Nitrogen transport ureides

Figure 25-18 Pathways of catabolism of purine nucleotides, nucleosides, and free bases. Spiders excrete xanthine while mammals and birds excrete uric acid. Spiders and birds convert all of their excess nitrogen via the de novo pathway of Fig. 25-15 into purines. Many animals excrete allantoin, urea, or NH4+. Some legumes utilize the pathway marked by green arrows in their nitrogen transport via ureides.

Plants also form the ureides allantoin and allantoic acid, and in some legumes, such as soy beans, these compounds account for 70-80% of the organic nitrogen in the xylem. They appear to function in nitrogen transport.337 As indicated in Fig. 25-18, the hydrolysis to glyoxylate, NH4+, and C02 follows a different pathway than in animals. See also Chapter 24, Section C. 

N2 Fixation The NH3 produced by the bacteroids has first to be exported via bacteroid and host membranes to cytosol of the nodule and then possibly assimilated within various organelles or cell types to form amino acids, amides, or ureides for export in the xylem. Considering the numbers of nitrogen atoms exported per compound, costs of 3, 2, and 1.75 ATP/nitrogen transported would appear to be reasonable estimates of the energy cost for the transport of amino acids, amides, and ureides, respectively. Added to this would be any costs in transmembrane transfers of ammonia from bacteroid to plant. 

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. 

A similar classification may be appropriate for N2-fixing actinorhizal species. Asparagine is the major product of N2 fixation in Myiica species (32), w hereas the ureide, citrulline, predominates in Alnus species (20, 33). The structures of the difiFerent nitrogenous components transported from N2-fixing plants are given in Figure 3. 

Ureides are a class of cyclic or acyclic nitrogenous organic compounds derived from or structurally related to urea. Representatives of this class which have been found in plants include allantoin, allantoic acid, citrulline, uric acid, hypoxanthine, xanthine, caffeine, hydroxycitruUine, and albizziine. The structures of allantoin (ALN), allantoic acid (ALA), and citrulline (CIT), ureides which occur throughout the plant kingdom, are presented in Fig. 1. These ureido compounds play an essential role in the assimilation, metabolism, transport, and storage of nitrogen in plants. In this chapter, attention will be focused on the occurrence, function, synthesis, and metabolism of these key metabolites. 

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.

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