Reactivation of glutamine synthetase from Escherichia coli after auto-inactivation with L-methionine-S-sulfoximine, ATP, and Mn2+

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.

Inhibition of human glutamine synthetase by L-methionine-S,R-sulfoximine-relevance to the treatment of neurological diseases

At high concentrations, the glutamine synthetase inhibitor L-methionine-S,R-sulfoximine (MSO) is a convulsant, especially in dogs. Nevertheless, sub-convulsive doses of MSO are neuroprotective in rodent models of hyperammonemia, acute liver disease, and amyotrophic lateral sclerosis and suggest MSO may be clinically useful. Previous work has also shown that much lower doses of MSO are required to produce convulsions in dogs than in primates. Evidence from the mid-20th century suggests that humans are also less sensitive. In the present work, the inhibition of recombinant human glutamine synthetase by MSO is shown to be biphasic-an initial reversible competitive inhibition (K i 1.19 mM) is followed by rapid irreversible inactivation. This K i value for the human enzyme accounts, in part, for relative insensitivity of primates to MSO and suggests that this inhibitor could be used to safely inhibit glutamine synthetase activity in humans.

'Irreversible' slow-onset inhibition of orotate phosphoribosyltransferase by an amidrazone phosphate transition-state mimic

A mimic of the putative transition-state intermediate has been synthesized and found to be a very slow-onset inhibitor of yeast orotate phosphoribosyltransferase. The mechanism of inhibition may involve a rate-determining isomerization of the enzyme to a form receptive to the inhibitor, which then remains tightly bound.

Thermodynamic and extrathermodynamic requirements of enzyme catalysis

An enzyme's affinity for the altered substrate in the transition state (symbolized here as S) matches the value of k(cat)/K(m) divided by the rate constant for the uncatalyzed reaction in water. The validity of this relationship is not affected by the detailed mechanism by which any particular enzyme may act, or on whether changes in enzyme conformation occur on the path to the transition state. It subsumes potential effects of substrate desolvation, H-bonding and other polar attractions, and the juxtaposition of several substrates in a configuration appropriate for reaction. The startling rate enhancements that some enzymes produce have only recently been recognized. Direct measurements of the binding affinities of stable transition-state analog inhibitors confirm the remarkable power of binding discrimination of enzymes. Several parts of the enzyme and substrate, that contribute to S binding, exhibit extremely large connectivity effects, with effective relative concentrations in excess of 10(8) M. Exact structures of enzyme complexes with transition-state analogs also indicate a general tendency of enzyme active sites to close around S in such a way as to maximize binding contacts. The role of solvent water in these binding equilibria, for which Walter Kauzmann provided a primer, is only beginning to be appreciated.

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.

Mutational analysis of Asp51 of Anabaena azollae glutamine synthetase. D51E mutation confers resistance to the active site inhibitors L-methionine-DL-sulfoximine and phosphinothricin

The role of Asp51 in the catalytic activity of glutamine synthetase from the cyanobacterium Anabaena azollae has been analyzed. Five mutant enzymes, D51S, D51A, D51E, D51N and D51R, were constructed by site-directed mutagenesis and characterized. Asp51 appears to participate in the binding of ammonium ion, as affinity for this substrate was affected in all cases, although it varied according to the charge and/or size of the amino-acid residue, decreasing in the order Glu > Asn > Ser > Ala. The replacement of Asp51 by Glu (D51E) conferred besides a high resistance to the herbicides L-methionine-DL-sulfoximine and phosphinothricin, as a result of a decreased phosphorylation ability.

Metal-dependent self-assembly of protein tubes from Escherichia coli glutamine synthetase. Cu(2+) EPR studies of the ligation and stoichiometry of intermolecular metal binding sites

Escherichia coli glutamine synthetase (GS) is a dodecameric assembly of identical subunits arranged as two back-to-back hexagonal rings. In the presence of divalent metal ions, the dodecamers "stack" along their six-fold axis of symmetry to yield elongated tubes. This self-assembly process provides a useful model for probing metal-dependent protein-protein interactions. However, no direct spectroscopic or structural data have confirmed the identity of the ligands to the shared metal ions in "stacked" GS. Here, 9-GHz Cu(2+) EPR studies have been used to probe the ligand structure and stoichiometry of the metal binding sites. The wild type protein, with N-terminal sequence (His-4)-X(3)-(Met-8)-X(3)-(His-12), exhibits a classic Cu(2+)-nitrogen spectrum, with g = 2.06 G, g = 2.24 G, and A = 19.3 x 10(-3) cm(-1). No superhyperfine structure is observed. The H4C mutant affords a spectrum that is the combination of two spectra at all stages of saturation. One of the overlapping spectra is nearly identical to the spectrum of wild type, and is due to His ligation. The second spectrum observed yields g = 2.28 and A = 17.1 x 10(-3) cm(-1). The linewidth and tensor values of the second component have been assigned to Cu(2+)-S ligation. In contrast, the H12C mutant exhibits an EPR spectrum at low Cu(2+) occupancy that is very similar to the second set of spectral features observed for H4C, and which is assigned to Cu(2+)-S ligation. No Cu(2+)-His ligation is apparent until the Cu(2+)/N-terminal helices ratio is >1.0. At saturation, the g = 2.00-2.06 region of the spectrum is essentially a mirror image of the spectrum obtained with H4C, and is due to overlapping Cu(2+)-N and Cu(2+)-S EPR spectra. The M8L and M8C mutants were also studied, in order to probe the role of position 8 in the N-terminal helix. Spectral parameters of these mutants are nearly identical to each other and to the wild type spectrum at saturating Cu(2+), suggesting that Met-8 does not act as a direct metal ligand. Together, the results provide the first direct evidence for a binuclear metal ion site between each N-terminal helix pair at the GS-GS interface, with both His-4 and His-12 providing metal ligands.

Supramolecular self-assembly of Escherichia coli glutamine synthetase: characterization of dodecamer stacking and high order association

Dodecameric glutamine synthetase (GS) from bacteria is formed from two face-to-face hexameric rings of identical subunits. These highly symmetrical aggregates from some bacteria, including Escherichia coli, "stack" in the presence of Zn2+ and other divalent ions to generate protein tubes (phase I) and subsequently associate side-to-side to yield "cables" and nonspecific aggregates (phase II). In order to understand the molecular mechanisms of recognition leading to this macromolecular self-assembly, the effects of solution conditions on the kinetics of these processes have been studied. These reactions have been monitored by changes in light scattering and by electron microscopy. Conditions have been established for isolation of phases I and II. At 0.04 mg of GS/mL, pH 7.0, 100 mM KCl, and 1 mM Mn2+, 25 degrees C, minimal side-to-side aggregation occurs, and the stacking reaction follows second-order kinetics, with respect to GS, at low extent of reaction. The second-order rate constants determined for phase I, initiated by Zn2+ or Co2+, demonstrate a pH optimum at 7.0-7.25, whereas phase II is favored at pHs below 6.5. The pH profile for the stacking reaction suggests that His residues are involved, and modification of 2-3 histidines/subunit with diethyl pyrocarbonate (DEPC) is sufficient to completely inhibit metal-dependent dodecamer stacking. The effect of ionic strength on GS stacking was also studied. Although hydrophobic interactions have previously been assumed to dominate this protein-protein association, both phase I and phase II of the assembly are inhibited by KCl and NaCl, suggesting that ionic interactions also play an essential role.(ABSTRACT TRUNCATED AT 250 WORDS)

Supramolecular self-assembly of Escherichia coli glutamine synthetase: effects of pressure and adenylylation state on dodecamer stacking

Escherichia coli glutamine synthetase is a dodecamer of identical subunits, consisting of two face-to-face hexameric rings. The enzymatic activity of GS is regulated by covalent attachment of an adenylyl group to each subunit, at the edge of the ring structure (Tyr-397). In the presence of Zn2+, Cu2+, Co2+, and other divalent metal ions, the free dodecamers self-organize into protein tabules [Miller et al. (1974) Arch. Biochem. Biophys. 163, 155-171]. Here, the temperature dependence and pressure dependence of the kinetics of Zn(2+)-induced self-assembly of GS tubules have been determined for the adenylylated and unadenylylated GS. The adenylylated enzyme exhibits a bimolecular rate constant for Zn(2+)-induced stacking that is 3-fold lower than for the unadenylylated GS at temperatures ranging from 0 to 25 degrees C. The enthalpy of activation, delta H++, for both adenylylated and unadenylylated GS increases from approximately 10 kcal/mol of dodecamer interface to 20 kcal/mol of dodecamer interface upon addition of 125 mM KCl to the reaction buffer. The delta H++ values for adenylylated and unadenylylated GS are nearly identical, at each concentration of KCl, suggesting that entropic factors are responsible for the differences in rate of stacking for these forms of GS. Hydrostatic pressure markedly inhibits the stacking reaction for both adenylylated and unadenylylated GS. The activation volumes, delta V++a, for stacking are increased from approximately 50 mL/mol of dodecamer interface in the absence of KCl to approximately 65 mL/mol of dodecamer interface in the presence of 125 mM KCl.(ABSTRACT TRUNCATED AT 250 WORDS)

Transition-state characterization: a new approach combining inhibitor analogues and variation in enzyme structure

A new strategy of potentially broad application for probing transition-state (TS) analogy in enzymatic systems is described in this paper. The degree to which a series of phosphonate inhibitors act as TS analogues of rat carboxypeptidase A1 has been determined for the wild-type enzyme, for the R127K, R127M, and R127A mutants, and for the R127A mutant in the presence of 0.5 M guanidine hydrochloride. The impact that the mutations have on the inverse second-order rate constants (Km/kcat) for substrate hydrolysis is mirrored by the effect on the inhibition constants (Ki) for the corresponding phosphonate inhibitors. These results demonstrate that the phosphonate moiety mimics some of the electronic as well as the geometric characteristics of the TS. A similar but distinctly separate correlation is observed for tripeptide analogues in comparison to analogues of the dipeptide Cbz-Gly-Phe, reflecting an anomalous mode of binding for the latter system. The selective rate increases and corresponding enhancement in inhibitor binding observed on addition of 0.5 M guanidine hydrochloride to the R127A mutant indicate that the exogenous cation can assume the role played by Arg-127 in stabilizing the TS and in providing substrate selectivity at the P2 position.

Synthesis and evaluation of an inhibitor of carboxypeptidase A with a Ki value in the femtomolar range

Comparative studies among a series of tripeptide phosphonate inhibitors of the zinc peptidase carboxypeptidase A indicate that incorporation of the phosphonic acid analogue of valine at the P1 position results in significantly higher affinity than the glycine, alanine, or phenylalanine analogues. When applied to the tripeptide analogue Cbz-Phe-ValP-(O)Phe [ZFVP(O)F], determination of the inhibition constant Ki was complicated by the very slow rate of dissociation. The rate of exchange of [3H]ZFVP(O)F with enzyme-bound [14C]ZFVP(O)F was followed for periods of 3-4 months to measure dissociation rate constants in the range of (1.7-4.4) x 10(-9) s-1, corresponding to half-lives of 5-13 years. Although the on- and off-rate constants differ for different carboxypeptidase isozymes, their ratios, corresponding to the inhibition constants Ki, are consistently in the range of 10-27 fM. Both the inhibition constants and the dissociation rate constants appear to be the lowest values yet determined for an enzyme-small inhibitor interaction.

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.

Novel subunit-subunit interactions in the structure of glutamine synthetase

We present an atomic model for glutamine synthetase, an enzyme of central importance in bacterial nitrogen metabolism, from X-ray crystallography. The 12 identical subunits are arranged as the carbon atoms in two face-to-face benzene rings, with unusual subunit contacts. Our model, which places the active sites at the subunit interfaces, suggests a mechanism for the main functional role of glutamine synthetase: how the enzyme regulates the rate of synthesis of glutamine in response to covalent modification and feedback inhibition.

Role of glutamine synthetase adenylylation in the self-protection of Pseudomonas syringae subsp. "tabaci" from its toxin, tabtoxinine-beta-lactam

Selected pathovars of Pseudomonas syringae produce an extracellular phytotoxin, tabtoxinine-beta-lactam, that irreversibly inhibits its known physiological target, glutamine synthetase (GS). Pseudomonas syringae subsp. "tabaci" retains significant amounts of glutamine synthetase activity during toxin production in culture. As part of our investigation of the self-protection mechanism(s) used by these pathovars, we have determined that GS becomes adenylylated after toxin production is initiated and that the serine released from the zinc-activated hydrolysis of tabtoxin is a factor in the initiation of this adenylylation. The adenylylation state of this GS was estimated to range from E5.0-7.5. The irreversible inactivation by tabtoxinine-beta-lactam of unadenylylated and adenylylated glutamine synthetase purified from P. syringae subsp. "tabaci" was investigated. Adenylylated GS was inactivated by tabtoxinine-beta-lactam at a slower rate than was unadenylylated enzyme. Adenylylated GS (E7.5-10.5) was significantly protected from this inactivation in the presence of the enzyme effectors, AMP, Ala, Gly, His, and Ser. Thus, the combination of the adenylylation of GS after toxin production is initiated and the presence of the enzyme effectors in vivo could provide part of the self-protection mechanism used by subsp. "tabaci".

Mg2+ is bound to glutamine synthetase extracted from bovine or ovine brain in the presence of L-methionine-S-sulfoximine phosphate

Purified glutamine synthetase from bovine or ovine brain had no tightly bound Mn2+. By extraction of bovine or ovine brain glutamine synthetase in the presence of L-Met-S-sulfoximine phosphate and ADP in metal ion-free water and 0.1 M KCl, only endogenously bound divalent cations were trapped on the enzyme. Enzyme complexes isolated by immunoprecipitation contained less than 0.05 Mn2+ and 1.5 +/- 0.2 Mg2+ per subunit. Without inactive complex formation, the enzyme immunoprecipitated from extracts contained undetectable Mn2+ (less than 0.01 eq per subunit) and 0.1-2.0 eq of Mg2+ per subunit. Direct binding measurements showed that the purified bovine brain enzyme contained two divalent cations bound at the active site of each subunit. Thus, although either Mg2+ or Mn2+ supports enzyme activity in vitro, Mg2+ rather than Mn2+ appears to be bound to brain glutamine synthetase in vivo.

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)

Biophysical studies of Escherichia coli glutamine synthetase

The purpose of the EPR and NMR studies presented in this article was to determine the spatial relationship between the n1 and n2 metal ion sites of E. coli glutamine synthetase. Table I presents the distances between these two metal ion sites in various complexes using Mn(II) bound to each site. These studies also employed the transition-state analog, methionine sulfoximine, as an active-site probe as well as various nucleotide complexes. Two primary conclusions result from these data. The Mn(II)-Mn(II) distance changes from approximately 10 to approximately 8 A when nucleotides bind to the enzyme presumably as the result of a protein conformational change. Two Mn(II) ions can bind to the enzyme in the presence of the substitution-inert Co(III)-ATP complex, implying that the metal ion [Co(III)] coordinated to the beta-gamma phosphoryl groups in the complex is displaced from the normal n2 metal ion site. A model showing the probable spatial relationships among components of the active site is shown in Fig. 6. This model comprises our current working hypothesis of the active site of glutamine synthetase. Further studies of distance relationships are presently underway in our laboratory and will be placed in the context of this model and the known kinetic mechanism.