Citric acid cycle, reductive

Reductive citric acid cycle 5 3 NAD(P)H, 1 unknown donor", 2 ferredoxin 2-Oxoglutarate synthase Isocitrate dehydrogenase6 Pyruvate synthase PEP carboxylase C02 C02 C02 HCOJ Acetyl-CoA, pyruvate, PEP, oxaloacetate, succinyl-CoA, 2-oxoglutarate 2-Oxoglutarate synthase, ATP-citrate lyase... 

The Reductive Citric Acid Cycle (Arnon-Buchanan Cycle)... 

Figure 3.2 Reductive citric acid cycle, ffi, ATP-citrate lyase 2-oxoglutarate ferredoxin oxidoreductase (2-oxoglutarate synthase)

The dicarboxylate/4-hydroxybutyrate cycle starts from acetyl-CoA, which is reductively carboxylated to pyruvate. Pyruvate is converted to PEP and then car-boxylated to oxaloacetate. The latter is reduced to succinyl-CoA by the reactions of an incomplete reductive citric acid cycle. Succinyl-CoA is reduced to 4-hydroxybu-tyrate, the subsequent conversion of which into two acetyl-CoA molecules proceeds in the same way as in the 3-hydroxypropionate/4-hydroxybutyrate cycle. The cycle can be divided into part 1 transforming acetyl-CoA, one C02 and one bicarbonate to succinyl-CoA via pyruvate, PEP, and oxaloacetate, and part 2 converting succinyl-CoA via 4-hydroxybutyrate into two molecules of acetyl-CoA. This cycle was shown to function in Igrticoccus hospitalis, an anaerobic autotrophic hyperther-mophilic Archaeum (Desulfurococcales) [40]. Moreover, this pathway functions in Thermoproteus neutrophilus (Thermoproteales), where the reductive citric acid cycle was earlier assumed to operate, but was later disproved (W.H. Ramos-Vera et al., unpublished results). 

The Formose Cycle and the Reductive Citric Acid Cycle. 199... 

It cannot be stressed too strongly that without exception, all known cellular life possesses an autocatalytic metabolism, even if the cells are het-erotrophic. Thus for the autocatalytic nature of the whole metabolic network it is not necessary to be able to identify a smaller autocatalytic core as the reductive citric acid cycle or the Calvin cycle. Imagine the following thought experiment. Take away all metabolites from a cell but leave all the water and the informational macromolecules in place. Can the network be recreated from the food materials only, or not Let us be generous and provide enough ATP also for the supposed kick-start. The fact is that no contemporary cell could resume its activity in this experiment. Consequently, all cells today possess a distributive autocatalytic network that cannot be seeded from outside, because some of its seed components cannot be taken up from medium. 

The reactive chemical system. Some scientists have attempted to specify key components of such a system but not the entire reactive system. See, for example, de Duve62,63 and Weber.64,65 Others have suggested complete chemical cycles. Modified versions of the reductive citric acid cycle, a carbon-fixation pathway that is used by several organisms today, have been proposed.66 70... 

According to a general rule of organic chemistry, reactions involving the smallest molecules are catalytically the most restrictive. This rule holds notably for the build-up of carbon skeletons with the arithmetic Cl -F Cl = C2 (e.g., C2 = glycine or acetyl thioester). Therefore, it may not come as a surprise that in the course of metabolic evolution, these most simple carbon fixation reactions may fall by the wayside. Under these conditions, an autotrophic carbon fixation metabolism can only be maintained by a metabolic cycle, which multiplies the C2 unit autocatalytically in the absence of its de novo synthesis. A prominent example is the reductive citric acid cycle (C2 -F Cl... 

Many of the sulphur-dependent thermophilic archaebacteria can grow autotrophically and, in the two genera where the mechanism of C02-fixation has been studied, there is evidence that it is via a reductive citric acid cycle. 

Fig. 5. The reductive citric acid cycle proposed for autotrophic carbon dioxide assimilation in...

Pathways of autotrophic CO2 fixation A Reductive citric acid cycle Reductive acetyl CoA pathway I Reductive hydroxypropionante pathway Cathin-Bonion cycle... 

Theoretical studies [21, 22] have postulated that emergence of autotrophic metabolic pathways, such as reductive citric acid cycle (RCC) may have occurred under high P-T hydrothermal conditions analogous to deep marine hydrothermal vents. Earlier experimental work established that citric acid decomposition under hydrothermal conditions leads to the... 

Figure 6. A compilation of the important decomposition pathways for citric acid based on Cody et al. [25], Each reaction is numbered to correspond with discussions in the text. A solid line encloses each distinct pathway, described in detail within the text. The end products of each pathway are acetone, acetic acid and propene with CO2. The various reactions within this reductive citric acid cycle are ...

The reductive citric acid cycle is generally considered to be the most energy-efficient CO2 fixation pathway ( 0.6 mol ATP/mol CO2 for pyruvate) [20, 30, 31]. This is reflected by the activity of the pathway s key carboxylases, isocitrate dehydrogenase, a-ketoglutarate ferredoxin oxidoreductase, and pyruvate ferre-doxin oxidoreductase. All three enzymes couple their carboxylation reaction to a subsequent reduction step. Whereas isocitrate dehydrogenase is an NAD(P)H-dependent enzyme, the latter two enzymes use ferredoxin as a reductant (Scheme 9.3) [32-34]. 

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