On the binding of L-S- and L-R-diastereoisomers of the substrate analog L-methionine sulfoximine to glutamine synthetase from Escherichia coli

Mechanistic Basis for ATP-Dependent Inhibition of Glutamine Synthetase by Tabtoxinine-β-lactam

Tabtoxinine-β-lactam (TβL), also known as wildfire toxin, is a time- and ATP-dependent inhibitor of glutamine synthetase produced by plant pathogenic strains of Pseudomonas syringae. Here we demonstrate that recombinant glutamine synthetase from Escherichia coli phosphorylates the C3-hydroxyl group of the TβL 3-(S)-hydroxy-β-lactam (3-HβL) warhead. Phosphorylation of TβL generates a stable, noncovalent enzyme-ADP-inhibitor complex that resembles the glutamine synthetase tetrahedral transition state. The TβL β-lactam ring remains intact during enzyme inhibition, making TβL mechanistically distinct from traditional β-lactam antibiotics such as penicillin. Our findings could enable the design of new 3-HβL transition state inhibitors targeting enzymes in the ATP-dependent carboxylate-amine ligase superfamily with broad therapeutic potential in many disease areas.

Evaluation of the amino acid binding site of Mycobacterium tuberculosis glutamine synthetase for drug discovery

A combination of a literature survey, structure-based virtual screening and synthesis of a small library was performed to identify hits to the potential antimycobacterial drug target, glutamine synthetase. The best inhibitor identified from the literature survey was (2S,5R)-2,6-diamino-5-hydroxyhexanoic acid (4, IC(50) of 610+/-15microM). In the virtual screening 46,400 compounds were docked and subjected to a pharmacophore search. Of these compounds, 29 were purchased and tested in a biological assay, allowing three novel inhibitors containing an aromatic scaffold to be identified. Based on one of the hits from the virtual screening a small library of 15 analogues was synthesized producing four compounds that inhibited glutamine synthetase.

Computer-aided analysis of the interactions of glutamine synthetase with its inhibitors

Mechanism of inhibition of glutamine synthetase (EC 6.3.1.2; GS) by phosphinothricin and its analogues was studied in some detail using molecular modeling methods. Among three possible conformations of phosphinothricin in the active site of GS, this compatible with binding mode of methionine sulfoximine, determined recently by crystallography, was found to be energetically favored. Basing on these results eleven inhibitors of GS were docked into its active site. Taking into consideration that phosphinothricin acts as suicide inhibitor, which is due to phosphorylation by the enzyme, seven of studied analogues were additionally analyzed in their phosphorylated forms. All the inhibitor-enzyme complexes were evaluated quantitatively by using eight scoring functions implemented in Insight and Sybyl program packages and significant correlation between the obtained scores and experimental pK(i) values was achieved. Computed surface charge distribution for five selected inhibitors in both free and phosphorylated forms and their comparison with electronic structure of enzymatic reaction transition state allowed us to determine important electronic features required to construct potent inhibitors of glutamine synthetase.

Thermodynamic effects of active-site ligands on the reversible, partial unfolding of dodecameric glutamine synthetase from Escherichia coli: calorimetric studies

Dodecameric glutamine synthetase (GS) from Escherichia coli undergoes reversible, thermally induced partial unfolding without subunit dissociation. A single endotherm for Mn.GS (+/- active-site ligands) in the presence of 1 mM free Mn2+ and 100 mM KCl at pH 7 is observed by differential scanning calorimetry (DSC). Previous deconvolutions of DSC data for Mn.GS showed only two two-state transitions (with similar tm values; 51.6 +/- 2 degrees C), and indicated that cooperative interactions link partial unfolding reactions of all subunits within the Mn.enzyme dodecamer [Ginsburg, A., & Zolkiewski, M. (1991) Biochemistry 30, 9421]. A net uptake of 8.0 equiv of H+ by Mn.GS occurs during partial unfolding, as determined in the present DSC experiments conducted with four buffers having different heats of protonation at 50 degrees C. These data gave a value of 176 +/- 12 kcal (mol of dodecamer)-1 for delta Hcal corrected for buffer protonation. L-Glutamine and L-Met-(SR)-sulfoximine stabilize the Mn.GS dodecamer through the free energies of ligand binding, and these were shown to be partially and totally released, respectively, from the 12 active sites at high temperature. Ligand effects on Tm values from DSC were similar to those from spectral measurements of Trp and Tyr exposures in two subunit domains. Effects of varying [ADP] on DSC profiles of Mn.GS were complex; Tm is increased by low [ADP] and decreased by > 100 microM free ADP. This is due to the exposure of an additional low-affinity ADP binding site per GS subunit at high temperature with log K1' = 4.3 and log K2' = 3.6 at 60 degrees C relative to log K' = 5.5 for ADP binding at 30 degrees C, as determined by isothermal calorimetric and fluorescence titrations. Moreover, delta Hcal at > 27% saturation with ADP (corrected for ADP binding/dissociation) is approximately 80-100 kcal/mol more than in the absence of ligands. Changes in domain interactions could result from ADP bridging subunit contacts in the dodecamer. Each of the active-site ligands investigated here produces different effects on DSC profiles without uncoupling the extremely cooperative, partial unfolding reactions in the Mn.GS dodecamer.

The crystal structure of phosphinothricin in the active site of glutamine synthetase illuminates the mechanism of enzymatic inhibition

Phosphinothricin is a potent inhibitor of the enzyme glutamine synthetase (GS). The resolution of the native structure of GS from Salmonella typhimurium has been extended to 2.5 A resolution, and the improved model is used to determine the structure of phosphinothricin complexed to GS by difference Fourier methods. The structure suggests a noncovalent, dead-end mechanism of inhibition. Phosphinothricin occupies the glutamate substrate pocket and stabilizes the Glu327 flap in a position which blocks the glutamate entrance to the active site, trapping the inhibitor on the enzyme. One oxygen of the phosphinyl group of phosphinothricin appears to be protonated, because of its proximity to the carboxylate group of Glu327. The other phosphinyl oxygen protrudes into the negatively charged binding pocket for the substrate ammonium, disrupting that pocket. The distribution of charges in the glutamate binding pocket is complementary to those of phosphinothricin. The presence of a second ammonium binding site within the active site is confirmed by its analogue thallous ion, marking the ammonium site and its protein ligands. The inhibition of GS by methionine sulfoximine can be explained by the same mechanism. These models of inhibited GS further illuminate its catalytic mechanism.

Structure-function relationships of glutamine synthetases

As a highly regulated enzyme at the core of nitrogen metabolism, glutamine synthetase has been studied intensively. We review structural and functional studies of both bacterial and eukaryotic glutamine synthetases, with emphasis on enzymatic inhibitors.

Advances in the enzymology of glutamine synthesis

Meister's proposal of a gamma-glutamyl-P intermediate in the glutamine synthetase reaction set the scene for understanding how the stepwise activation of the carboxyl group greatly increased its susceptibility toward nucleophilic attack and amide bond synthesis. Topics covered in this review include: the discovery of the enzymatic synthesis of glutamine; the role of glutamine synthetase in defining the thermodynamics of ATPases; early isotopic tracer studies of the synthetase reaction; the proposed intermediacy of gamma-glutamyl-phosphate; the mechanism of methionine sulfoximine inhibition; stereochemical mapping of the enzyme's active site; detection of enzyme reaction cycle intermediates; borohydride trapping of gamma-glutamyl-P; positional isotope exchanges catalyzed by glutamine synthetase; regulation of bacterial enzyme; and a brief account of how knowledge of the atomic structure of bacterial glutamine synthetase has clarified ligand binding interactions. Concluding remarks also address how the so-called "Protein Ligase Problem" may be solved by extending the catalytic versatility of carboxyl-group activating enzymes.

Investigation of the mechanism of phosphinothricin inactivation of Escherichia coli glutamine synthetase using rapid quench kinetic technique

A number of slow tight-binding inhibitors are known for glutamine synthetase that resemble the geometry of the tetrahedral intermediate formed during the enzyme-catalyzed condensation of gamma-glutamyl phosphate and ammonia. One of these inhibitors, phosphinothricin [L-2-amino-4-(hydroxymethyl-phosphinyl)butanoic acid], has been investigated by rapid kinetic methods. Phosphinothricin not only exhibits the kinetic properties of a slow tight-binding inhibitor but also undergoes phosphorylation during the course of the ATP-dependent inactivation. The acid lability of phosphinothricin phosphate enabled investigation of the kinetics of glutamine synthetase inactivation using rapid quench kinetic techniques. The rate-limiting step in the inhibition reaction is the binding of inhibitor (0.004-0.014 microM-1 s-1) and/or a conformational change associated with binding, which is several orders of magnitude slower than the binding of ATP. The association rate of phosphinothricin depends on which metal ion is bound to the enzyme (Mn2+ or Mg2+). With Mn2+ bound to glutamine synthetase the rate of association and the phosphorylation rate are faster than when Mg2+ is bound. The data are interpreted with use of a model in which the binding of a substrate analogue with a tetrahedral moiety enhances the phosphorylation rate of the reaction intermediate; however, the initial binding interaction is retarded because the enzyme has to bind a molecule that has a "transition-state" geometry rather than a ground-state substrate structure. During the course of the inactivation, progressively slower rates for binding and phosphoryl transfer were observed, indicating communication between active sites.

Adenosine 5'-triphosphate analogues as structural probes for Escherichia coli glutamine synthetase

Introduction of specific structural probes into substrate binding sites of Escherichia coli glutamine synthetase is now possible. Various analogues of ATP substituted with an amino or sulfhydryl moiety at the 6- or 8-position of the purine ring have been found to substitute for ATP in the autoinactivation reaction of the manganese enzyme with L-Met-(S)-sulfoximine at pH 7. Dissociation of enzyme complexes containing an ADP analogue, L-Met-(S)-sulfoximine phosphate, and 2 equiv of Mn2+ is negligible at neutral pH. Prior to binding of the mercapto nucleotides to active sites, 6-mercaptopurine ribonucleoside triphosphate (6-S-ATP) and 8-mercaptoadenosine 5'-triphosphate (8-S-ATP) also have been further modified with fluorescent and chromogenic probes for energy-transfer measurements [Maurizi, M. R., Kasprzyk, P. G., & Ginsburg, A. (1986) Biochemistry (following paper in this issue)] or with electron-dense markers for electron microscopic and X-ray crystallographic structural analyses. Binding 6-S-ATP or 8-S-ATP to enzyme active sites at pH 7.1 produced red shifts of approximately 6 nm in nucleotide spectra characteristic for transfer of these nucleotide analogues into more acidic and hydrophobic environments. The spectrum of 6-S-ADP at active sites was more red-shifted than that of 6-S-AMP attached to adenylylation sites. The thiol group at the 6- or 8-position of the purine ring of the bound nucleotides was accessible for reactions with alkylating or mercurial reagents. Alkylation or mercaptide formation produced large blue shifts in the spectrum of enzyme-bound 6-S-ADP or 8-S-ADP at active sites or of 6-S-AMP covalently bound at adenylylation sites. At least one of two tryptophanyl residues in each subunit is very near the nucleotide binding site, as evidenced by changes in tryptophanyl residue fluorescence on binding ATP, mercaptonucleotides, or other ATP analogues.

Distances between active site probes in glutamine synthetase from Escherichia coli: fluorescence energy transfer in free and in stacked dodecamers

Probes for fluorescence energy transfer measurements were introduced into active sites of dodecameric glutamine synthetase from Escherichia coli by substituting appropriate ATP analogues for ATP in the autoinactivation reaction of this enzyme with L-Met-(S)-sulfoximine and Mn2+ [Maurizi, M. R., & Ginsburg, A. (1986) Biochemistry (preceding paper in this issue)]. Two fluorescent donors, 8-mercapto-ATP alkylated with either 5-[[[(iodoacetyl)amino]ethyl]amino]naphthalene-1-sulfonic acid (AEDANS-ATP) or 1,N6-etheno-2-aza-ATP (aza-epsilon-ATP), and two acceptors, 6-mercaptopurine ribonucleotide triphosphate or 8-mercapto-ATP alkylated with the chromophore 4'-[[4-(dimethylamino)-phenyl]azo]-2-iodoacetanilide (6-Y- or 8-Y-ATP), were used. Fluorescence emissions of enzyme derivatives with 1 or 2 equiv of fluorescent donor per dodecamer and either an acceptor (Y) or ADP at the remaining active sites were compared at pH 7.0. The results, together with the known geometry of the enzyme, indicate that active site probes in the dodecamer are widely separated and that energy transfer occurs from a single donor to two or three acceptors on adjacent subunits. The calculated distance between equidistant active site probes on heterologously bonded subunits within the same hexagonal ring is 56-60 A. Probes on isologously bonded subunits are no closer than 60 A and may be as far apart as 78 A. Thus, active sites are away from the 6-fold axis of symmetry toward the outer edges of the dodecamer and are located greater than or equal to 30 A from the plane separating the hexagonal rings. During Zn2+-induced stacking of the same enzyme derivatives along the 6-fold axes of symmetry, additional quenches of fluorescent probes were dependent on the presence of acceptors on separate dodecamers. The Zn2+-induced face to face aggregation of dodecamers in the presence of 46 microM ZnCl2 and 9 mM MgCl2 at pH 7.0 had an Arrhenius activation energy of 22.3 +/- 0.2 kcal/mol and a second-order rate constant at 25 degrees C of approximately 10(5) M-1 s-1 at early stages. Time-dependent fluorescence quenches correlated well with the degree of linear polymer formation and reached maximum values of 47-70% quench when the average n-mer was six dodecamers. After correction for unquenched polymer ends, a fluorescent donor and an acceptor probe in layered dodecamers were estimated to be approximately 36 A apart--an average value if there is some twisting of single strands.(ABSTRACT TRUNCATED AT 400 WORDS)