If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Synonyms: Wood pathway, acetate synthesis, acetyl-CoA pathway, carbon monoxide dehydrogenase pathway, carbon fixation, CO2 fixation, Ljungdahl-Wood pathway, Wood-Ljungdahl pathway, reductive acetyl CoA pathway
|Superclasses:||Degradation/Utilization/Assimilation → C1 Compounds Utilization and Assimilation → CO2 Fixation → Autotrophic CO2 Fixation|
Some taxa known to possess this pathway include : Acetitomaculum ruminis , Acetobacterium carbinolicum , Acetobacterium woodii , Blautia producta , Clostridium formicaceticum , Eubacterium limosum , Methanothermobacter thermautotrophicus , Moorella thermoacetica , Moorella thermoautotrophica , Sporomusa malonica , Sporomusa termitida , Syntrophococcus sucromutans , [Butyribacterium] methylotrophicum
This pathway describes autotrophic production of acetyl-CoA from two CO2 molecules. The pathway has been documented in several types of organisms, including homoacetogenic bacteria and methanogenic archaea [Jansen82, Stupperich83]. Homoacetogenic bacteria are strict anaerobes that can synthesize acetate from H2 and CO2 [Diekert94, Ljungdahl86].
Two key enzymes in this pathway activate CO2. One enzyme is NADP-dependent formate dehydrogenase, which reduces CO2 to formate, and the second is carbon monoxide dehydrogenase, which reduces CO2 to carbon monoxide.
The formate formed by NADP-dependent formate dehydrogenase is incorporated into tetrahydropteroyl mono-L-glutamate, and goes through a series of rearrangements resulting in the formation of a methyl group, in the form of N5-methyl-tetrahydropteroyl mono-L-glutamate. This methyl group then reacts with the CO formed by carbon monoxide dehydrogenase, producing acetyl-CoA, which is then used for carbohydrate synthesis in methanogens (see gluconeogenesis II (Methanobacterium thermoautotrophicum)) or converted to acetate in acetogenic bacteria (see Acetate Formation).
The pathway ends with the formation of acetate from acetyl-CoA via acetyl phosphate in an ATP-generating reaction (see acetate formation from acetyl-CoA I). Since all of the ATP that is formed by EC 184.108.40.206, acetate kinase is consumed by EC 220.127.116.11, formate—tetrahydrofolate ligase, growth with H2 and CO2 is only possible if the pathway is coupled to electron-transport phosphorylation. Of the redox reactions involved only the reduction of 5,10-methylenetetrahydropteroyl mono-L-glutamate to N5-methyl-tetrahydropteroyl mono-L-glutamate (E0' = -200 mV) [Wohlfarth91] with H2 (E0' = -414 mV) is exergonic enough to be coupled with energy conservation [Hugenholtz90, Buckel13].
In Clostridium formicaceticum the MetVF proteins interact with ferredoxin [Clark84]. Cell extracts of Acetobacterium woodii show methylene-H4F reductase activity with NADH [Buchenau01], and the metFV genes that encode methylene-H4F reductase in that organism are clustered with rnfC2 gene, which encodes an NADH dehydrogenase. In Moorella thermoacetica the MetVF proteins form a complex with HdrCBA and MvhD proteins, suggesting some sort of electron bifurcation [Mock14]. Thus, it seems that different acetogens, although all of them use the Wood-Ljungdahl pathway of CO2 reduction to acetic acid, have evolved different ways to directly or indirectly conserve the energy associated with methylene-H4F reduction [Mock14].
This pathway operates in the opposite direction in sulfate reducing bacteria, where electrons originating from organic molecules are shuttled via acetyl-CoA to NADH, oxidizing acetyl-CoA to CO2 [Rabus06].
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Clark84: Clark JE, Ljungdahl LG (1984). "Purification and properties of 5,10-methylenetetrahydrofolate reductase, an iron-sulfur flavoprotein from Clostridium formicoaceticum." J Biol Chem 259(17);10845-9. PMID: 6381490
Stupperich83: Stupperich E, Hammel KE, Fuchs G, Thauer RK (1983). "Carbon monoxide fixation into the carboxyl group of acetyl coenzyme A during autotrophic growth of Methanobacterium." FEBS Lett 152(1);21-3. PMID: 6840273
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Drake81: Drake HL, Hu SI, Wood HG (1981). "Purification of five components from Clostridium thermoaceticum which catalyze synthesis of acetate from pyruvate and methyltetrahydrofolate. Properties of phosphotransacetylase." J Biol Chem 1981;256(21);11137-44. PMID: 7287757
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