Wood–Ljungdahl pathway

The Wood–Ljungdahl pathway is a set of biochemical reactions used by some bacteria. It is also known as the reductive acetyl-coenzyme A (acetyl-CoA) pathway. This pathway enables these organisms to use hydrogen (H₂) as an electron donor, and carbon dioxide (CO₂) as an electron acceptor and as a building block to generate acetate for biosynthesis.

In this pathway carbon dioxide is reduced to carbon monoxide (CO) and formic acid (HCOOH) or directly into a formyl group (R−CH=O), the formyl group is reduced to a methyl group (−CH₃) 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 CO₂ and the latter combines the resulting CO with a methyl group to give acetyl-CoA.

Some anaerobic bacteria use the Wood–Ljungdahl pathway in reverse to break down acetate. For example, sulfate-reducing bacteria (SRB) transform acetate completely into CO₂ and H₂ coupled with the reduction of sulfate to sulfide. When operating in the reverse direction, the acetyl-CoA synthase is sometimes called acetyl-CoA decarbonylase.

An evolutionarily related but biochemically distinct pathway named the Wolfe Cycle occurs exclusively in some methanogenic archaea called methanogens. In these anaerobic archaea, the Wolfe Cycle functions as a methanogenesis pathway to reduce CO₂ into methane (CH₄) with electron donors such as hydrogen (H₂) and formate (HCOO^–).