Nitrogen fixation and assimilation

Instant Notes in Biochemistry 2nd Edition, B.D. Hames N.M. Hooper, (c) 2000 BIOS Scientific Publishers Ltd, Oxford. 

The chemical unreactivity of the N=N bond is clearly seen when one considers the industrial process of nitrogen fixation. This process, devised by Fritz Haber in 1910 and still used today in fertilizer factories, involves the reduction of N2 in the presence of H2 gas over an iron catalyst at a temperature of 500°C and a pressure of 300 atmospheres. 

The eight high-potential electrons come from reduced ferredoxin that is produced either in chloroplasts by the action of photosystem I or in oxidative electron transport (Fig. 2) (see Topics L2 and L3). The overall reaction of biological nitrogen fixation  

Nitrogen The next step in the nitrogen cycle is the assimilation of inorganic nitrogen, in 

Glutamate dehydrogenase catalyzes the reductive amination of the citric acid cycle intermediate a-ketoglutarate (Fig. 3a) (see Topic LI). Although the reaction is reversible, the reductant used in the biosynthetic reaction is NADPH. This enzyme is also involved in the catabolism of amino acids (see Topic M2). 

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Mulder, E. G., Bakema, K., and Van Veen, W. L. (1959). Molybdenum in symbiotic nitrogen fixation and in nitrate assimilation. Plant Soil 10 319-34. 

Fig. 7. Model for the subcellular localization of reactions of purine synthesis and ureide biogenesis in nodules of ureide-exportlng legumes. The model is based on results of subcellular fractionation and ultrastructural studies. The processes (shown in the hatched boxes) involved in ureide biogenesis (i.e., nitrogen fixation, ammonium assimilation, precursor synthesis, purine synthesis, energy-yielding metabolism, and purine oxidation and catabolism) may occur in more than one subcellular compartment. The location of the enzymes involved in the conversion of IMP to xanthine is not certain. We have proposed that in soybean nodules these reactions [shown in bold-face type with bold arrows] occur in the plastid while in other species such as cowpea these reactions may take place in the ground cytoplasm. In all cases the intermediate exported from the plastid is uncertain. This uncertainty is indicated with the dashed lines and question marks.

Fig. 9. Proposed model for the cellular compartmentalization of the reactions of nitrogen fixation, ammonium assimilation, purine synthesis, and ureide biogenesis in infected and uninfected cells of soybean root nodules. Uncertainty still exists with respect to the nature of the intermediate (e.g., IMP, XMP, xanthine, glutamine ) transported from the infected cell to the uninfected cell as well as the site of purine synthesis. In addition, as discussed in the text the site(s) of PRPP synthesis (plastid and/or cytosolic) and the path and site of synthesis (de novo from the PPP or via salvage) of tibose S-phosphate (R-S-P) are s not defined, lliese uncertainties are indicated with question marks and/or dashed lines. Lb, leghemoglobin.

Ammonia is oxidized in nature to nitrate via several intermediates in the process of nitrification. Nitrate may be reduced to nitrite by either a dissimilatory or an assimilatory process. Nitrite may be assimilated into the cell via reduction to ammonia, or it may be reduced by microorganisms to N20 and N2 in denitrification. A major part of the total nitrogen in this pathway is lost to the atmosphere. However, in turn, atmospheric dinitrogen is converted to ammonia by various bacteria in nitrogen fixation. 

Ammonium, the primary product of nitrogen fixation, is transported to the host cell cytoplasm where it is assimilated into amides and, in some cases, further converted into ureides before being transported to the shoot. Since the physiological environment within the nodule is apparently different from the other parts of the plant, nodule-specific or nodule-abundant forms of several enzymes of the nitrogen and carbon assimilation pathways have evolved, and are induced to improve the efficiency of nitrogen and carbon metabolism in nodules. 

The nitrogen supplies on land consist of the assimilable nitrogen in the soil VS2 0.19-104tkm-2, in plants (12 1091), and living organisms (0.2 1091). A diversity of nitrogen fluxes is formed here of the processes of nitrification, denitrification, ammonification, fixation, and river run-off. The intensities of these fluxes depend on climatic conditions, temperature regime, moisture, as well as the chemical and physical properties of soil. Many qualitative and quantitative characteristics of these dependences have been described in the literature (Hellebrandt et al., 2003). Let us consider some of them. 

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