1
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Characterization and improved properties of Glutamine synthetase from Providencia vermicola by site-directed mutagenesis. Sci Rep 2018; 8:15640. [PMID: 30353099 PMCID: PMC6199252 DOI: 10.1038/s41598-018-34022-5] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2017] [Accepted: 06/19/2018] [Indexed: 11/08/2022] Open
Abstract
In this study, a novel gene for Glutamine synthetase was cloned and characterized for its activities and stabilities from a marine bacterium Providencia vermicola (PveGS). A mutant S54A was generated by site directed mutagenesis, which showed significant increase in the activity and stabilities at a wide range of temperatures. The Km values of PveGS against hydroxylamine, ADP-Na2 and L-Glutamine were 15.7 ± 1.1, (25.2 ± 1.5) × 10-5 and 32.6 ± 1.7 mM, and the kcat were 17.0 ± 0.6, 9.14 ± 0.12 and 30.5 ± 1.0 s-1 respectively. In-silico-analysis revealed that the replacement of Ser at 54th position with Ala increased the catalytic activity of PveGS. Therefore, catalytic efficiency of mutant S54A had increased by 3.1, 0.89 and 2.9-folds towards hydroxylamine, ADP-Na2 and L-Glutamine respectively as compared to wild type. The structure prediction data indicated that the negatively charged pocket becomes enlarged and hydrogen bonding in Ser54 steadily promotes the product release. Interestingly, the residual activity of S54A mutant was increased by 10.7, 3.8 and 3.8 folds at 0, 10 and 50 °C as compared to WT. Structural analysis showed that S54A located on the loop near to the active site improved its flexibility due to the breaking of hydrogen bonds between product and enzyme. This also facilitated the enzyme to increase its cold adaptability as indicated by higher residual activity shown at 0 °C. Thus, replacement of Ala to Ser54 played a pivotal role to enhance the activities and stabilities at a wide range of temperatures.
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2
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Kazmiruk NV, Boronovskiy SE, Nartsissov YR. Modeling the Regulation of the Activity of Glutamine Synthetase from Escherichia coli by Magnesium Ions. Biophysics (Nagoya-shi) 2018. [DOI: 10.1134/s0006350918030090] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
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3
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Wang L, Lai L, Ouyang Q, Tang C. Flux balance analysis of ammonia assimilation network in E. coli predicts preferred regulation point. PLoS One 2011; 6:e16362. [PMID: 21283535 PMCID: PMC3026816 DOI: 10.1371/journal.pone.0016362] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2010] [Accepted: 12/22/2010] [Indexed: 11/18/2022] Open
Abstract
Nitrogen assimilation is a critical biological process for the synthesis of biomolecules in Escherichia coli. The central ammonium assimilation network in E. coli converts carbon skeleton α-ketoglutarate and ammonium into glutamate and glutamine, which further serve as nitrogen donors for nitrogen metabolism in the cell. This reaction network involves three enzymes: glutamate dehydrogenase (GDH), glutamine synthetase (GS) and glutamate synthase (GOGAT). In minimal media, E. coli tries to maintain an optimal growth rate by regulating the activity of the enzymes to match the availability of the external ammonia. The molecular mechanism and the strategy of the regulation in this network have been the research topics for many investigators. In this paper, we develop a flux balance model for the nitrogen metabolism, taking into account of the cellular composition and biosynthetic requirements for nitrogen. The model agrees well with known experimental results. Specifically, it reproduces all the (15)N isotope labeling experiments in the wild type and the two mutant (ΔGDH and ΔGOGAT) strains of E. coli. Furthermore, the predicted catalytic activities of GDH, GS and GOGAT in different ammonium concentrations and growth rates for the wild type, ΔGDH and ΔGOGAT strains agree well with the enzyme concentrations obtained from western blots. Based on this flux balance model, we show that GS is the preferred regulation point among the three enzymes in the nitrogen assimilation network. Our analysis reveals the pattern of regulation in this central and highly regulated network, thus providing insights into the regulation strategy adopted by the bacteria. Our model and methods may also be useful in future investigations in this and other networks.
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Affiliation(s)
- Lu Wang
- School of Physics, Peking University, Beijing, China
- Center for Theoretical Biology, Peking University, Beijing, China
| | - Luhua Lai
- Center for Theoretical Biology, Peking University, Beijing, China
- College of Chemistry and Molecular Engineering, Peking University, Beijing, China
| | - Qi Ouyang
- School of Physics, Peking University, Beijing, China
- Center for Theoretical Biology, Peking University, Beijing, China
- Department of Physics, Hong Kong Baptist University, Hong Kong, China
- * E-mail: (QQ); (CT)
| | - Chao Tang
- Center for Theoretical Biology, Peking University, Beijing, China
- Department of Bioengineering and Therapeutic Sciences, University of California San Francisco, San Francisco, California, United States of America
- * E-mail: (QQ); (CT)
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4
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Pearson JT, Dabrowski MJ, Kung I, Atkins WM. The central loop of Escherichia coli glutamine synthetase is flexible and functionally passive. Arch Biochem Biophys 2005; 436:397-405. [PMID: 15797252 DOI: 10.1016/j.abb.2005.02.008] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2005] [Revised: 02/02/2005] [Indexed: 10/25/2022]
Abstract
Bacterial glutamine synthetases (GSs) are dodecameric aggregates comprised of two face-to-face hexameric rings, which form a cylindrical aqueous channel. Available crystal structures indicate that each subunit provides a 'central loop' that protrudes into this channel. Residues on either side of this loop contribute directly to substrate or metal ion cofactor binding. Although it has been suggested that this conspicuous structural feature may be functionally important, a systematic structure-function analysis of this loop has not been done. Here, we examine the behavior of a cysteine mutant, E165C, which yields inter-subunit disulfide bonds connecting the central loops. The inter-subunit disulfide bonds are readily detected by electrospray ionization mass spectrometry. Based on molecular models, the disulfide bonds would form only if the engineered cysteines on adjacent subunits moved approximately 5 A. Surprisingly, inter-subunit disulfide bonds between the central loops caused no detectable changes in the KMs for glutamate or ATP, nor the KD for either ATP or the transition state analog (L)-methionine sulfoximine (MSOX). Furthermore, covalent and quantitative adduction of the E165C mutant with iodo-acetamido-pyrene yielded nearly fully active enzyme bearing fluorescent pyrene excimers. The relative contribution of pyrene monomers to excimers in the steady state fluorescence is temperature dependent, suggesting thermal equilibrium between loop conformational states. However, the monomer-excimer ratio is independent of ligands such as MSOX, glutamate, or Mn2+. These results validate the suspected flexibility of the central loop, but raise significant doubt about its direct functional role in GS catalysis via conformational switching, including the proposed regulation of GS via ADP-ribosylation within this loop.
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Affiliation(s)
- Josh T Pearson
- Department of Medicinal Chemistry, University of Washington, Box 357610, Seattle, WA 98195-7610, USA
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5
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Mehta R, Pearson JT, Mahajan S, Nath A, Hickey MJ, Sherman DR, Atkins WM. Adenylylation and catalytic properties of Mycobacterium tuberculosis glutamine synthetase expressed in Escherichia coli versus mycobacteria. J Biol Chem 2004; 279:22477-82. [PMID: 15037612 DOI: 10.1074/jbc.m401652200] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
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.
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Affiliation(s)
- Ranjana Mehta
- Departments of Pathobiology and Medicinal Chemistry, University of Washington, Seattle, Washington 98195-7610, USA
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6
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Okeson CD, Riley MR. Glutamine replenishment and ammonia removal in hybridoma cell cultures via immobilized glutamine synthetase. Biochem Eng J 2001. [DOI: 10.1016/s1369-703x(01)00134-6] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
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7
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Eisenberg D, Gill HS, Pfluegl GM, Rotstein SH. Structure-function relationships of glutamine synthetases. BIOCHIMICA ET BIOPHYSICA ACTA 2000; 1477:122-45. [PMID: 10708854 DOI: 10.1016/s0167-4838(99)00270-8] [Citation(s) in RCA: 268] [Impact Index Per Article: 11.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
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.
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Affiliation(s)
- D Eisenberg
- UCLA-DOE Laboratory of Structural Biology and Molecular Medicine, Departments of Chemistry, Biochemistry and Biological Chemistry, University of California, Los Angeles, 201 MBI, Box 951570, Los Angeles, CA 90095-1570, USA.
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8
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Richards NG, Schuster SM. Mechanistic issues in asparagine synthetase catalysis. ADVANCES IN ENZYMOLOGY AND RELATED AREAS OF MOLECULAR BIOLOGY 1998; 72:145-98. [PMID: 9559053 DOI: 10.1002/9780470123188.ch5] [Citation(s) in RCA: 45] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
The enzymatic synthesis of asparagine is an ATP-dependent process that utilizes the nitrogen atom derived from either glutamine or ammonia. Despite a long history of kinetic and mechanistic investigation, there is no universally accepted catalytic mechanism for this seemingly straightforward carboxyl group activating enzyme, especially as regards those steps immediately preceding amide bond formation. This chapter considers four issues dealing with the mechanism: (a) the structural organization of the active site(s) partaking in glutamine utilization and aspartate activation; (b) the relationship of asparagine synthetase to other amidotransferases; (c) the way in which ATP is used to activate the beta-carboxyl group; and (d) the detailed mechanism by which nitrogen is transferred.
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Affiliation(s)
- N G Richards
- Department of Chemistry, University of Florida, Gainesville 32611, USA
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9
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Reynaldo LP, Villafranca JJ, Horrocks WD. Investigating the effects of posttranslational adenylylation on the metal binding sites of Escherichia coli glutamine synthetase using lanthanide luminescence spectroscopy. Protein Sci 1996; 5:2532-44. [PMID: 8976562 PMCID: PMC2143323 DOI: 10.1002/pro.5560051216] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
Lanthanide luminescence was used to examine the effects of posttranslational adenylylation on the metal binding sites of Escherichia coli glutamine synthetase (GS). These studies revealed the presence of two lanthanide ion binding sites of GS of either adenylylation extrema. Individual emission decay lifetimes were obtained in both H2O and D2O solvent systems, allowing for the determination of the number of water molecules coordinated to each bound Eu3+. The results indicate that there are 4.3 +/- 0.5 and 4.6 +/- 0.5 water molecules coordinated to Eu3+ bound to the n1 site of unadenylylated enzyme, GS0, and fully adenylylated enzyme, GS12, respectively, and that there are 2.6 +/- 0.5 water molecules coordinated to Eu3+ at site n2 for both GS0 and GS12. Energy transfer measurements between the lanthanide donor-acceptor pair Eu3+ and Nd3+, obtained an intermetal distance measurement of 12.1 +/- 1.5 A. Distances between a Tb3+ ion at site n2 and tryptophan residues were also performed with the use of single-tryptophan mutant forms of E. coli GS. The dissociation constant for lanthanide ion binding to site n1 was observed to decrease from Kd = 0.35 +/- 0.09 microM for GS0 to Kd = 0.06 +/- 0.02 microM for GS12. The dissociation constant for lanthanide ion binding to site n2 remained unchanged as a function of adenylylation state; Kd = 3.8 +/- 0.9 microM and Kd = 2.6 +/- 0.7 microM for GS0 and GS12, respectively. Competition experiments indicate that Mn2+ affinity at site n1 decreases as a function of increasing adenylylation state, from Kd = 0.05 +/- 0.02 microM for GS0 to Kd = 0.35 +/- 0.09 microM for GS12. Mn2+ affinity at site n2 remains unchanged (Kd = 5.3 +/- 1.3 microM for GS0 and Kd = 4.0 +/- 1.0 microM for GS12). The observed divalent metal ion affinities, which are affected by the adenylylation state, agrees with other steady-state substrate experiments (Abell LM, Villafranca JJ, 1991, Biochemistry 30:1413-1418), supporting the hypothesis that adenylylation regulates GS by altering substrate and metal ion affinities.
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Affiliation(s)
- L P Reynaldo
- Pennsylvania State University, Department of Chemistry, University Park 16802, USA
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10
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Affiliation(s)
- William N. Lipscomb
- Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138
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11
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Sträter N, Lipscomb WN, Klabunde T, Krebs B. Enzymatische Acyl- und Phosphoryltransferreaktionen unter Beteiligung von zwei Metallionen. Angew Chem Int Ed Engl 1996. [DOI: 10.1002/ange.19961081804] [Citation(s) in RCA: 84] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
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12
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Atkins WM. Supramolecular self-assembly of Escherichia coli glutamine synthetase: effects of pressure and adenylylation state on dodecamer stacking. Biochemistry 1994; 33:14965-73. [PMID: 7999752 DOI: 10.1021/bi00254a003] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
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)
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Affiliation(s)
- W M Atkins
- Department of Medicinal Chemistry, School of Pharmacy, University of Washington, Seattle 98195
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Witmer MR, Palmieri-Young D, Villafranca JJ. Probing the catalytic roles of n2-site glutamate residues in Escherichia coli glutamine synthetase by mutagenesis. Protein Sci 1994; 3:1746-59. [PMID: 7849593 PMCID: PMC2142605 DOI: 10.1002/pro.5560031015] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
The contribution of metal ion ligand type and charge to catalysis and regulation at the lower affinity metal ion site (n2 site) of Escherichia coli glutamine synthetase (GS) was tested by mutagenesis and kinetic analysis. The 2 glutamate residues at the n2 site, E129 and E357, were changed to E129D, E129H, E357H, E357Q, and E357D, representing conservative and nonconservative alterations. Unadenylylated and fully adenylylated enzyme forms were studied. The Mn(2+)-KD values, UV-cis and fluorescence emission properties were similar for all mutants versus WTGS, except E129H. For kinetic determinations with both Mn2+ and Mg2+, nonconservative mutants (E357H, E129H, E357Q) showed lower biosynthetic activities than conservative mutants (E129D, E357D). Relative to WTGS, all the unadenylylated Mn(2+)-activated enzymes showed reduced kcat/Km values for ATP (> 7-fold) and for glutamate (> 10-fold). Of the unadenylylated Mg(2+)-activated enzymes, only E129D showed kinetic parameters competitive with WTGS, and adenylylated E129D was a 20-fold better catalyst than WTGS. We propose the n2-site metal ion activates ADP for departure in the phosphorylation of glutamate by ATP to generate gamma-glutamyl phosphate. Alteration of the charge density at this metal ion alters the transition-state energy for phosphoryl group transfer and may affect ATP binding and/or ADP release. Thus, the steady-state kinetic data suggest that modifying the charge density increases the transition-state energies for chemical steps. Importantly, the data demonstrate that each ligand position has a specialized spatial environment and the charge of the ligand modulates the catalytic steps occurring at the metal ion. The data are discussed in the context of the known X-ray structures of GS.
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Affiliation(s)
- M R Witmer
- Department of Chemistry, Pennsylvania State University, University Park 16802
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14
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Dhalla AM, Li B, Alibhai MF, Yost KJ, Hemmingsen JM, Atkins WM, Schineller J, Villafranca JJ. Regeneration of catalytic activity of glutamine synthetase mutants by chemical activation: exploration of the role of arginines 339 and 359 in activity. Protein Sci 1994; 3:476-81. [PMID: 7912599 PMCID: PMC2142696 DOI: 10.1002/pro.5560030313] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
In order to understand the nature of ATP and L-glutamate binding to glutamine synthetase, and the involvement of Arg 339 and Arg 359 in catalysis, these amino acids were changed to cysteine via site-directed mutagenesis. Individual mutations (Arg-->Cys) at positions 339 and 359 led to a sharp drop in catalytic activity. Additionally, the Km values for the substrates ATP and glutamate were elevated substantially above the values for wild-type (WT) enzyme. Each cysteine was in turn chemically modified to an arginine "analog" to attempt to "rescue" catalytic activity by covalent modification; 2-chloroacetamidine (CA) (producing a thioether) and 2,2'-dithiobis (acetamidine)(DTBA) (producing a disulfide) were the reagents used to effect these chemical transformations. Upon reaction with CA, both R339C and R359C mutants showed a significant regain of catalytic activity (50% and 70% of WT, respectively) and a drop in Km value for ATP close to that for WT enzyme. With DTBA, chemically modified R339C had a greater kcat than WT glutamine synthetase, but chemically modified R359C only regained a small amount of activity. Modification with DTBA was quantitative for each mutant and each modified enzyme had similar Km values for both ATP and glutamate. The high catalytic activity of DTBA-modified R339C could be reversed to that of unmodified R339C by treatment with dithiothreitol, as expected for a modified enzyme containing a disulfide bond. Modification of each cysteine-containing mutant to a lysine "analog" was accomplished using 3-bromopropylamine (BPA).(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- A M Dhalla
- Department of Chemistry, Pennsylvania State University, University Park 16802
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15
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Alibhai M, Villafranca JJ. Kinetic and mutagenic studies of the role of the active site residues Asp-50 and Glu-327 of Escherichia coli glutamine synthetase. Biochemistry 1994; 33:682-6. [PMID: 7904829 DOI: 10.1021/bi00169a008] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
The role of Asp-50 and Glu-327 of Escherichia coli glutamine synthetase in catalysis and substrate binding has been interrogated by construction of site-directed mutants at these positions. Steady-state and rapid-quench kinetic methods were used to elucidate contributions of Asp-50 and Glu-327 to the Km values of all three substrates, ATP, glutamate, and NH4+, as well as to the enzymatic kcat value. Kinetic constants were obtained for the D50A enzyme using both Mg2+ and Mn2+ as activating metal ions; the data reveal that Asp-50 has a significant role in both substrate binding and catalysis as reflected by the increases in the Km values for NH4+ and the destabilization of both the ground state and the transition state for phosphoryl transfer. The D50E mutant was found to have activity with Mn2+ but very low activity with Mg2+, the physiologically important metal ion. The kcat/Km values for all three substrates were substantially altered by changing Asp to Glu. The steady-state results for the E327A mutant indicate a decreased kcat/Km value for NH4+ compared to that of the wild-type enzyme. The E327A-Mg2+ enzyme destabilizes the ground state of the ternary complex (E-ATP-Glu-NH4+) and the transition state for phosphoryl transfer while the E327A-Mn2(+)-enzyme provides greater stabilization for the ATP and glutamate complexes but destabilizes phosphoryl transfer steps in the ternary complex. Overall, these results suggest that Asp-50 is likely involved in binding NH4+ and may also play a role in catalyzing deprotonation of NH4+ to form NH3.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- M Alibhai
- Department of Chemistry, Pennsylvania State University, University Park 16802
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16
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Atkins WM, Villafranca JJ. Time-resolved fluorescence studies of tryptophan mutants of Escherichia coli glutamine synthetase: conformational analysis of intermediates and transition-state complexes. Protein Sci 1993; 1:342-55. [PMID: 1363912 PMCID: PMC2142202 DOI: 10.1002/pro.5560010306] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
Single tryptophan-containing mutants of low adenylylation state Escherichia coli glutamine synthetase have been studied by frequency-domain fluorescence spectroscopy in the presence of various substrates and inhibitors. At pH 6.5, the Mn-bound wild-type enzyme (wild type has two tryptophans/subunit) and the mutant enzymes exhibit heterogeneous fluorescence decay kinetics; the individual tryptophans are adequately described by a triple exponential decay scheme. The recovered lifetime values are 5.9 ns, 2.6 ns, and 0.4 ns for Trp-57 and 5.8 ns, 2.3 ns, and 0.4 ns for Trp-158. These values are nearly identical to the previously reported results at pH 7.5 (Atkins, W.M., Stayton, P.S., & Villafranca, J.J., 1991, Biochemistry 30, 3406-3416). In addition, Trp-57 and Trp-158 both exhibit an ATP-induced increase in the relative fraction of the long lifetime component, whereas only Trp-57 is affected by this ligand at pH 7.5. The transition-state analogue L-methionine-(R,S)-sulfoximine (MSOX) causes a dramatic increase in the fractional intensity of the long lifetime component of Trp-158. This ligand has no effect on the W158S mutant protein and causes a small increase in the fractional intensity of the long lifetime component of the W158F mutant protein. Addition of glutamate to the ATP complex, which affords the gamma-glutamylphosphate-ADP complex, results in the presence of new lifetime components at 7, 3.2, and 0.5 ns for Trp-158, but has no effect on Trp-57. Similar results were obtained when ATP was added to the MSOX complex; Trp-57 exhibits heterogeneous fluorescence decay with lifetimes of 7, 3.5, and 0.8 ns. Decay kinetics of Trp-158 are best fit to a nearly homogeneous decay with a lifetime of 5.5 ns in the MSOX-ATP inactivated complex. These results provide a model for the sequence of structural and dynamic changes that take place at the Trp-57 loop and the central loop (Trp-158) during several intermediate stages of catalysis.
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Affiliation(s)
- W M Atkins
- Department of Chemistry, Pennsylvania State University, University Park 16802
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17
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Atkins WM, Cader BM, Hemmingsen J, Villafranca JJ. Time-resolved fluorescence and computational studies of adenylylated glutamine synthetase: analysis of intersubunit interactions. Protein Sci 1993; 2:800-13. [PMID: 8098638 PMCID: PMC2142491 DOI: 10.1002/pro.5560020510] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
Adenylylation of Tyr-397 of each subunit of Escherichia coli glutamine synthetase (GS) down-regulates enzymatic activity in vivo. The overall structure of the enzyme consists of 12 subunits arranged as two hexamers, face to face. Research reported in this paper addresses the question of whether the covalently attached adenylyl group interacts with neighboring amino acid residues to produce the regulatory phenomenon. Wild-type GS has two Trp residues (positions 57 and 158) and the adenylylation site lies within 7-8 A of the Trp-57 loop in the adjacent subunit of the same hexameric ring; Trp-158 is about 35 A from the site of adenylylation. Fluorescence lifetimes and quantum yields have been determined for two fluorophores with wild-type and mutant GS. One fluorophore is epsilon-AMP adenylylated GS (at Tyr-397), and the other fluorophore is the intrinsic protein residue Trp-57. These experiments were conducted in order to detect possible intersubunit interactions between adenylyl groups and the neighboring Trp-57 to search for a role for the Trp-57 loop in the regulation of GS. The fluorescence due to epsilon-AMP of two adenylylated enzymes, wild-type GS and the W158F mutant, exhibits heterogeneous decay kinetics; the data adequately fit to a double exponential decay model with recovered average lifetime values of 18.2 and 2.1 ns, respectively. The pre-exponential factors range from 0.66 to 0.73 for the long lifetime component, at five emission wavelengths. The W57L-epsilon-AMP enzyme yields longer average lifetime values of 19.5 and 2.4 ns, and the pre-exponential factors range from 0.82 to 0.85 for the long lifetime component. An additional residue in the Trp-57 loop, Lys-58, has been altered and the K58C mutant enzyme has been adenylylated with epsilon-AMP on Tyr-397. Lys-58 is near the ATP binding site and may represent a link by which the adenylyl group controls the activity of GS. The fluorescence of epsilon-AMP-adenylylated K58C mutant GS is best described by a triple exponential decay with average recovered lifetime values of 19.9, 4.6, and 0.58 ns, with the largest fraction being the median lifetime component. Relative quantum yields of epsilon-AMP-Tyr-397 were measured in order to determine if static quenching occurs from adenine-indole stacking in the wild-type GS. The relative quantum yield of the epsilon-AMP-adenylylated W57L mutant is larger than the wild-type protein by the amount predicted from the difference in lifetime values: thus, no static quenching is evident.(ABSTRACT TRUNCATED AT 400 WORDS)
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Affiliation(s)
- W M Atkins
- Department of Medicinal Chemistry, University of Washington, Seattle 98195
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Abell LM, Villafranca JJ. Investigation of the mechanism of phosphinothricin inactivation of Escherichia coli glutamine synthetase using rapid quench kinetic technique. Biochemistry 1991; 30:6135-41. [PMID: 1676298 DOI: 10.1021/bi00239a008] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
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.
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Affiliation(s)
- L M Abell
- Department of Chemistry, Pennsylvania State University, University Park 16802
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