Active Site Of Glutamine Synthetase Research Articles

2D-QSAR, Docking Studies, andIn SilicoADMET Prediction of Polyphenolic Acetates as Substrates for Protein Acetyltransferase Function of Glutamine Synthetase ofMycobacterium tuberculosis

A novel transacetylase (TAase) function of glutamine synthetase (GS) in bacterial species such asMycobacterium smegmatisandMycobacterium tuberculosisH37Rv was established by us, termed as mycobacterial TAase (MTAase). Several polyphenolic acetates (PAs) were found to be substrates for MTAase by inhibiting certain receptor proteins such as glutathione S-transferase by way of acetylation. The present work describes the descriptor-based 2D-QSAR studies developed for a series of PA synthesized by us and evaluated for MTAase and antimycobacterial activity using stepwise multiple linear regression method with the kinetic constants and the minimum inhibitory constant (MIC) as the dependent variables, to address the fact that TAase activity was leading to the antimycobacterial activity. Further, blind docking methods using AutoDock were carried out to study the interaction of potent PA with the crystal structure ofM. tuberculosisGS. PAs were predicted to bindM. tuberculosisGS on the protein surface away from the known active site of GS. Subsequent focussed/refined docking of potent PA with GS showed that theε-amino group of Lys4 of GS formed a cation-πinteraction with the benzene ring of PA. Also, ADMET-related descriptors were calculated to predict the pharmacokinetic properties for the selection of the effective and bioavailable compounds.

Oxidation of the active site of glutamine synthetase: Conversion of arginine-344 to γ-glutamyl semialdehyde

Metal-catalyzed oxidative modification of proteins is implicated in a number of physiologic and pathologic processes. The reaction is presumed to proceed via a site-specific free radical mechanism, with the site-specificity conferred by a cation-binding site on the protein. The oxidation of bacterial glutamine synthetase has been studied in detail, providing the opportunity to examine whether the oxidation is consistent with a site-specific radical reaction. Oxidation leads to the appearance of carbonyl groups in amino acid side chains of the protein, and labeling of those carbonyl groups with fluoresceinamine facilitated purification of the oxidized peptide from a tryptic digest. The oxidized residue was arginine-344, which was converted to a γ-glutamyl semialdehyde residue. Histidine-269 had previously been shown to be converted to asparagine during metal-catalyzed oxidation. Both arginine-344 and histidine-269 are situated at the metal-nucleotide binding pocket of the enzyme's active site, thus establishing the site-specificity of the oxidation.

Labeling of specific lysine residues at the active site of glutamine synthetase.

Glutamine synthetase (Escherichia coli) was incubated with three different reagents that react with lysine residues, viz. pyridoxal phosphate, 5'-p-fluorosulfonylbenzoyladenosine, and thiourea dioxide. The latter reagent reacts with the epsilon-nitrogen of lysine to produce homoarginine as shown by amino acid analysis, nmr, and mass spectral analysis of the products. A variety of differential labeling experiments were conducted with the above three reagents to label specific lysine residues. Thus pyridoxal phosphate was found to modify 2 lysine residues leading to an alteration of catalytic activity. At least 1 lysine residue has been reported previously to be modified by pyridoxal phosphate at the active site of glutamine synthetase (Whitley, E. J., and Ginsburg, A. (1978) J. Biol. Chem. 253, 7017-7025). By varying the pH and buffer, one or both residues could be modified. One of these lysine residues was associated with approximately 81% loss in activity after modification while modification of the second lysine residue led to complete inactivation of the enzyme. This second lysine was found to be the residue which reacted specifically with the ATP affinity label 5'-p-fluorosulfonylbenzoyladenosine. Lys-47 has been previously identified as the residue that reacts with this reagent (Pinkofsky, H. B., Ginsburg, A., Reardon, I., Heinrikson, R. L. (1984) J. Biol. Chem. 259, 9616-9622; Foster, W. B., Griffith, M. J., and Kingdon, H. S. (1981) J. Biol. Chem. 256, 882-886). Thiourea dioxide inactivated glutamine synthetase with total loss of activity and concomitant modification of a single lysine residue. The modified amino acid was identified as homoarginine by amino acid analysis. The lysine residue modified by thiourea dioxide was established by differential labeling experiments to be the same residue associated with the 81% partial loss of activity upon pyridoxal phosphate inactivation. Inactivation with either thiourea dioxide or pyridoxal phosphate did not affect ATP binding but glutamate binding was weakened. The glutamate site was implicated as the site of thiourea dioxide modification based on protection against inactivation by saturating levels of glutamate. Glutamate also protected against pyridoxal phosphate labeling of the lysine consistent with this residue being the common site of reaction with thiourea dioxide and pyridoxal phosphate.

Cysteine residues at the active site of glutamine synthetase from spinach leaves

Titration of cysteine residues of spinach glutamine synthetase with 5-5′ dithiobis (2-nitrobenzoic acid) indicates that there are five such residues per monomer of enzyme and that two of these five are on the surface of the molecule. The presence of substrates, or either of the competitive inhibitors methionine sulfoximine or phosphinothricin, completely protects both of the surface sulfhydryls from titration. This suggests that both are located at the active site. In the absence of Mg 2+ and ATP, both surface sulfhydryls must be modified before loss of activity. We conclude that while both of the cysteine residues are located at the active site, only one of them may be involved in catalysis. Because the cysteine residue which is implicated in catalysis can be protected by Mg 2+ and ATP, we believe that it may be located at or near the binding site of these ligands.

Labeling of a specific arginine residue at the active site of glutamine synthetase ( [formula omitted

Chemical modification of a specific arginine residue of Escherichia coli glutamine synthetase has been accomplished by the use of the arginine-specific reagents p-hydroxyphenylglyoxal, phenylglyoxal, and methylglyoxal. Modification of one arginine residue results in complete inactivation of the enzyme and the modified enzyme seems to be extremely stable since no reactivation is observed upon addition of free arginine or dialysis. Saturating levels of ATP but not L-glutamate, L-methionine sulfoximine, or inorganic phosphate provide substantial protection against inactivation of the enzyme suggesting the modified amino acid is at or near the ATP substrate binding site. However, an ATP affinity analog is not prevented from binding upon modification of the arginine residue indicating that the reduction in catalytic activity is not solely due to alteration in substrate binding but may also reflect a catalytic role for the arginine residue.

Evidence for a glutamine synthetase-chromatophore association in the phototroph Rhodospirillum rubrum: purification, properties, and regulation of the enzyme.

The characteristics of soluble and membrane-bound glutamine synthetase (GS) from Rhodospirillum rubrum were compared with those of the enzyme located in situ (measured in detergent-treated cells). The results suggest that in vivo GS may be associated with, or bound to, the chromatophore membranes. GS was found to reversibly associate and dissociate from purified chromatophores as a function of the ionic strength of the buffer or the Mg2+ concentration. Solubilized GS was purified to homogeneity and found to be similar to the GS of enteric bacteria in that its molecular weight was about 600,000 and it had one type of subunit of 51,000 molecular weight. Removal of GS from the membrane had no effect on the Km values for the substrates of the biosynthetic reaction, but it did have a substantial effect on both its Mg2+ requirement (the Km increased 10-fold) and the sensitivity of the gamma-glutamyl transferase reaction to the inhibitor methionine sulfoximine (the I0.5 decreased from 1,500 to 60 microM). Both observations suggest that the active site of GS is influenced by its association with the membrane. GS activity was shown to respond to NH4+, phosphodiesterase, Mg2+, and adenylylation cofactors in a manner identical to that of the GS of the coliform bacteria, suggesting that the former may also respond to adenylylation and deadenylylation. Finally, R. rubrum GS was also inhibited by NH4+ by a newly observed, as yet undefined, system.

Rat Liver Glutamine Synthetase: PREPARATION, PROPERTIES, AND MECHANISM OF INHIBITION BY CARBAMYL PHOSPHATE

Abstract Glutamine synthetase, purified from rat liver, is homogeneous on acrylamide gel electrophoresis and on ultracentrifugation (s20,w, 15.0 S). The enzyme, which consists of eight subunits (subunit molecular weight, 44,000), resembles ovine brain glutamine synthetase in its physical properties, amino acid composition, and substrate specificity. Complete inhibition of the liver enzyme by methionine sulfoximine is associated with the binding of 4 moles of inhibitor per mole of enzyme. The enzyme is activated by α-ketoglutarate (in the presence of Mg++ or Mn++) and is inhibited by glycine, l-alanine, l-serine, and carbamyl phosphate (with Mn++ only). Evidence is presented that inhibition by carbamyl phosphate is produced by the binding of this compound to the active site of glutamine synthetase. Thus, the enzyme can catalyze the synthesis of ATP from carbamyl phosphate and ADP. When glutamine synthetase is incubated with ADP, carbamyl phosphate, and glutamate, glutamine is formed. Enzyme inactivated by treatment with methionine sulfoximine and ATP did not utilize carbamyl phosphate. Liver glutamine synthetase also catalyzes ATP synthesis from ADP and acetyl phosphate; this reaction is competitively inhibited by glutamate. Glutamine synthetases from ovine brain, rat brain, and Escherichia coli also catalyze ATP synthesis from carbamyl phosphate and ADP.

Computer analysis of the active site of glutamine synthetase.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTComputer analysis of the active site of glutamine synthetaseJerald D. Gass and Alton MeisterCite this: Biochemistry 1970, 9, 6, 1380–1390Publication Date (Print):March 1, 1970Publication History Published online1 May 2002Published inissue 1 March 1970https://doi.org/10.1021/bi00808a012RIGHTS & PERMISSIONSArticle Views91Altmetric-Citations78LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts