Fixation reduction cycle

Interestingly, autotrophic growth of S. brierleyi may also involve C02-fixation via a reductive cycle [52]. Wood et al. [13] could provide no further evidence for such a pathway, but suggest that the observed simultaneous use of acetate and CO2 for cellular biosyntheses indicates that the C02-fixing cycle must operate simultaneously with both assimilatory and oxidative pathways. 

These reactivities encouraged the investigation of reduction into NH3 and spectacular results were obtained with triphosphine-borane Fe complexes. Various V-bound Fe complexes 65 relevant to the catalytic reduction of into NH were prepared. Most significant are the protonation of the [FeJ-NH complex into [Fe]-NHj and the reductive displacement of NH by N, which are the toal steps proposed to account for the fixation/reduction of at Fe (Scheme 25). This stoichiometric reaction sequence was adapted into a catalytic transformation. Treatment of the anionic complex 65d with excess and KCj at -78°C results in NHj production. Under these conditions, up to 7 NH3 molecules were generated per Fe complex and more than 40% of the furnished protons were delivered to Nj. The borane moiety plays a major role in this process, being capable of accommodating the various [FelN H, )] species involved in the catalytic cycle due to its versatile coordination properties. This is further supported by the inability of the related triphosphine-silyl complex to promote the reduction of Nj under similar conditions. 

The Calvin cycle is a series of light-independent reactions in which C02 is incorporated into organic molecules. The Calvin cycle reactions occur in three phases carbon fixation, reduction, and regeneration. Photorespiration is a wasteful process in which photosynthesizing cells evolve C02. 

The pathway of carbon dioxide fixation and assimilation to the level of sugar consists of a cyclic series of reactions sometimes referred to as the reductive pentose cycle (in contrast to the dissimilatory oxidative pentose cycle), the carbon reduction cycle or the Calvin cycle. ... 

Organic-inorganic hybrid microparticles, 337 Oxidation-reduction cycle, 227 Oxygen enrichment efiect, 182 Oxygen enrichment type fixation composite material, 184... 

One of these alternate models, postulated by Gunter Wachtershanser, involves an archaic version of the TCA cycle running in the reverse (reductive) direction. Reversal of the TCA cycle results in assimilation of CO9 and fixation of carbon as shown. For each turn of the reversed cycle, two carbons are fixed in the formation of isocitrate and two more are fixed in the reductive transformation of acetyl-CoA to oxaloacetate. Thus, for every succinate that enters the reversed cycle, two succinates are returned, making the cycle highly antocatalytic. Because TCA cycle intermediates are involved in many biosynthetic pathways (see Section 20.13), a reversed TCA cycle would be a bountiful and broad source of metabolic substrates. 

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. 

Figure 3. The general nitrogen model for illustrating the bio geochemical cycling in Forest ecosystems. Explanations for the fluxes 1, ammonia volatilization 2, forest fertilization 3, N2-fixation 4, denitrification 5, nitrate respiration 6, nitrification 7, immobilization 8, mineralization 9, assimilatory and dissimilatory nitrate reduction to ammonium 10, leaching 11, plant uptake 12, deposition N input 13, residue composition, exudation 14, soil erosion 15, ammonium fixation and release by clay minerals 16, biomass combustion 17, forest harvesting 18, litterfall (Bashkin, 2002).

The most noteworthy multistage element cycles in which bacteria play important roles are the nitrogen and sulfur redox cycles. The fixation of nitrogen is a reductive process that provides organisms with nitrogen in a form usable for the synthesis of amino acids, nucleic acids, and other cell constituents. In essence, the overall conversion to the key intermediate, ammonia, can be represented as ... 

The biogeochemical cycling of nitrogen is very much controlled by redox reactions. This perspective is presented in Figure 24.3 for the redox reactions that take place in the water column and sediments. The major pathways of reduction are nitrogen fixation, assimilatory nitrogen reduction, and denitrification. The major oxidation processes are nitrification and anaerobic ammonium oxidation (anammox). Each of these is described next in further detail. 

Bacterial ferredoxins function primarily as electron carriers in ferredoxin-mediated oxidation reduction reactions. Some examples are reduction of NAD, NADP, FMN, FAD, sulfite and protons in anaerobic bacteria, CO -fixation cycles in photosynthetic bacteria, nitrogen fixation in anaerobic nitrogen fixing bacteria, and reductive carboxylation of substrates in fermentative bacteria. The roles of bacterial ferredoxins in these reactions have been summarized by Orme-Johnson (2), Buchanan and Arnon (3), and Mortenson and Nakos (31). 

Photosynthesis in vascular plants takes place in chloroplasts. In the C02-assimilating reactions (the Calvin cycle), ATP and NADPH are used to reduce C02 to triose phosphates. These reactions occur in three stages the fixation reaction itself, catalyzed by rubisco reduction of the resulting 3-phosphoglycerate to glyceraldehyde 3-phosphate and regeneration of ribulose 1,5-bisphosphate from triose phosphates. 

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... 

A quantitatively much more important pathway of C02 fixation is the reductive pentose phosphate pathway (ribulose bisphosphate cycle or Calvin-Benson cycle Fig. 17-14). This sequence of reactions, which takes place in the chloroplasts of green plants and also in many chemiautotrophic bacteria, is essentially a way of reversing the oxidative pentose phosphate pathway (Fig. 17-8). The latter accomplishes the complete oxidation of glucose or of glucose 1-phosphate by NADP+ (Eq. 17-48) ... 

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