Ammonia assimilation enzymes

Jain, V., Garg, N., and Nainawatee, H.S., Effect of nod regulators on the ammonia assimilation enzymes of Rhizobium sp vigna and Rhizobium meliloti, Natl. Acad. Sci. Lett., 15, 345, 1992. 

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... 

Although the antibacterial and antifungal activities of bialaphos and phosphinothricin were not found to be usehil, the two agents were later used as biodegradeable, relatively nonselective, postemergent herbicides. Glutamine synthetase inhibition is toxic to plants because the enzyme is key to ammonia assimilation. There is some selectivity for individual plant species as shown by the LD for bialaphos ranging from 0.125 to 8.5 kg/ha (301—303). 

GS in plants is the key enzyme of the GS/GOGAT (glutamine synthetase/g1utamine 2-oxyg1utarate aminotransferase) pathway and thus plays a crucial role in ammonia assimilation/reassimilation (117. 118). GS inhibition by phosphinothricin causes accumulation of toxic levels of ammonia. Since ammonia production is increased by photorespiration and conversion of nitrite to ammonia (also light-dependent), nitrogen fertilizers and light act to promote phosphinothricin efficacy (119). 

Although there are many cautionary tales regarding the use of inhibitors they have their uses, provided that they are reasonably specific and the results are critically assessed. Fortunately there are some well-known compounds which have proved useful in ammonia assimilation. The most common one is L-methionine-5-sulphoximine (MSO) this compound has been shown to be an irreversible Inhibitor of glutamine synthetase (Ronzio et al. 1969 Tate and Meister, 1973). In the presence of ATP and metal ions the MSO becomes tightly bound to the enzyme in the form of MSO-phosphate and this is analogous to the formation of y-glutamyl phosphate by the en-... 

This hypothesis is interesting from the point of view of the role of mitochondrial GDH enzymes in higher plant tissues, which are considered to assimilate ammonia via the GS/GOGAT pathway (Miflin and Lea, 1976). Thus the late evolution of the higher plant enzyme suggests that it could have a role in the mitochondrion and that it is not simply an obsolete ancestral enzyme of ammonia assimilation. 

Seeds of pea (Thurman et al., 1965) and Medicago sativa (Hartmann et al., 1973) possess 7 GDH isoenzymes (pattern 1) which decrease in number during germination, while a new pattern of isoenzymes (pattern 2) becomes visible on gel electrophoresis. Pattern 1 is characteristic of the seed, pattern 2 is characteristic of the root, while the shoot possesses some pattern 1 and some pattern 2 isoenzymes. Prior to the discovery of the GS/GOGAT pathway in higher plant tissues, it was postulated that pattern 1 enzymes were involved in deamination reactions, while pattern 2 enzymes were involved in ammonia assimilation. 

The chloroplast NADP linked GDH of Lactuca has a for ammonia of 5.2 mM, and the observation that ammonia uncouples chloroplasts at concentrations of 2 mM makes a role for this enzyme in ammonia assimilation unlikely (Miflin and Lea, 1976). It is, however, possible that the values of GDH enzymes within the cell may be lower than the values determined in vitro, or that high concentrations of ammonia may be localized at the site of the enzyme in the cell. Alternatively, GDH may function primarily in deamination reactions in higher plants, and alternative pathways for ammonia assimilation have been proposed (Miflin and Lea 1976 and this volume. Chapter 4). 

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). 

An alternative approach to estimating the metabolic capabilities of chloroplasts entails measurement of the light-dependent metabolism of radioactive tracers. Using isolated pea chloroplasts. Mills and Wilson (1978a) found that lysine, methionine, threonine, and isoleucine were synthesized from [ C]aspartate. Further evidence that aspartate was being metabolized via the anticipated pathways was provided by the demonstration that the synthesis of homoserine was inhibited by lysine and threonine (Lea et al., 1979). These results, combined with those relating to enzyme localization, lead to the concept that chloroplasts contain a complete functional sequence of enzymes which can facilitate the synthesis of the aspartate family and at least some of the branched-chain amino acids. This is consistent with the importance of chloroplasts in ammonia assimilation (Miflin and Lea, this volume. Chapter 4) and with the evidence that protein can be synthesized from CO2 in isolated plastids (Shepard and Leven, 1972 Huberer al., 1977). The actual fraction of [ ]02 which is utilized for amino acid biosynthesis in isolated plastids is usually quite small. Thus, reactions which normally occur outside of chloroplasts are considered to be of major importance in the synthesis of carbon skeletons such as oxaloacetate or pyruvate (Kirk and Leech, 1972 Leech and Murphy, 1976). 

Glutamine synthetase see Ammonia assimilation, Covalent modification of enzymes, L-Glutamine. 

NKrogtn catabolite raprassion repression by ammonia of the synthesis of various enzymes of nitrogen metabolism. See Ammonia assimilation. 

Nitrogen fixation a process in which atmospheric nitrogen is converted into ammonia. Activation of molecular nitrogen and its reduction to ammonia depend on the catalytic activity of the enzyme Nitrogen-ase (see). The ammonia is then incorporated into the various nitrogenous compounds of the cell by the processes of Ammonia assimilation (see). Nitrogenase is a very unstable enzyme, especially in anaerobic organisms. N.f. is fundament ly important for the ni-... 

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