Glutamine synthetase, action

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... 

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. 

Both compounds inhibit glutamine synthetase, which is necessary for the production of glutamine and for ammonia detoxification. Plants that are exposed to glufosinate have reduced glutamine and increased ammonia levels in their tissues, which stops photosynthesis and results in death within a few days. Other natural products (e.g. tabtoxine- -lactam, oxetin, and methionine sufoximine) are known to have this mode of action, but they have not been developed as commercial products. 

Elaborate cascades initiate the clotting of blood (Chapter 12) and the action of the protective complement system (Chapter 31). Cascades considered later in the book are involved in controlling transcription (Fig. 11-13) and in the regulation of mammalian pyruvate dehydrogenase (Eq. 17-9), 3-hydroxy-3-methyl-glutaryl-CoA reductase and eicosanoids (Chapter 21), and glutamine synthetase (Chapter 24). 

A second ammonium ion is incorporated into glutamate to form glutamine by the action of glutamine synthetase. This amidation is driven by the hydrolysis of ATP. ATP participates directly in the reaction by phosphorylating the side chain of glutamate to form an acyl-phosphate intermediate, which then reacts with ammonia to form glutamine. 

The mechanism of action of glutamine synthetase has been extensively studied, and both the bacterial and mammalian enzymes were reviewed in Volume X of this series (83, 84). Many experimental approaches have been applied to the question of the chemical activation of the y-carboxyl group of glutamate and its reaction with ammonia. All of these experiments supported the stepwise chemical mechanism outlined in reactions (24a) and (24b) (8J),... 

The intracellular levels of ammonium ions, in S. noursei, are also influenced by the action of alanine dehydrogenase [102]. Ammonium liberated by alanine oxidation is available for the biosynthesis of glutamine via glutamine synthetase. 0-aminobenzoic acid represses both enzymes while stimulating formation of glutamate dehydrogenase and the antibiotic. Cephalosporin biosynthesis appears to be regulated by the same mechanism [103]. 

The natural glutamine synthetase inhibitor phosphinothricin as well as its synthetic racemate glufosinate are broad spectrum post-emergent herbicides that will play a role in future agriculture due to the unique mode of action. Because these GS inhibitors fully control weeds that have evolved resistances against other types of herbicides, the use of phosphinothricin-containing herbicides in tolerant crops will remain an important option for future sustainable agriculture. 

Glufosinate (GA) is a post-emergence and nonselective herbicide [91, 92], The L-enantiomer of this herbicide, which is also called phos-phinothricin, is a natural microbial phytotoxin, produced by Strepto-myces viridochromogenes and Streptomyces hygroscopicus, and acts as an inhibitor of glutamine synthetase, while the D-enantiomer shows no activity on this enzyme. The herbicidal action involves a rapid accumulation of ammonia, a deficiency in several amino acids, an inhibition of photosynthesis and, finally, the death of the plant cell [91, 93]. 

Glutamine synthetase, the enzyme catalysing this reaction, has been purified from sheep brain . The possible mechanism of its action has been reviewed by Meister, but none of the intermediate steps. 

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