Wood–Ljungdahl pathway
The Wood–Ljungdahl pathway is a set of biochemical reactions used by some bacteria and archaea called acetogens and methanogens, respectively. It is also known as the reductive acetyl-coenzyme A (Acetyl-CoA) pathway. This pathway enables these organisms to use hydrogen as an electron donor, and carbon dioxide as an electron acceptor and as a building block for biosynthesis. In this pathway carbon dioxide is reduced to carbon monoxide and formic acid or directly into a formyl group, the formyl group is reduced to a methyl group and then combined with the carbon monoxide and Coenzyme  A to produce acetyl-CoA. Two specific enzymes participate on the carbon monoxide side of the pathway: CO Dehydrogenase and acetyl-CoA synthase. The former catalyzes the reduction of the CO and the latter combines the resulting CO with a methyl group to give acetyl-CoA. Some anaerobic bacteria and archaea use the Wood–Ljungdahl pathway in reverse to break down acetate. For example, some methanogens break down acetate to a methyl group and carbon monoxide, and then reduce the methyl group to methane while oxidizing the carbon monoxide to carbon dioxide. Sulfate reducing bacteria, meanwhile, oxidize acetate completely to  CO and H coupled with the reduction of sulfate to sulfide. When operating in the reverse direction, the acetyl-CoA synthase is sometimes called acetyl-CoA decarbonylase. The pathway occurs in both bacteria (e.g. acetogens) and archaea (e.g. methanogens). Unlike the Reverse Krebs cycle and the Calvin cycle, this process is not cyclic. A recent study of the genomes of a set of bacteria and archaea suggests that the last universal common ancestor (LUCA) of all cells was using the Wood–Ljungdahl pathway in a hydrothermal setting. Phylometabolic reconstructions also supports this. However, recent experiments have tried to replicate this  pathway by attempting to reduce CO, with very little pyruvate observed using native iron as a reducing agent (<0.03 mM), and even less so under hydrothermal settings with H (10 μM).