If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Synonyms: reductive tricarboxylic acid cycle, reductive tricarboxylic acid pathway, reductive citric acid cycle, reverse citric acid cycle, carbon fixation, CO2 fixation, reductive carboxylic acid cycle
|Superclasses:||Degradation/Utilization/Assimilation → C1 Compounds Utilization and Assimilation → CO2 Fixation → Autotrophic CO2 Fixation → Reductive TCA Cycles|
Some taxa known to possess this pathway include : Aquifex pyrophilus, Candidatus Arcobacter sulfidicus, Chlorobaculum tepidum, Chlorobaculum thiosulfatiphilum, Chlorobium limicola, Desulfobacter hydrogenophilus, Pyrobaculum islandicum, Pyrobaculum neutrophilum, Sulfurimonas denitrificans, Thermoproteus tenax
The reductive tricarboxylic acid (TCA) cycle is a carbon dioxide fixation pathway found in autotrophic eubacteria and archaea (there is also a report of the pathway also operating in a strain of the green algae Chlamydomonas reinhardtii [Chen92]). It is considered to be a primordial pathway for production of starting organic molecules for biosynthesis of sugars, lipids, amino acids, pyrimidines and pyrroles [Smith04a, Romano96, Buchanan90].
The reductive TCA cycle is largely the oxidative, catabolic TCA cycle in reverse. Most of the enzymes of the TCA cycle work reversibly and could catalyze both directions. Only three counteracting enzyme pairs are thought to determine the oxidative or reductive direction of the cycle. These three enzymes are ATP-citrate lyase; 2-oxoglutarate synthase; and fumarate reductase (the names provided here are the names of the enzyme catalyzing the reductive direction) [Schauder87a, Shiba85, Siebers04]. It should be noted that these enzymes may participate in other pathways (for example, 2-oxoglutarate synthase also participates in the oxidative TCA cycle found in Helicobacteraceae). Nonetheless, the presence of these enzyme activities in autotrophically grown bacteria and archaea is considered indicative of the presence of the reductive TCA cycle.
Activities of the key enzymes in the reductive TCA cycle have been demonstrated in the following autotrophic bacteria and archaea: the green sulfur bacterium Chlorobium limicola [Kanao01, Fuchs80]; the δ proteobacterium Desulfobacter hydrogenophilus [Schauder87a]; the α proteobacterium strain MC-1 [Williams06]; the ε proteobacteria Sulfurimonas denitrificans and Candidatus Arcobacter sulfidicus [Hugler05]; seven other ε proteobacteria isolated from deep-sea hydrothermal environments [Takai05]; the aquificales Hydrogenobacter thermophilus and Aquifex pyrophilus [Beh93, Shiba85]; and the Crenarchaeota Pyrobaculum neutrophilum [Schafer89, Beh93] and Pyrobaculum islandicum [Hugler03].
About This Pathway
The pathway described here was first discovered in the green sulfur bacterium Chlorobaculum thiosulfatiphilum [Evans66]. The key step of converting citrate to oxaloacetate and acetyl-coA is catalyzed by the enzyme ATP-citrate lyase.
The basic cycle results in the fixation of two molecules of carbon dioxide and the production of one molecule of acetyl-CoA [Hugler05, Takai05, Siebers04]. Another molecule of carbon dioxide can be fixed by the carboxylation of acetyl-coA to pyruvate [Kanao01]. Pyruvate can be converted to phosphoenolpyruvate, which can enter gluconeogenesis I [Beh93], or, as shown here, can alternatively assimilate a fourth carbon dioxide molecule by carboxylation to oxaloacetate. This carboxylation is catalyzed by phosphoenolpyruvate carboxylase [Kanao01, Romano96, Buchanan90, Schauder87a, Evans66].
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