Acetogenesis and the Wood–Ljungdahl pathway of CO2 fixation

Abstract

I. SummaryConceptually, 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.

Genomic reconstruction of a novel, deeply branched sediment archaeal phylum with pathways for acetogenesis and sulfur reduction

“…The first CO 2 is converted to a methyl group through sequential transfer of six electrons. The methyl group is then transferred to convert a second CO 2 to acetyl-CoA (Ragsdale and Pierce, 2008). Although SMTZ-45 did not show any genes for enzymes in this pathway, near-complete gene sets were found in the SMTZ1-45 and SMTZ1-83 bins.…”

“…However, it is possible that the pathway is in fact incomplete in these archaea. The presence of this pathway is not direct evidence of autotrophy, as CO 2 could serve primarily as an acceptor for electrons derived from a wide range of fermentative reactions within the organism, rather than an autotrophic carbon source that sustains the biosynthetic needs of the cell (Ragsdale and Pierce, 2008).…”

“…The CO 2 -reducing enzymes lacked formate dehydrogenase for the initial CO 2 reduction step, and methylene-tetrahydrofolate reductase (metF), which converts 5,10-methylene-tetrahydrofolate to 5-methyl-tetrahydrofolate. Genes for acetyl-CoA synthesis were nearly complete except for the acsE gene, which codes for the methyltransferase that transfers the methyl group from methyltetrahydroformate to a corrinoid iron/sulfur protein, an intermediate methyl carrier; from here, the methyl group is accepted by the key enzyme of the pathway, CO-dehydrogenase/acetyl-CoA synthase, to produce acetyl-CoA and acetate (Ragsdale and Pierce, 2008). If the missing genes are accounted for by the incomplete sequence coverage of bins SMTZ1-45 and SMTZ1-83, these Thorarchaeaota may be able to use the Wood-Ljungdahl pathway of carbon fixation and support themselves by autotrophic acetogenesis, depending on the need.…”

Biogas process parameters—energetics and kinetics of secondary fermentations in methanogenic biomass degradation

“…This ratio may be shifted under specific conditions (enhanced temperature, high nitrogen loads) by homoacetogenesis (Ragsdale and Pierce 2008) or syntrophic acetate oxidation (Schnürer et al 1996). This link could also be used as a side channel to the opposite side in case of a limitation or promotion of only one group of the methanogenic partners involved.…”

Application of metabolic controls for the maximization of lipid production in semicontinuous fermentation

“…Gasification of biomass and organic waste (3), recycling of industrial off-gases (4), and thermochemical dissociation of CO 2 and H 2 O using solar energy (5) will also be available to provide plentiful amounts of renewable syngas. By using the Wood-Ljungdahl pathway, acetogenic bacteria are able to convert the syngas into acetic acid as the main product (6,7). Additionally, as the key intermediate in this process, acetic acid can also be generated renewably from other sources, expanding the range of feedstocks that can drive bio-GTL operation.…”