Autotrophic carbon fixation

Autotrophic Carbon Fixation in Biology Pathways, Rules, and Speculations... 

The question is therefore, what are the principal requirements of an autotrophic carbon-fixation mechanism An organic molecule serves as a C02 acceptor molecule, which becomes carboxylated by a carboxylase enzyme. This C02 acceptor molecule needs to be regenerated in a reductive autocatalytic cycle. The product that can be drained off from such a metabolic cycle should be a central cellular metabolite, from which all cellular building blocks for polymers can be derived examples of such central metabolites are acetyl-CoA, pyruvate, oxaloacetate, 2-oxoghitarate, phosphoe-nolpyruvate, and 3-phosphoglycerate. Importantly, the intermediates should not be toxic to the cell. The irreversible steps of the pathway are driven by ATP hydrolysis, while the reduction steps are driven by low-potential reduced coenzymes. 

There are various aspects of autotrophic carbon fixation linked to chemistry ... 

The only cultivated AOA, Nitrospumilus maritimus, depends on CO2 as its only carbon source and the presence of even low levels of organic carbon were inhibitory to growth. The pathway of CO2 fixation is, however, unknown. Hyperthermophilic Crenarchaeota generally utilize a 3-hydroxypropionate pathway or a reductive TCA cycle for autotrophic carbon fixation. Another cultivated marine Crenarchaeota strain, Cenarchaeum symbiosum, a sponge symbiont, appears to use the 3-hydroxypropionate pathway. It cannot be concluded on this basis which pathway is used by the AOA, but it very likely that is not the Calvin cycle. N. maritimus had a minimal generation time of 21 h, longer but roughly on the same scale as AOB. 

The synthesis of acetyl-CoA by the Ljungdahl-Wood pathway of autotrophic carbon fixation in diverse bacteria and archaea is catalyzed by a Co- and Fe-containing corrinoid iron-sulfur protein (CoFeSP). This protein participates in the transfer of a methyl group from A -methyltetrahydrofolate to the cob(I)amide of CoFeSP to give a methylcob(III)amide, from which the methyl group is transferred to the reduced Ni-Ni-(4Fe-4S) active site cluster A of acetyl-CoA synthase (27). 

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... 

Mendez C., Bauer A., Huber H., Gad on N., Stetter K. O., and Fuchs G. (1999) Presence of acetyl coenzyme A (CoA) carboxylase and propionyl-CoA carboxylase in autotrophic Crenarchaeota and indication for operation of a 3-hydro-xypropionate cycle in autotrophic carbon fixation. J. Bacterial. 181, 1088-1098. 

Overall, the hydrogen stored in NADPH is used to reduce C02 to carbohydrate units (0H2O). This is not a direct reaction because the C02 is first combined with a C5 compound, ribulose diphosphate (RDP), which then spontaneously splits into two identical C3 molecules, phosphoglyceric acid (PGA). Most of the PGA is used to synthesize further RDP but some is reduced by NADPH, using energy supplied by the ATP/ADP system, to give triose phosphate, which in turn is converted into the glucose phosphate from which various carbohydrates are synthesized. This assimilatory path is known as the Calvin cycle and is involved in all autotrophic carbon fixation, whether photosynthetic or chemosynthetic. 

Alber B, Olinger M, Rieder A, Kockelkom D, Jobst B, Hugler M, Fuchs G (2006) Malonyl-coenzyme A reductase in the modified 3-hydroxypropionate cycle for autotrophic carbon fixation in Archaeal Metallosphaera and Sulfolobus spp. J Bacterid 188 8551-8559... 

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