Carbon fixation cycle

A detailed theoretical study of the properties of the redox system FeS/FeS2 was carried out in the Department of Geosciences of SUNY Stony Brook (Schoonen et al., 1999). The authors conclude that the hypothetical reduction of CO2 (by the FeS/FeS2 redox pair) formulated in Wachtershauser s early work, and the carbon fixation cycle on the primeval Earth associated with it, probably could not have occurred. This judgement is made on the basis of a theoretical analysis of thermodynamic data other conditions would naturally have been involved if CO had reacted rather than C02. It is not known whether free CO existed in the hydrosphere, or if so, at what concentrations. 

In addition, cyanobacteria possess a second carbon fixation mechanism through phosphoenolpyruvate carboxylase (PPC) that is responsible for close to 25% of CO2 fixation [102]. PPC fixes HCOg" rather than CO2 and combines it with phosphoenolpyruvate to form oxaloacetate and inorganic phosphate. An in silico modeling study has proposed to couple PPC with the core C4 plant carbon fixation cycle to Increase the overall carbon fixation rate compare to the CBB cycle [103]. 

US biochemist Melvin Calvin (1911-97) publishes details of the photosynthetic carbon-fixation cycle (Calvin cycle). 

Keywords Nucleobase Kaolinite DPT Adsorption Tautomerism Carbon fixation cycle... 

All the above mentioned studies have been performed experimentally and they have not been supported by ab initio techniques capable to determine physico-chemical conditions favorable (thermodynamically and kinetically) for the formation of components of the primordial pathway catalyzed by the transition metal sulfides using the carbon fixation cycle. Therefore, recently the investigations of thermodynamic aspects of this cycle using DFT approaches and simple models of Ni-Fe sulfide as catalysts at 373 K temperature were carried out [138] (this temperature was shown at hot oceanic vents on the early Earth). The viability of this cycle and its possible influence on the origin of low-molecular bioorganic compounds of primary archaic metabolism was examined in this theoretical study. 

The light independent reactions take place in the stroma with the help of ATP and NADPH. In a process called the Calvin-Benson cycle, or carbon fixation, carbon dioxide from the atmosphere is captured and converted into carbohydrates [135]. The reaction is catalyzed by the enzyme RuBisCO (ribulose-1,5-biphosphate... 

The dark reaction involves the fixation of carbon dioxide to form carbohydrates. The ATP and the NADPH produced in the light reaction drive this carbon fixation. It might be thought that the interruption of the Calvin cycle would also produce effective herbicides but this is not the case. There are two reasons why. First, the reaction is not an energetic reaction whose interruption would lead to the destruction of cellular components and second, the enzymes involved in the process are present in very high amounts. If an enzyme is to be targeted as a key step in the metabolism of a plant, it is important that it is present in small amounts and that it is not turned over very quickly. If an enzyme is abundant,... 

Triose phosphates produced by the Calvin cycle in bright sunlight, as we have noted, may be stored temporarily in the chloroplast as starch, or converted to sucrose and exported to nonphotosynthetic parts of the plant, or both. The balance between the two processes is tightly regulated, and both must be coordinated with the rate of carbon fixation. Five-sixths of the triose... 

Calvin cycle 752 plastids 752 chloroplast 752 amyloplast 752 carbon-fixation reaction 753 ribulose 1,5-bisphosphate 753 3-phosphoglycerate 753 pentose phosphate pathway 753 reductive pentose phosphate pathway 753 C3 plants 754 ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) 754 rubisco activase 757... 

Flow of Electrons from H20 to NADP+ Drives Proton Transport into the Thylakoid Lumen Protons Return to the Stroma through an ATP-Synthase Carbon Fixation Utilizes the Reductive Pentose Cycle Ribulose-Bisphosphate Carboxylase-Oxygenase Photorespiration and the C-4 Cycle... 

Carbon Fixation Utilizes the Reductive Pentose Cycle... 

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. 

The pathway can be divided into two metabolic cycles (Figure 3.4). In the first cycle, acetyl-CoA is carboxylated to malonyl-CoA, which is subsequently reduced and converted into propionyl-CoA via 3-hydroxypropionate as a free intermediate. Propionyl-CoA is carboxylated to methylmalonyl-CoA, which is subsequently converted to succinyl-CoA the latter is then used to activate L-malate by succinyl-CoA L-malate coenzyme A transferase, which forms L-malyl-CoA and succinate. Succinate is oxidized to L-malate via conventional steps. L-Malyl-CoA, the second characteristic intermediate of this cycle, is cleaved by L-malyl-CoA/P-methylmalyl-CoA lyase, thus regenerating the starting molecule acetyl-CoA and releasing gly-oxylate as a first carbon-fixation product [27]. 

The dark reactions (carbon-fixation reactions) use the ATP and NADPH produced by the light reactions to fix carbon dioxide as carbohydrate sucrose and starch. The reactions form a cycle (the Calvin cycle) in which the enzyme ribulose bisphosphate carboxylase (rubisco), located in the stroma, condenses a C02 molecule with ribulose 1,5-bisphosphate to produce two molecules of 3-phosphoglycerate. Other reactions then regenerate the ribulose... 

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... 

Bicarbonate ion is important not only as a substrate for carbon fixation in the Calvin cycle but also for its role in PSII ET efficiency and photoassembly of the OEC cluster in PSII [29],... 

Calvin cycle (aka Calvin-Benson Cycle or Carbon Fixation) Series of biochemical, enzyme-mediated reactions during which atmospheric carbon dioxide is reduced and incorporated into organic molecules, eventually some of this forms sugars. In eukaryotes, this occurs in the stroma of the chloroplast. 

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. 

Walsh, J. J. (1996). Nitrogen fixation within a tropical upweUing ecosystem Evidence for a Redfield budget of carbon/nitrogen cycling by the total phytoplankton community. J. Geophys. Res. 101 (C9), 20607-20616. 

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