“Acetogens are obligately anaerobic bacteria that use the reductive acetyl-CoA or Wood–Ljungdahl pathway as their main mechanism for energy conservation and for synthesis of acetyl-CoA and cell carbon from CO2 [2,3]. An acetogen is sometimes called a ‘homoacetogen’ (meaning that it produces only acetate as its fermentation product) or a ‘CO2-reducing acetogen’…organisms that house acetogens in their digestive systems [include] humans, termites, and ruminants. Since the build-up of H2 inhibits biodegradation by creating an unfavorable thermodynamic equilibrium, acetogens enhance biodegradative capacity by coupling the oxidation of hydrogen gas to the reduction of CO2 to acetate.” (Ragsdale 2008)

The Wood–Ljungdahl pathway (or reductive acetyl–CoA pathway) is a series of biochemical reactions used by these anaerobic bacteria to synthesize acetyl-CoA. This pathway allows the organism to use hydrogen as an electron donor and carbon dioxide as an electron acceptor. Unlike other energy production pathways (e.g., the Calvin cycle) this process is not cyclic. For details on how this process works, please see the image gallery.

Depiction of how the Wood–Ljungdahl pathway produces acetyl-CoA. Artist: Yikrazuul. See gallery for molecular structure of acetyl-CoA.

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“Conceptually, the simplest way to synthesize an organic molecule is to construct it one carbon at a time. The Wood-Ljungdahl pathway of CO(2) fixation involves this type of stepwise process. The biochemical events that underlie the condensation of two one-carbon units to form the two-carbon compound, acetate, have intrigued chemists, biochemists, and microbiologists for many decades. We begin this review with a description of the biology of acetogenesis. Then, we provide a short history of the important discoveries that have led to the identification of the key components and steps of this usual mechanism of CO and CO(2) fixation. In this historical perspective, we have included reflections that hopefully will sketch the landscape of the controversies, hypotheses, and opinions that led to the key experiments and discoveries. We then describe the properties of the genes and enzymes involved in the pathway and conclude with a section describing some major questions that remain unanswered” (Ragsdale 2008:1873).

Journal article
Acetogenesis and the Wood–Ljungdahl pathway of CO2 fixationBiochimica et Biophysica Acta (BBA) - Proteins and ProteomicsSeptember 7, 2008
Stephen W. Ragsdale, Elizabeth Pierce

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