The Wood–Ljungdahl pathway is a set of biochemical reactions used by some

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

. It is also known as the reductive acetyl-coenzyme A (

Acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidized for ...

) pathway. This pathway enables these organisms to use hydrogen as an electron donor, and carbon dioxide as an

electron acceptor An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process. Electron acceptors are sometimes mista ...

and as a building block for

biosynthesis Biosynthesis is a multi-step, enzyme-catalyzed process where substrates are converted into more complex products in living organisms. In biosynthesis, simple compounds are modified, converted into other compounds, or joined to form macromolecul ...

. In this pathway

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

is reduced to

carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...

and

formic acid Formic acid (), systematically named methanoic acid, is the simplest carboxylic acid, and has the chemical formula HCOOH and structure . It is an important intermediate in chemical synthesis and occurs naturally, most notably in some ants. Est ...

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 Coenzyme A (CoA, SHCoA, CoASH) is a coenzyme, notable for its role in the synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle. All genomes sequenced to date encode enzymes that use coenzyme A as a subs ...

to produce acetyl-CoA. Two specific enzymes participate on the carbon monoxide side of the pathway: CO Dehydrogenase and

acetyl-CoA synthase Acetyl-CoA synthase (ACS), not to be confused with Acetyl-CoA synthetase or Acetate-CoA ligase (ADP forming), is a nickel-containing enzyme involved in the metabolic processes of cells. Together with Carbon monoxide dehydrogenase (CODH), it forms ...

. 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 An acetate is a salt formed by the combination of acetic acid with a base (e.g. alkaline, earthy, metallic, nonmetallic or radical base). "Acetate" also describes the conjugate base or ion (specifically, the negatively charged ion called ...

. For example, Sulfate reducing bacteria oxidize acetate completely to CO₂ and H₂ coupled with the reduction of sulfate to

sulfide Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds la ...

. When operating in the reverse direction, the

is sometimes called acetyl-CoA decarbonylase. Not to be confused with the Wood-Ljungdahl pathway, an evolutionarily related but biochemically distinct pathway named the Wolfe Cycle occurs exclusively in some methanogenic

archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaeba ...

called

methanogen Methanogens are microorganisms that produce methane as a metabolic byproduct in hypoxic conditions. They are prokaryotic and belong to the domain Archaea. All known methanogens are members of the archaeal phylum Euryarchaeota. Methanogens are c ...

s. In these anaerobic

, the Wolfe Cycle functions as a

methanogenesis Methanogenesis or biomethanation is the formation of methane coupled to energy conservation by microbes known as methanogens. Organisms capable of producing methane for energy conservation have been identified only from the domain Archaea, a group ...

pathway to reduce CO₂ into methane with electron donors such as hydrogen and formate. Similar to the Reverse Krebs cycle and the

Calvin cycle The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis is a series of chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into ...

, the Wolfe Cycle is cyclic, but the Wood-Ljungdahl pathway is not.

Evolution

Last universal common ancestor

A 2016 study of the genomes of a set of bacteria and archaea suggested that the last universal common ancestor (LUCA) of all cells was using an ancient Wood–Ljungdahl pathway in a hydrothermal setting, but more recent work challenges this conclusion. Phylometabolic reconstructions also support this. However, recent experiments have tried to replicate this pathway by attempting to reduce CO₂, with very little pyruvate observed using native iron as a reducing agent (<0.03 mM), and even less so under hydrothermal settings with H₂ (10 μM).

Evolution

Last universal common ancestor

See also

References

Other reading