Glutamine synthetase

3 Glutamine Synthetase. The enzyme glutamine synthetase is very important in the control of nitrogen metabolism in plants. It catalyses 

A close analogue of glufosinate, bilanafos, that is the L-homoalanyl-L-alanyl-L-alanyl amide of glufosinate, is produced by the soil strepto- 

The mechanism of the GS reaction has been extensively studied. The initial step of the reaction involves formation of y-glutamyl phosphate 

PPT has two actions under initial rate conditions, it behaves as a reversible inhibitor of GS competitive with glutamate Kj = 18 fiM), but in the presence of ATP a time-dependent irreversible inhibition occurs. Studies with a range of substituted PPT analogues show that phosphorylation is important to the inhibition. The second stage is analogous to that 

coli enzyme-inhibitor complex contains PPT, ADP, and inorganic phosphate incubation at acid pH and at high ionic strength releases the ADP, phosphate, and PPT and restores enzymatic activity. With higher plants, the GS-phosphorylated PPT complex may not be so labile, but it is not clear that the conditions required to reactivate the enzyme have been fully explored. -  

GS activity from a wide spectrum of mono- and dicotyledonous species is inhibited by PPT, whether or not both isozymes are present very similar kinetic parameters occur in the different plant species studied. - Neither GOG AT nor glutamate dehydrogenase is affected by PPT in uifro.  

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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). 

Many enzymes (see Chapters 14 to 16) derive at least some of their catalytic power from oligomeric associations of monomer subunits. This can happen in several ways. The monomer may not constitute a complete enzyme active site. Formation of the oligomer may bring ail the necessary catalytic groups together to form an active enzyme. For example, the active sites of bacterial glutamine synthetase are formed from pairs of adjacent subunits. The dissociated monomers are inactive. 

As mentioned in Section II., Meister and his co-workers (64) have studied extensively the substrate specificity of the enzyme glutamine synthetase from... 

Ahmad, I., Larher, F., Mann, A.F., McNally, S.F. Stewart, G.R. (1982). Nitrogen metabolism of halophytes. IV. Characteristics of glutamine synthetase from Triglochin maritima L. New Phytologist, 91, 585-95. 

The enzymes glutamate dehydrogenase, glutamine synthetase, and aminotransferases occupy central positions in amino acid biosynthesis. The combined effect of... 

Figure 28-4. The asparagine synthetase reaction. Note similarities to and differences from the glutamine synthetase reaction (Figure 28-2).

Oliver, C.N., Starke-Reed, P.E., Stadtman, E.R., Liu, G.J., Carney, J.M. and Floyd, R.A. (1990). Oxidative damage to brain proteins, loss of glutamine synthetase activity, and production of free radicals during ischemia/reperfusion-induced injury to gerbil brain. Proc. Natl Acad. Sd. USA 87, 5144-5147. 

S. E. Smith, B. L. St John, F. A. Smith, and D. j. D. Nicholas, Activity of glutamine synthetase and glutamate dehydrogenase in Trifolium subterraneum and Allium cepa L., effects of mycorrhizal infection and phosphorus nutrition. New Phytologist 99 211 (1985). 

Glucosidase and 0-galactosidase (sweet almond emulsin)[36] Glutamine synthetase (native octameric brain) 371 Glycogen phosphorylase At3 ]... 

Such enzymes catalyse the condensation of specific compounds, accompanied by the breakdown of a pyrophosphate bond in adenosine triphosphate (10.64). Adenosine is the condensation product of a pentose (D-ribofuranose) and a purine (adenine). Scheme 10.15 shows the action of glutamine synthetase on a mixture of L-glutamic acid (10.65) and... 

Iida T., Kawabe T., Noguchi F., Mitamura T., Nagata K., Tomita K., ISFET-type -glutamate sensor using thermophilic glutamine-synthetase from a thermophile, Nippon-Kagaku-Kaishi 1987 10 1817-1821. 

The key reaction that links primary and secondary metabolism is provided by the enzyme phenylalanine ammonia lyase (PAL) which catalyzes the deamination of l-phenylalanine to form iran.v-cinnamic acid with the release of NH3 (see Fig. 3.3). Tyrosine is similarly deaminated by tyrosine ammonia lyase (TAL) to produce 4-hydroxycinnamic acid and NH3. The released NH3 is probably fixed by the glutamine synthetase reaction. These deaminations initiate the main phenylpropanoid pathway. 

In earlier studies the in vitro transition metal-catalyzed oxidation of proteins and the interaction of proteins with free radicals have been studied. In 1983, Levine [1] showed that the oxidative inactivation of enzymes and the oxidative modification of proteins resulted in the formation of protein carbonyl derivatives. These derivatives easily react with dinitrophenyl-hydrazine (DNPH) to form protein hydrazones, which were used for the detection of protein carbonyl content. Using this method and spin-trapping with PBN, it has been demonstrated [2,3] that protein oxidation and inactivation of glutamine synthetase (a key enzyme in the regulation of amino acid metabolism and the brain L-glutamate and y-aminobutyric acid levels) were sharply enhanced during ischemia- and reperfusion-induced injury in gerbil brain. 

In contrast to the findings obtained in Ref. [34], it was concluded [36,37] that exposure of proteins to peroxynitrite leads to a very small increase in carbonyl content at physiological pH and C02 concentration. At the same time, carbonyl contents in glutamine synthetase and BSA increased in the absence of C02. These data show the importance of C02 in the... 

Cervera and Levine [81] studied the mechanism of oxidative modification of glutamine synthetase from Escherichia coli. It was found that active oxygen species initially caused inactivation of the enzyme and generated a more hydrophilic protein, which still was not a substrate for the protease. Continuous action of oxygen species resulted in the formation of oxidized protein subjected to the proteolytic attack of protease. 

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