Structure-function relationships of glutamine synthetases

Different nitrogen sources modulate activity but not expression of glutamine synthetase in arbuscular mycorrhizal fungi

Glutamine synthetase (GS) is a central enzyme of nitrogen metabolism that allows assimilation of nitrogen and biosynthesis of glutamine. We isolated the cDNA encoding GS from two arbuscular mycorrhizal fungi, Glomus mosseae (GmGln1) and Glomus intraradices (GiGln1). The deduced protein orthologues have a high degree of similarity (92%) with each other as well as with GSs from other fungi. GmGln1 was constitutively expressed during all stages of the fungal life cycle, i.e., spore germination, intraradical and extraradical mycelium. Feeding experiments with different nitrogen sources did not induce any change in the mRNA level of both genes independent of the symbiotic status of the fungus. However, GS activity of extraradical hypahe in G. intraradices was considerably modulated in response to different nitrogen sources. Thus, in a N re-supplementation time-course experiment, GS activity responded quickly to addition of nitrate, ammonium or glutamine. Re-feeding with ammonium produced a general increase in GS activity when compared with hyphae grown in nitrate as a sole N source.

Structural characterization of glutamine synthetase fromAzospirillum brasilense

CD spectroscopic study of the secondary structure of partly adenylylated glutamine synthetase (GS) of the bacterium Azospirillum brasilense showed both the native and cation-free (EDTA-treated) enzyme to be highly structured (58 and 49% as alpha-helices, 10 and 20% as beta-structure, respectively). Mg(2+), Mn(2+), or Co(2+), when added to the native GS, had little effect on its CD spectrum, whereas their effects on the cation-free GS were more pronounced. Emission ((57)Co) Mössbauer spectroscopic (EMS) study of (57)Co(2+)-doped cation-free GS in frozen solution and in the dried state gave similar spectra and Mössbauer parameters for the corresponding spectral components, reflecting the ability of the Co(2+)-enzyme complex to retain its properties upon drying. The EMS data show that (a) A. brasilense GS has 2 cation-binding sites per active center and (b) one site has a higher affinity to Co(2+) than the other, in line with the data on other bacterial GSs.

Adenylylation and catalytic properties of Mycobacterium tuberculosis glutamine synthetase expressed in Escherichia coli versus mycobacteria

Bacterial glutamine synthetases (GSs) are complex dodecameric oligomers that play a critical role in nitrogen metabolism, converting ammonia and glutamate to glutamine. Recently published reports suggest that GS from Mycobacterium tuberculosis (MTb) may be a therapeutic target (Harth, G., and Horwitz, M. A. (2003) Infect. Immun. 71, 456-464). In some bacteria, GS is regulated via adenylylation of some or all of the subunits within the aggregate; catalytic activity is inversely proportional to the extent of adenylylation. The adenylylation and deadenylylation of GS are catalyzed by adenylyl transferase (ATase). Here, we demonstrate via electrospray ionization mass spectrometry that GS from pathogenic M. tuberculosis is adenylylated by the Escherichia coli ATase. The adenylyl group can be hydrolyzed by snake venom phosphodiesterase to afford the unmodified enzyme. The site of adenylylation of MTb GS by the E. coli ATase is Tyr-406, as indicated by the lack of adenylylation of the Y406F mutant, and, as expected, is based on amino acid sequence alignments. Using electrospray ionization mass spectroscopy methodology, we found that GS is not adenylylated when obtained directly from MTb cultures that are not supplemented with glutamine. Under these conditions, the highly related but non-pathogenic Mycobacterium bovis BCG yields partially ( approximately 25%) adenylylated enzyme. Upon the addition of glutamine to the cultures, the MTb GS becomes significantly adenylylated ( approximately 30%), whereas the adenylylation of M. bovis BCG GS does not change. Collectively, the results demonstrate that MTb GS is a substrate for E. coli ATase, but only low adenylylation states are accessible. This parallels the low adenylylation states observed for GS from mycobacteria and suggests the intriguing possibility that adenylylation in the pathogenic versus non-pathogenic mycobacteria is differentially regulated.

Nitrogen assimilation and global regulation in Escherichia coli

Nitrogen limitation in Escherichia coli controls the expression of about 100 genes of the nitrogen regulated (Ntr) response, including the ammonia-assimilating glutamine synthetase. Low intracellular glutamine controls the Ntr response through several regulators, whose activities are modulated by a variety of metabolites. Ntr proteins assimilate ammonia, scavenge nitrogen-containing compounds, and appear to integrate ammonia assimilation with other aspects of metabolism, such as polyamine metabolism and glutamate synthesis. The leucine-responsive regulatory protein (Lrp) controls the synthesis of glutamate synthase, which controls the Ntr response, presumably through its effect on intracellular glutamine. Some Ntr proteins inhibit the expression of some Lrp-activated genes. Guanosine tetraphosphate appears to control Lrp synthesis. In summary, a network of interacting global regulators that senses different aspects of metabolism integrates nitrogen assimilation with other metabolic processes.

Glucocorticoids induce glutamine synthetase expression in human osteoblastic cells: a novel observation in bone

Glucocorticoids have marked effects on bone metabolism, and continued exposure of skeletal tissue to excessive amounts of these steroids results in osteoporosis. Therefore, in the present proteomic study, we characterized the potential effects of glucocorticoids on protein expression in human osteoblastic cells. Using two-dimensional gel electrophoresis and mass spectrometry, we identified an increased expression of glutamine synthetase (GS) in dexamethasone (Dex)-treated human MG-63 osteosarcoma cells. GS is an enzyme catalyzing the conversion of glutamate and ammonia to glutamine. Intracellular and extracellular glutamate levels may be important in cell signalling mediated by glutamate transporters and receptors which have recently been found in bone cells. The induction of GS protein by Dex was accompanied by an increase in mRNA level and enzyme activity. Dex induction of GS was also mediated by glucocorticoid receptors (GRs) because it was blocked by the GR antagonist RU-38486. In addition, Dex induction of GS expression was partially blocked by cyclohexamide indicating that it at least partly required new protein synthesis. GS induction by Dex was not associated with apoptosis as determined by Bax/Bcl-2 ratio and DNA staining. In addition to MG-63 cells, Dex induction of GS was also observed in human G-292 osteosarcoma cells as well as conditionally immortalized human preosteoblastic (HOB-03-C5) and mature osteoblastic (HOB-03-CE6) cells. However, in two other human osteosarcoma cell lines, SaOS-2 and U2-OS, GS expression was not affected by Dex. This observation may be explained by the lower levels of GR protein in these cells. In summary, this is the first report of the regulation of GS expression by glucocorticoids in bone cells. The role of GS in bone cell metabolism and glucocorticoid action on the skeleton is not yet known, but as a modulator of intracellular glutamate and glutamine levels, it may have an important role in these processes.

Loss of glutamine synthetase in the human epileptogenic hippocampus: possible mechanism for raised extracellular glutamate in mesial temporal lobe epilepsy

BACKGROUND

High extracellular glutamate concentrations have been identified as a likely trigger of epileptic seizures in mesial temporal lobe epilepsy (MTLE), but the underlying mechanism remains unclear. We investigated whether a deficiency in glutamine synthetase, a key enzyme in catabolism of extracellular glutamate in the brain, could explain the perturbed glutamate homoeostasis in MTLE.

METHODS

The anteromedial temporal lobe is the focus of the seizures in MTLE, and surgical resection of this structure, including the hippocampus, leads to resolution of seizures in many cases. By means of immunohistochemistry, western blotting, and functional enzyme assays, we assessed the distribution, quantity, and activity of glutamine synthetase in the MTLE hippocampus.

FINDINGS

In western blots, the expression of glutamine synthetase in the hippocampus was 40% lower in MTLE than in non-MTLE samples (median 44 [IQR 30-58] vs 69 [56-87]% of maximum concentration in standard curve; p=0.043; n=8 and n=6, respectively). The enzyme activity was lower by 38% in MTLE vs non-MTLE (mean 0.0060 [SD 0.0031] vs 0.0097 [0.0042] U/mg protein; p=0.045; n=6 and n=9, respectively). Loss of glutamine synthetase was particularly pronounced in areas of the MTLE hippocampus with astroglial proliferation, even though astrocytes normally have high content of the enzyme. Quantitative immunoblotting showed no significant change in the amount of EAAT2, the predominant glial glutamate transporter in the hippocampus.

INTERPRETATION

A deficiency in glutamine synthetase in astrocytes is a possible molecular basis for extracellular glutamate accumulation and seizure generation in MTLE. Further studies are needed to define the cause, but the loss of glutamine synthetase may provide a new focus for therapeutic interventions in MTLE.

N-terminal extension of canine glutamine synthetase created by splicing alters its enzymatic property

It was found that an extra exon exists in the first intron of glutamine synthetase gene, generated by means of alternative splicing. Inclusion of this exon decreased the translation of glutamine synthetase (GS) in human, dog, and mouse. When translated in vitro with the canine GS transcript containing the exon, we obtained two different species of GS enzymes. Besides the known 45-kDa protein, the extended form of GS was identified with additional 40 amino acids on its N-terminal end. An upstream ATG in the extra exon served as a translation initiator for the long form of GS. When the long transcript was translated in vivo in animal cells, only the long GS was expressed. On the other hand, the long GS is less predominant relative to the short one in canine tissues including brain and liver. Subcellular fractionation of canine brain revealed that the long GS is present in all cellular compartments as is the short one, which is consistent with fluorescence microscopy data obtained with green fluorescent protein fused to GS. The short (SGS) and long (LGS) forms of canine GS were purified in Escherichia coli and shown to have similar Km values for l-glutamate and hydroxylamine. However, the Km values for ATP were slightly altered, 1.3 and 1.9 mm for the short and long GSs, respectively. The Kis for l-methionine-S-sulfoximine (MSOX), a highly potent ATP-dependent inactivator of GS, were considerably different such that the values are 0.067 and 0.124 mm for the short and long forms, respectively. When the intrinsic fluorescences of tryptophans were monitored upon bindings of chloride and metal ions without any effect on the oligomeric state, the pattern of quenching in LGS was significantly different from that of SGS. Taken together, the N-terminal extension in the long isoform of GS induces a conformational change of core enzyme, leading to a change in affinity to its substrates as well as in the effector-induced conformational alterations.

Phenotypic changes resulting from distinct point mutations in the Azospirillum brasilense glnA gene, encoding glutamine synthetase

Sequencing the glnA genes of two chemically induced Azospirillum brasilense glutamine synthetase mutants revealed an Arg-->Cys mutation, corresponding to the glutamate binding site, in one mutant and an Asp-->Asn mutation, corresponding to the ammonium binding site, in the second mutant. The phenotypic changes in these mutants are discussed in relation to their genotypes.

Distinctive properties and expression profiles of glutamine synthetase from a plant symbiotic fungus

The nucleotide sequences reported in this paper have been submitted to the GenBank(R)/EBI Nucleotide Sequence Databases with accession numbers AF462037 (glutamine synthetase) and AF462032 (glutamate synthase). Nitrogen retrieval and assimilation by symbiotic ectomycorrhizal fungi is thought to play a central role in the mutualistic interaction between these organisms and their plant hosts. Here we report on the molecular characterization of the key N-assimilation enzyme glutamine synthetase from the mycorrhizal ascomycete Tuber borchii (TbGS). TbGS displayed a strong positive co-operativity ( n =1.7+/-0.29) and an unusually high S(0.5) value (54+/-16 mM; S(0.5) is the substrate concentration value at which v =(1/2) V (max)) for glutamate, and a correspondingly low sensitivity towards inhibition by the glutamate analogue herbicide phosphinothricin. The TbGS mRNA, which is encoded by a single-copy gene in the Tuber genome, was up-regulated in N-starved mycelia and returned to basal levels upon resupplementation of various forms of N, the most effective of which was nitrate. Both responses were accompanied by parallel variations of TbGS protein amount and glutamine synthetase activity, thus indicating that TbGS levels are primarily controlled at the pre-translational level. As revealed by a comparative analysis of the TbGS mRNA and of the mRNAs for the metabolically related enzymes glutamate dehydrogenase and glutamate synthase, TbGS is not only the sole messenger that positively responds to N starvation, but also the most abundant under N-limiting conditions. A similar, but even more discriminating expression pattern, with practically undetectable glutamate dehydrogenase mRNA levels, was observed in fruitbodies. The TbGS mRNA was also found to be expressed in symbiosis-engaged hyphae, with distinctively higher hybridization signals in hyphae that were penetrating among and within root cells.

Diastereoisomerism, contact points, and chiral selectivity: a four-site saga

In biology, chiral recognition usually implies the ability of a protein, such as an enzyme or a drug receptor, to distinguish between the two enantiomeric forms of a chiral substrate or drug. Both diastereoisomerism and specific contacts between enzyme/receptor and substrate/drug are necessary. The minimum requirement is for four contact points including four nonplanar atoms (or groups of atoms) in both probe and target. The molecular models described by Easson and Stedman and by Ogston require three binding sites in a plane. A modified model with three binding sites in three dimensions is described. Under certain circumstances this model allows binding of both enantiomeric forms of a substrate or a drug. Enantiomer superposition of two enantiomers at an active site occurs in some specific cases (e.g., phenylalanine ammonia-lyase, isocitrate dehydrogenase) and is likely in others. The nature of enantiomer binding to racemase enzymes is discussed.

Multicopy crystallographic refinement of a relaxed glutamine synthetase from Mycobacterium tuberculosis highlights flexible loops in the enzymatic mechanism and its regulation

The crystal structure of glutamine synthetase (GS) from Mycobacterium tuberculosis determined at 2.4 A resolution reveals citrate and AMP bound in the active site. The structure was refined with strict 24-fold noncrystallographic symmetry (NCS) constraints and has an R-factor of 22.7% and an R-free of 25.5%. Multicopy refinement using 10 atomic models and strict 24-fold NCS constraints further reduced the R-factor to 20.4% and the R-free to 23.2%. The multicopy model demonstrates the range of atomic displacements of catalytic and regulatory loops in glutamine synthesis, simulating loop motions. A comparison with loop positions in substrate complexes of GS from Salmonella typhimurium shows that the Asp50 and Glu327 loops close over the active site during catalysis. These loop closures are preceded by a conformational change of the Glu209 beta-strand upon metal ion or ATP binding that converts the enzyme from a relaxed to a taut state. We propose a model of the GS regulatory mechanism based on the loop motions in which adenylylation of the Tyr397 loop reverses the effect of metal ion binding, and regulates intermediate formation by preventing closure of the Glu327 loop.

Mutations in Bacillus subtilis glutamine synthetase that block its interaction with transcription factor TnrA

In Bacillus subtilis, the activity of the nitrogen regulatory factor TnrA is regulated through a protein- protein interaction with glutamine synthetase. During growth with excess nitrogen, the feedback-inhibited form of glutamine synthetase binds to TnrA and blocks DNA binding by TnrA. Missense mutations in glutamine synthetase that constitutively express the TnrA-regulated amtB gene were characterized. Four mutant proteins were purified and shown to be defective in their ability to inhibit the in vitro DNA-binding activity of TnrA. Two of the mutant proteins exhibited enzymatic properties similar to those of wild-type glutamine synthetase. A model of B. subtilis glutamine synthetase was derived from a crystal structure of the Salmonella typhimurium enzyme. Using this model, all the mutated amino acid residues were found to be located close to the glutamate entrance of the active site. These results are consistent with the glutamine synthetase protein playing a direct role in regulating TnrA activity.

Transformation of isopropylamine to L-alaninol by Pseudomonas sp. strain KIE171 involves N-glutamylated intermediates

Pseudomonas sp. strain KIE171 was able to grow with isopropylamine or L-alaninol [S-(+)-2-amino-1-propanol] as the sole carbon source, but not with D-alaninol. To investigate the hypothesis that L-alaninol is an intermediate in the degradation of isopropylamine, two mini-Tn5 mutants unable to utilize both isopropylamine and L-alaninol were isolated. Whereas mutant KIE171-BI transformed isopropylamine to L-alaninol, mutant KIE171-BII failed to do so. The two genes containing a transposon insertion were cloned, and the DNA regions flanking the insertions were sequenced. Two clusters, one comprising eight ipu (isopropylamine utilization) genes (ipuABCDEFGH) and the other encompassing two genes (ipuI and orf259), were identified. Comparisons of sequences of the deduced Ipu proteins and those in the database suggested that isopropylamine is transported into the cytoplasm by a putative permease, IpuG. The next step, the formation of gamma-glutamyl-isopropylamide from isopropylamine, ATP, and L-glutamate, was shown to be catalyzed by IpuC, a gamma-glutamylamide synthetase. gamma-Glutamyl-isopropylamide is then subjected to stereospecific monooxygenation by the hypothetical four-component system IpuABDE, thereby yielding gamma-glutamyl-L-alaninol [gamma(L-glutamyl)-L-hydroxy-isopropylamide]. Enzymatic hydrolysis by a hydrolase, IpuF, was shown to finally liberate L-alaninol and to regenerate L-glutamate. No gene(s) encoding an enzyme for the next step in the degradation of isopropylamine was found in the ipu clusters. Presumably, L-alaninol is oxidized by an alcohol dehydrogenase to yield L-2-aminopropionaldehyde or it is deaminated by an ammonia lyase to propionaldehyde. Genetic evidence indicated that the aldehyde formed is then further oxidized by the hypothetical aldehyde dehydrogenases IpuI and IpuH to either L-alanine or propionic acid, compounds which can be processed by reactions of the intermediary metabolism.

Substrate binding determinants of Trypanosoma brucei gamma-glutamylcysteine synthetase

gamma-Glutamylcysteine synthetase (gamma-GCS) catalyzes the ATP-dependent ligation of L-Glu and L-Cys, which is the first step in de novo biosynthesis of the tripeptide glutathione. Recently it was demonstrated that gamma-GCS is a structural homologue of glutamine synthetase (GS), providing the basis to build a model for the gamma-GCS active site [Abbott et al. (2001) J. Biol. Chem. 276, 42099-42107]. Substrate binding determinants in the active site of gamma-GCS have been identified and characterized in the enzyme from the parasitic protozoa Trypanosoma brucei using this model as a guide for site-directed mutagenesis. R366 and R491 were identified as key determinants of L-Glu binding. Mutation of R366 to Ala increases the K(d) for L-Glu by 160-fold and eliminates the positive cooperativity observed for the binding of L-Glu and ATP to the wild-type enzyme, based on a rapid equilibrium random mechanism of substrate binding. Unlike the wild-type enzyme, the R366A mutant enzyme was able to form product using the substrate analogue gamma-aminobutyric acid, suggesting that R366 interacts with the alpha-carboxylate of L-Glu. Mutation of R491 to Ala decreased k(cat) for ATP hydrolysis by 70-fold; however, dipeptide product was only formed in 5% of these turnovers. These data suggest that R491 stabilizes the phosphorylated gamma-carboxylate of L-Glu during nucleophilic attack by the L-Cys to form the dipeptide product. T323, R474, and R487 were predicted to be ATP binding determinants. Mutation of each of these residues to Ala increased the apparent K(m) for ATP by 20-100-fold while having only modest effects on k(cat) or the apparent K(m)'s for the other substrates. Finally, mutation of R179, a conserved residue that is present in gamma-GCS, but not in GS, increased the apparent K(m) for both L-Cys and L-Glu.

Escherichia coli gamma-glutamylcysteine synthetase. Two active site metal ions affect substrate and inhibitor binding

Gamma-glutamylcysteine synthetase (gamma-GCS, glutamate-cysteine ligase), which catalyzes the first and rate-limiting step in glutathione biosynthesis, is present in many prokaryotes and in virtually all eukaryotes. Although all eukaryotic gamma-GCS isoforms examined to date are rapidly inhibited by buthionine sulfoximine (BSO), most reports indicate that bacterial gamma-GCS is resistant to BSO. We have confirmed the latter finding with Escherichia coli gamma-GCS under standard assay conditions, showing both decreased initial binding affinity for BSO and a reduced rate of BSO-mediated inactivation compared with mammalian isoforms. We also find that substitution of Mn2+ for Mg2+ in assay mixtures increases both the initial binding affinity of BSO and the rate at which BSO causes mechanism-based inactivation. Similarly, the specificity of E. coli gamma-GCS for its amino acid substrates is broadened in the presence of Mn2+, and the rate of reaction for some very poor substrates is improved. These results suggest that divalent metal ions have a role in amino acid binding to E. coli gamma-GCS. Electron paramagnetic resonance (EPR) studies carried out with Mn2+ show that E. coli gamma-GCS binds two divalent metal ions; Kd values for Mn2+ are 1.1 microm and 82 microm, respectively. Binding of l-glutamate or l-BSO to the two Mn2+/gamma-GCS species produces additional upfield and downfield X-band EPR hyperfine lines at 45 G intervals, a result indicating that the two Mn2+ are spin-coupled and thus apparently separated by 5 A or less in the active site. Additional EPR studies in which Cu2+ replaced Mg2+ or Mn2+ suggest that Cu2+ is bound by one N and three O ligands in the gamma-GCS active site. The results are discussed in the context of the catalytic mechanism of gamma-GCS and its relationship to the more fully characterized glutamine synthetase reaction.

Plasma glutamine and ammonia concentrations in ornithine carbamoyltransferase deficiency and citrullinaemia

A retrospective review of the plasma ammonia and glutamine concentrations during the long-term management of 7 patients with ornithine carbamoyltransferase (OCT) deficiency and three with citrullinaemia is presented. The relationship between the plasma concentrations of ammonia and glutamine is not a simple linear one and there are significant differences between the two disorders. Patients with citrullinaemia tend to have higher plasma ammonia concentrations for a given plasma glutamine concentration compared to those with OCT deficiency. The possible reasons for these differences and the implications for management are discussed.

Bacillus subtilis glutamine synthetase controls gene expression through a protein-protein interaction with transcription factor TnrA

Bacillus subtilis TnrA, a global regulator of transcription, responds to nitrogen availability, but the specific signal to which it responds has been elusive. Genetic studies indicate that glutamine synthetase is required for the regulation of TnrA activity in vivo. We report here that the feedback-inhibited form of glutamine synthetase directly interacts with TnrA and blocks the DNA binding activity of TnrA. Mutations in the tnrA gene (tnrA(C)) that allow constitutive high level expression of tnrA-activated genes were isolated and characterized. Feedback-inhibited glutamine synthetase had a significantly reduced ability to block the in vitro DNA binding by three of the TnrA(C) proteins. Thus, glutamine synthetase, an enzyme of central metabolism, directly interacts with and regulates the DNA binding activity of TnrA.

Structure prediction and active site analysis of the metal binding determinants in gamma -glutamylcysteine synthetase

gamma-Glultamylcysteine synthetase (gamma-GCS) catalyzes the first step in the de novo biosynthesis of glutathione. In trypanosomes, glutathione is conjugated to spermidine to form a unique cofactor termed trypanothione, an essential cofactor for the maintenance of redox balance in the cell. Using extensive similarity searches and sequence motif analysis we detected homology between gamma-GCS and glutamine synthetase (GS), allowing these proteins to be unified into a superfamily of carboxylate-amine/ammonia ligases. The structure of gamma-GCS, which was previously poorly understood, was modeled using the known structure of GS. Two metal-binding sites, each ligated by three conserved active site residues (n1: Glu-55, Glu-93, Glu-100; and n2: Glu-53, Gln-321, and Glu-489), are predicted to form the catalytic center of the active site, where the n1 site is expected to bind free metal and the n2 site to interact with MgATP. To elucidate the roles of the metals and their ligands in catalysis, these six residues were mutated to alanine in the Trypanosoma brucei enzyme. All mutations caused a substantial loss of activity. Most notably, E93A was able to catalyze the l-Glu-dependent ATP hydrolysis but not the peptide bond ligation, suggesting that the n1 metal plays an important role in positioning l-Glu for the reaction chemistry. The apparent K(m) values for ATP were increased for both the E489A and Q321A mutant enzymes, consistent with a role for the n2 metal in ATP binding and phosphoryl transfer. Furthermore, the apparent K(d) values for activation of E489A and Q321A by free Mg(2+) increased. Finally, substitution of Mn(2+) for Mg(2+) in the reaction rescued the catalytic deficits caused by both mutations, demonstrating that the nature of the metal ligands plays an important role in metal specificity.

Biochemical characterization of a phosphinate inhibitor of Escherichia coli MurC

The bacterial UDP-N-acetylmuramyl-L-alanine ligase (MurC) from Escherichia coli, an essential, cytoplasmic peptidoglycan biosynthetic enzyme, catalyzes the ATP-dependent ligation of L-alanine (Ala) and UDP-N-acetylmuramic acid (UNAM) to form UDP-N-acetylmuramyl-L-alanine (UNAM-Ala). The phosphinate inhibitor 1 was designed and prepared as a multisubstrate/transition state analogue. The compound exhibits mixed-type inhibition with respect to all three enzyme substrates (ATP, UNAM, Ala), suggesting that this compound forms dead-end complexes with multiple enzyme states. Results from isothermal titration calorimetry (ITC) studies supported these findings as exothermic binding was observed under conditions with free enzyme (K(d) = 1.80-2.79 microM, 95% CI), enzyme saturated with ATP (K(d) = 0.097-0.108 microM, 95% CI), and enzyme saturated with the reaction product ADP (K(d) = 0.371-0.751 microM, 95% CI). Titrations run under conditions of saturating UNAM or the product UNAM-Ala did not show heat effects consistent with competitive compound binding to the active site. The potent binding affinity observed in the presence of ATP is consistent with the inhibitor design and the proposed Ordered Ter-Ter mechanism for this enzyme; however, the additional binding pathways suggest that the inhibitor can also serve as a product analogue.

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.

Characterization of proteins in latex of the opium poppy (Papaver somniferum) using two-dimensional gel electrophoresis and microsequencing

The opium poppy (Papaver somniferum) belongs to the group of latex-containing plants. Latex is the milky-like fluid within laticifer cells. In this study, poppy latex was analyzed with respect to ultrastructure, alkaloid, and protein content. The main goal of this project was the examination of the proteins by two-dimensional gel electrophoresis. In a proteomics approach, we investigated two main fractions of the latex, namely the cytosolic serum and the sedimented fraction containing the alkaloid-accumulating vesicles. Of the serum, representing the protein-rich part of the latex, 75 spots were analyzed by internal peptide microsequencing, followed by a database searching. For 69 proteins a function could be assigned due to homology to known proteins, whereas six spots could not be identified. Furthermore, codeinone reductase, a representative of the specific enzyme system in morphine biosynthesis, could be detected within the cytosolic serum fraction. In the vesicle-containing pellet, 23 protein spots were analyzed. An attempt was also made to separate the vesicle pellet by density centrifugation, followed by investigation of the alkaloid content, ultrastructure, and protein pattern. This study describes the first database of soluble proteins present in the latex of P. somniferum