Inhibition glutamine synthetase

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). 

As the dipeptide J itself does not inhibit purified Glutamine synthetase in vitro [143], the amino acid K is considered to be the active form of J and hence, the actual toxin of wildfire disease. Since, the detailed mechanism of Glutamine synthetase inhibition by tabtoxinine-(3-lactam attracts chemical interest, a synthetic approach to the toxin K and its analogs, is of increasing importance. Spiro-(3-lactam L (Fig. 11) has been found to be an efficient precursor of toxin K. 

Figure 3. Comparison of structures of several compounds with potent iQ vitro glutamine synthetase-inhibiting properties.

Glufosinate Amino acid Glutamine synthetase Inhibition of amino acid synthesis 1986 Pre- and postemergence Nonselective 200-750... 

Willard-Mack, C. L., Koehler,R. C.,Hirata,T. etal. Inhibition of glutamine synthetase reduces ammonia-induced astrocyte swelling in rat. Neuroscience 71 589-599,1996. 

Glutamine synthetase, a brain Mn enzyme, is located mainly in astrocytes, and its synthesis may be modulated by nitric oxide (496). Inhibition of this enzyme could be relevant to aging diseases (497). There is evidence that human NT2-N neurons die via ionotropic glutamate receptor-mediated mechanisms when exposed to hypoxia in the presence of glutamate (498). 

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. 

Cumulative Feedback Inhibition In cumulative feedback inhibition, the end products can inhibit the reaction of the target enzyme separately. Many textbooks erroneously indicate that the cumulative feedback inhibition of E. coli glutamine synthetase involves separate regulatory sites for each feedback inhibitor. See Cumulative Feedback Inhibition... 

Allosteric regulation can be considerably more complex. An example is the remarkable set of allosteric controls exerted on glutamine synthetase of E. coli (Fig. 22-6). Six products derived from glutamine serve as negative feedback modulators of the enzyme, and the overall effects of these and other modulators are more than additive. Such regulation is called concerted inhibition. 

The glutamine synthetase of E. coli is independently modulated by various products of glutamine metabolism (see Fig. 22-6). In this concerted inhibition, the extent of enzyme inhibition is greater than the sum of the separate inhibitions caused by each product. For E. coli grown in a medium rich in histidine, what would be the advantage of concerted inhibition ... 

Bacterial glutamine synthetase is feedback inhibited by serine, glycine, and alanine. Explain specifically the connection between these amino acids and glutamine that would account for the logic of this inhibition. 

Rowe et al. (97) found that glutamine synthetase was irreversibly inhibited by methionine sulfoximine in the presence of ATP. Also, they discovered... 

Our studies of the substrates [glutamate (I) and ATP] and of substrate analogs [AMP-P-(CH2)-P and methionine sulfoximine] reveal interactions between both substrate sites and both metal ion sites. Previously mentioned studies by Meister s group showed that the irreversible inhibition of glutamine synthetase in the presence of L-methionine (S)-sulfoximine and ATP was due to formation of the sulfoximine phosphate (IV). The tetrahedral geometry at the sulfur atom of the sulfoximine was suggested to be a mimic of the active structure of the adduct of y-glutamyl phosphate and ammonia (III). Data in our laboratory provide spectroscopic evidence that methionine... 

Tabtoxin J is a dipeptide exotoxin produced by Pseudomonas tabaci, the organism responsible for the wildfire disease of tobacco plants [141]. When hydrolyzed by peptidase, in vivo, this exotoxin releases tabtoxinine-p-lactam K, which inhibits Glutamine synthetase of the photorespiratory nitrogen cycle, causing chlorosis and death of tobacco plants [142]. 

In two other cases the addition of topiramate was thought to have precipitated valproate-induced hyperammonemic encephalopathy (1172). Recovery occurred after withdrawal of valproate or topiramate. The authors suggested that topiramate may have contributed to the hyperammonemia by inhibiting carbonic anhydrase and cerebral glutamine synthetase. 

Sakamoto, A., Takeba, G., Shibata, D. Tanaka, K. (1990). Phytochrome-mediated activation of the gene for cytosolic glutamine synthetase (GSi) during inhibition of photosensitive lettuce seeds. Plant Molecular Biology 15, 317-23. 

Some plant pathogenic bacteria and their phytotoxins have been screened in bioassays that monitor the effects of their toxins (antibiotic and phytotoxic) on other sensitive bacteria. For example, several fluorescent Pseudomonas syringae pvs. produce extracellular phytotoxins.76,106,116 Tabtoxin is produced by P. syringae pv. tabaci and pv. coronafacines, and this natural product inhibits glutamine synthetase.34,46,116 Phaseolotoxin, produced by P. syringae pv. phaseolicola... 

G Harth, MA Horwitz (2003) Inhibition of Mycobacterium tuberculosis glutamine synthetase as a novel antibiotic strategy against tuberculosis demonstration of efficacy in vivo, Infect Immun 71(l) 456-464... 

J A Colanduoni, J J Villafranca (1986) Inhibition of E coli glutamine synthetase by phosphino-thricin, Bioorg Chem 14 163-169... 

H S Gill, D Eisenberg (2001) The Crystal Structure of Phosphinothricin in the Active Site of Glutamine Synthetase Illuminates the Mechanism of Enzymatic Inhibition, Biochemistry 40(7) 1903-1912... 

Figure 21. Inhibition of glutamine synthetase by wildfire toxin (42)...

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