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reductive TCA cycle I

Summary from MetaCyc: General Background The reductive tricarboxylic acid (TCA) cycle is a carbon dioxide fixation pathway found in autotrophic eubacteria and archaea (there is a report of the pathway also operating in a strain of the green algae : TAX-3055 ). It is considered to be a primordial pathway for production of starting organic molecules for biosynthesis of sugars, lipids, amino acids, pyrimidines and pyrroles . Other pathways for carbon dioxide fixation include the : CALVIN-PWY, the : CODH-PWY, and the : PWY-5743. The reductive TCA cycle is largely the oxidative, catabolic : TCA "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 :CPLX-7013; :CPLX-7011; and :MONOMER-11860 (the names provided here are the names of the enzyme catalyzing the reductive direction) . It should be noted that these enzymes may participate in other pathways (for example, :CPLX-7011 also participates in the oxidative TCA cycle found in : TAX-72293). 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 : TAX-1092 ; the δ proteobacterium : TAX-2291 ; the alpha proteobacterium strain MC-1 ; the ε proteobacteria : TAX-39766 and : TAX-162154 ; seven other ε proteobacteria isolated from deep-sea hydrothermal environments ; the aquificales : TAX-940 and : TAX-2714 ; and the Crenarchaeota : TAX-70771 and : TAX-2277 . In additon, the genes of the reductive TCA cycle have been demonstrated in : TAX-1097 TLS , and : TAX-2271 . In a few cases, the presence of both the oxidative and reductive TCA cycles has been shown in the same organism, as in : TAX-2271 and : TAX-2277 . About This Pathway The pathway described here was first discovered in the green sulfur bacterium : TAX-115852 . The key step of converting citrate to oxaloacetate and acetyl-coA is catalyzed by the enzyme :CPLX-7013. The basic cycle results in the fixation of two molecules of carbon dioxide and the production of one molecule of acetyl-CoA . Another molecule of carbon dioxide can be fixed by the carboxylation of acetyl-coA to pyruvate . Pyruvate can be converted to phosphoenolpyruvate, which can enter : GLUCONEO-PWY , or, as shown here, can alternatively assimilate a fourth carbon dioxide molecule by carboxylation to oxaloacetate. This carboxylation is catalyzed by :MONOMER-11858 .

from BIOCYC source record: CHLAMY_P23-PWY
Type: pathway
Taxonomic scope
organism-specific biosystem
Chlamydomonas reinhardtii

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