Glutamate ammonia assimilation

L. J. Reitzer, Ammonia Assimilation and the Biosynthesis of Glutamine, Glutamate, Aspartate, Asparagine, L-alanine, and D-alanine. In Escherichia coll and Salmonella, Cellular and Molecular Biology F. C. Neidhardt, Ed. ASM Press Washington, DC, 1996 Vol. 1, pp 391 07. 

Inhibitor studies (Leaet al., 1979) have shown that the synthesis of protein from glutamine by isolated pea cotyledons is unaffected by MSO but inhibited by azaserine, consistent with the action of glutamate synthase but not very conclusive because of the side effects of azaserine. Little if any labeling data on ammonia assimilation has so far been produced using developing seeds. 

Although similar studies of the activities of the ammonia assimilating enzymes have been done in Azolla (Ray al., 1978) a much less clear-cut picture has emerged. GS and glutamate synthase are present at similar or greater quantities in the symbiotic as compared to the free-living whereas... 

Certain organisms, for example fungi of the higher orders, possess two GDH enzymes, one specific for NAD, the other for NADP. A. variety of evidence (Smith et al., 1975) has indicated that in these cases, the NADP-linked enzyme is predominantly concerned with ammonia assimilation, while the NAD-linked enzyme is predominantly concerned with deamination of glutamate to 2-oxcglutarate. Control of the relative rates of amination and deamination to reactions in the cell has been observed to be exerted through control of the relative levels of the biosynthetic NADP enzyme and the catabolic NAD enzyme. This control mechanism therefore bypasses the necessity for the individual enzymes to possess elaborate allosteric control mechanisms. 

However, in organisms which have only one GDH enzyme, which is responsible for both ammonia assimilation and deamination of glutamate, there has been a necessity for the enzyme to evolve complex regulatory properties so that the relative rates of amination and deamination reactions can be finely controlled by the concentrations of certain metabolites, and by the energy status of the cell. This is seen in the complex regulatory properties of GDH enzymes from bovine liver and from the Phycomycetes (fungi). 

Many other shuttles can be contrived theoretically, but they lack experimental support. Liver cells are probably served by several shuttles, rather than one major system. In contrast to the C C, the ARC in the cytoplasm is low. The high concentration of NADPH relative to that of NADP helps to drive biosyntheses forward by mass action among them are the reduction of glycerate 3-phosphate to glycer-aldehyde 3-phosphate in the photosynthetic assimilation of CO2 (see Calvin cycle), reductive synthesis of glutamate by glutamate synthase (see Ammonia assimilation), and Fatty acid biosyntl is. 

Figure 2.7. Glutamine synthetase and glutamate synthase in ammonia assimilation.

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