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.[1][2]
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.[3] When operating in the reverse direction, the
acetyl-CoA synthase is sometimes called acetyl-CoA decarbonylase.
It has been proposed that the reductive acetyl-CoA pathway might have begun at
deep seaalkalinehydrothermal vents where
metal sulfides and
transition metalscatalyze the
prebiotic reactions of the reductive acetyl-CoA pathway.[7] Recent experiments have tried to replicate this pathway by attempting to reduce CO2, with very little
pyruvate observed using
native iron (Fe0,
zerovalent Fe) as a
reducing agent (< 30 μM),[8] and even less so under hydrothermal settings with H2 (10 μM).[9] Joseph Moran and colleagues state that "it has been proposed that either the complete or “horseshoe” forms of the r
TCA cycle may have once been united with the acetyl CoA pathway in an ancestral, possibly prebiotic,
carbon fixation network".[8]
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,[10] but more recent work challenges this conclusion as they argued that the previous study had "undersampled protein families, resulting in incomplete
phylogenetic trees which do not reflect
protein family evolution".[11] However geological
evidence and
phylogenomic reconstructions of the
metabolic network of the common ancestors of archaea and bacteria support that LUCA fixed CO2 and relied on H2.[12][9]
Ljungdahl LG (1986). "The autotrophic pathway of acetate synthesis in acetogenic bacteria". Annual Review of Microbiology. 40 (1): 415–50.
doi:
10.1146/annurev.micro.40.1.415.
PMID3096193.
^
abRagsdale Stephen W (2006). "Metals and Their Scaffolds To Promote Difficult Enzymatic Reactions". Chem. Rev. 106 (8): 3317–3337.
doi:
10.1021/cr0503153.
PMID16895330.
^Paul A. Lindahl "Nickel-Carbon Bonds in Acetyl-Coenzyme A Synthases/Carbon Monoxide Dehydrogenases" Met. Ions Life Sci. 2009, volume 6, pp. 133–150.
doi:
10.1039/9781847559159-00133
^Spormann, Alfred M.; Thauer, Rudolf K. (1988). "Anaerobic acetate oxidation to CO2 by Desulfotomaculum acetoxidans". Archives of Microbiology. 150 (4): 374–380.
doi:
10.1007/BF00408310.
ISSN0302-8933.
S2CID2158253.