotropic pathway for acetate synthesis among the acetogenic bacteria has been examined (67). Quantitative fermentation of one mole of glucose [50-99-7] yields three moles of acetic acid, while two moles of xylose [58-86-6] C H qO, yields five moles. The glucose reaction is... 

CODH from acetogens found to catalyze acetyl-CoA synthesis and nickel proposed to be the active site of CO oxidation and acetyl-CoA synthesis. CODH found to contain a heterometallic cluster consisting of nickel and iron that binds CO and proposed to be the active site of acetyl-CoA synthesis. Growth of acetogens on nitrate disables the acetyl-CoA pathway. ... 

Hsu T, MF Lux, HL Drake (1990) Expression of an aromatic-dependent decarboxylase which provides growth-essential COj equivalents for the acetogenic (Wood) pathway of Clostridium thermoaceticum. J Bacteriol 172 5901-5907. 

Figure 13.2. The preferred flow of reductant from aromatic aldehydes to the acetyl-CoA pathway by the acetogen C. formicoaceticum. THF, tetrahydrofolate brackets, the Ci unit is bound to a cofactor or structurally associated with an enzyme.

For many decades, the capacity of acetogens to reduce CO2 to acetate and the thermodynamics of the acetyl-CoA pathway casts the acetogens as somewhat obscure specialists and thermodynamically disadvantaged... 

Drake HL. 1994. Acetogenesis, acetogenic bacteria, and the acetyl-CoA Wood/ Ljungdahl pathway past and current perspectives. In Drake HL, editor. Acetogenesis. New York Chapman Hall. 

Electron-Transport Pathway Linked to the Use of CO2 as a Terminal Electron Acceptor in Acetogens... 

Figure 14.1. Proposed electron-transport pathway in the acetogenic bacteria M. thermoacetica and M. thermoautotrophica. Cyt., cytochrome MTi/FD//, methylene tetrahydrofolate dehydrogenase Fd, ferredoxin Fp, flavoprotein H2ase, hydrogenase.

Ljungdahl LG. 1986. The autotrophic pathway of acetate synthesis in acetogenic bacteria. Annu Rev Microbiol 40 415-50. 

Outline the metabolic pathways that are utilized by acetic acid-producing bacteria (acetogens) in the stoichiometric conversion of one molecule of glucose into three molecules of acetic acid. Indicate briefly the nature of any unusual coenzymes or metalloproteins that are required. 

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). 

This is a noncyclic pathway that also results in the fixation of two molecules of C02 to form acetyl-CoA. It was elucidated by Wood, Ljungdahl, Thauer and others as a pathway which is used by acetogenic bacteria to synthesize acetate from C02 in their energy metabolism [21]. The acetyl-CoA pathway resembles the Monsanto process of acetate synthesis from CO and methanol, with one molecule of C02 being reduced to the level of methyltetrahydropterin, while another C02 molecule is reduced to the level of carbon monoxide in the reaction catalyzed by the nickel-dependent carbon monoxide dehydrogenase (Figure 3.3). 

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