Glutamine synthetase: Difference between revisions

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'''Glutamine synthetase''' (GS) ({{EC number|6.3.1.2}})<ref name="pmid2900091">{{cite journal |vauthors=Eisenberg D, Almassy RJ, Janson CA, Chapman MS, Suh SW, Cascio D, Smith WW | title = Some evolutionary relationships of the primary biological catalysts glutamine synthetase and RuBisCO | journal = Cold Spring Harb. Symp. Quant. Biol. | volume = 52 | issue = | pages = 483–90 | year = 1987 | pmid = 2900091 | doi = 10.1101/sqb.1987.052.01.055| url = | issn = }}</ref> is an [[enzyme]] that plays an essential role in the [[metabolism]] of [[nitrogen]] by catalyzing the condensation of [[glutamate]] and [[ammonia]] to form [[glutamine]]:
'''Glutamine synthetase''' ('''GS''') ({{EC number|6.3.1.2}})<ref name="pmid2900091">{{cite journal |vauthors=Eisenberg D, Almassy RJ, Janson CA, Chapman MS, Suh SW, Cascio D, Smith WW | title = Some evolutionary relationships of the primary biological catalysts glutamine synthetase and RuBisCO | journal = Cold Spring Harb. Symp. Quant. Biol. | volume = 52 | issue = | pages = 483–90 | year = 1987 | pmid = 2900091 | doi = 10.1101/sqb.1987.052.01.055| url = | issn = }}</ref> is an [[enzyme]] that plays an essential role in the [[metabolism]] of [[nitrogen]] by catalyzing the condensation of [[glutamate]] and [[ammonia]] to form [[glutamine]]:


Glutamate + [[Adenosine triphosphate|ATP]] + NH<sub>3</sub> → Glutamine + [[Adenosine diphosphate|ADP]] + phosphate  
Glutamate + [[Adenosine triphosphate|ATP]] + NH<sub>3</sub> → Glutamine + [[Adenosine diphosphate|ADP]] + phosphate  
[[File:GS reacction.PNG|center|alt=Glutamine Synthetase reaction. |Glutamine Synthetase Catalyzed Reaction.]]
[[File:Glutamine_synthetase_reaction.svg|center|alt=Glutamine Synthetase reaction. |Glutamine Synthetase Catalyzed Reaction.]]


Glutamine Synthetase uses ammonia produced by nitrate reduction, [[amino acid]] degradation, and [[photorespiration]].<ref name="pmid8563633">{{cite journal |vauthors=Liaw SH, Kuo I, Eisenberg D | title = Discovery of the ammonium substrate site on glutamine synthetase, a third cation binding site | journal = Protein Sci. | volume = 4 | issue = 11 | pages = 2358–65 |date=Nov 1995 | pmid = 8563633 | pmc = 2143006 | doi = 10.1002/pro.5560041114| url = | issn = }}</ref> The amide group of glutamate is a nitrogen source for the synthesis of glutamine pathway [[metabolites]].<ref name="pmid8099447">{{cite journal |vauthors=Liaw SH, Pan C, Eisenberg D | title = Feedback inhibition of fully unadenylylated glutamine synthetase from Salmonella typhimurium by glycine, alanine, and serine | journal = Proc. Natl. Acad. Sci. USA | volume = 90 | issue = 11 | pages = 4996–5000 |date=Jun 1993 | pmid = 8099447 | pmc = 46640 | doi = 10.1073/pnas.90.11.4996| url = | issn = }}</ref>
Glutamine Synthetase uses ammonia produced by nitrate reduction, [[amino acid]] degradation, and [[photorespiration]].<ref name="pmid8563633">{{cite journal |vauthors=Liaw SH, Kuo I, Eisenberg D | title = Discovery of the ammonium substrate site on glutamine synthetase, a third cation binding site | journal = Protein Sci. | volume = 4 | issue = 11 | pages = 2358–65 |date=Nov 1995 | pmid = 8563633 | pmc = 2143006 | doi = 10.1002/pro.5560041114| url = | issn = }}</ref> The amide group of glutamate is a nitrogen source for the synthesis of glutamine pathway [[metabolites]].<ref name="pmid8099447">{{cite journal |vauthors=Liaw SH, Pan C, Eisenberg D | title = Feedback inhibition of fully unadenylylated glutamine synthetase from Salmonella typhimurium by glycine, alanine, and serine | journal = Proc. Natl. Acad. Sci. USA | volume = 90 | issue = 11 | pages = 4996–5000 |date=Jun 1993 | pmid = 8099447 | pmc = 46640 | doi = 10.1073/pnas.90.11.4996| url = | issn = }}</ref>
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== Structure ==
== Structure ==


[[File:Gs1.png|thumb|left | alt=GS Dodecamer |Glutamine Synthetase – 12 subunits.<ref name="pmid11329256"/>]]
[[File:Gs1.png|thumb|left | alt=GS Dodecamer |Glutamine synthetase – 12 subunits.<ref name="pmid11329256"/>]]


Glutamine Synthetase can be composed of 8, 10, or 12 identical subunits separated into two face-to-face rings.<ref name="pmid10708854"/><ref name="Berg_2007">{{cite book |vauthors=Stryer L, Berg JM, Tymoczko JL | title = Biochemistry | edition = 6th | language = | publisher = W.H. Freeman | location = San Francisco | year = 2007 | origyear = | pages = 679–706 | quote = | isbn = 0-7167-8724-5 | oclc = | doi = | url = | accessdate = }}</ref><ref name="url_PDB_MOFT_GS">{{cite web | url = http://www.rcsb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb30_1.html | title = Glutamine Synthetase | author = Goodsell DS | date = June 2002 | work = Molecule of the month | publisher = RCSB Protein Data Bank | accessdate = 2010-05-08 }}</ref><ref name="pmid18005987">{{cite journal |vauthors=Krajewski WW, Collins R, Holmberg-Schiavone L, Jones TA, Karlberg T, Mowbray SL | title = Crystal structures of mammalian glutamine synthetases illustrate substrate-induced conformational changes and provide opportunities for drug and herbicide design | journal = J Mol Biol | volume = 375 | issue = 1 | pages = 317–28 |date=Jan 2008 | pmid = 18005987 | pmc =  | doi = 10.1016/j.jmb.2007.10.029| url = | issn = }}</ref> Bacterial GS are dodecamers with 12 active sites between each [[monomer]].<ref name="pmid10708854"/> Each active site creates a ‘tunnel’ which is the site of three distinct substrate binding sites: [[nucleotide]], ammonium ion, and amino acid.<ref name="pmid8563633"/><ref name="pmid10708854"/><ref name="pmid18005987"/><ref name="pmid4906326">{{cite journal |vauthors=Ginsburg A, Yeh J, Hennig SB, Denton MD | title = Some effects of adenylylation on the biosynthetic properties of the glutamine synthetase from Escherichia coli | journal = Biochemistry | volume = 9 | issue = 3 | pages = 633–49 |date=Feb 1970 | pmid = 4906326 | pmc =  | doi = 10.1021/bi00805a025| url = | issn = }}</ref> ATP binds to the top of the bifunnel that opens to the external surface of GS.<ref name="pmid8563633"/> Glutamate binds at the bottom of the active site.<ref name="pmid7904828"/>  The middle of the bifunnel contains two sites in which divalent [[cations]] bind (Mn+2 or Mg+2). One cation binding site is involved in phosphoryl transfer of ATP to glutamate, while the second stabilizes active GS and helps with the binding of glutamate.<ref name="pmid10708854"/>  
Glutamine Synthetase can be composed of 8, 10, or 12 identical subunits separated into two face-to-face rings.<ref name="pmid10708854"/><ref name="Berg_2007">{{cite book |vauthors=Stryer L, Berg JM, Tymoczko JL | title = Biochemistry | edition = 6th | language = | publisher = W.H. Freeman | location = San Francisco | year = 2007 | origyear = | pages = 679–706 | quote = | isbn = 0-7167-8724-5 | oclc = | doi = | url = | accessdate = }}</ref><ref name="url_PDB_MOFT_GS">{{cite web | url = http://www.rcsb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb30_1.html | title = Glutamine Synthetase | author = Goodsell DS | date = June 2002 | work = Molecule of the month | publisher = RCSB Protein Data Bank | accessdate = 2010-05-08 }}</ref><ref name="pmid18005987">{{cite journal |vauthors=Krajewski WW, Collins R, Holmberg-Schiavone L, Jones TA, Karlberg T, Mowbray SL | title = Crystal structures of mammalian glutamine synthetases illustrate substrate-induced conformational changes and provide opportunities for drug and herbicide design | journal = J Mol Biol | volume = 375 | issue = 1 | pages = 317–28 |date=Jan 2008 | pmid = 18005987 | pmc =  | doi = 10.1016/j.jmb.2007.10.029| url = | issn = }}</ref> Bacterial GS are dodecamers with 12 active sites between each [[monomer]].<ref name="pmid10708854"/> Each active site creates a ‘tunnel’ which is the site of three distinct substrate binding sites: [[nucleotide]], ammonium ion, and amino acid.<ref name="pmid8563633"/><ref name="pmid10708854"/><ref name="pmid18005987"/><ref name="pmid4906326">{{cite journal |vauthors=Ginsburg A, Yeh J, Hennig SB, Denton MD | title = Some effects of adenylylation on the biosynthetic properties of the glutamine synthetase from Escherichia coli | journal = Biochemistry | volume = 9 | issue = 3 | pages = 633–49 |date=Feb 1970 | pmid = 4906326 | pmc =  | doi = 10.1021/bi00805a025| url = | issn = }}</ref> ATP binds to the top of the bifunnel that opens to the external surface of GS.<ref name="pmid8563633"/> Glutamate binds at the bottom of the active site.<ref name="pmid7904828"/>  The middle of the bifunnel contains two sites in which divalent [[cations]] bind (Mn+2 or Mg+2). One cation binding site is involved in phosphoryl transfer of ATP to glutamate, while the second stabilizes active GS and helps with the binding of glutamate.<ref name="pmid10708854"/>  
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ATP binds first to the top of the active site near a cation binding site, while glutamate binds near the second cation binding site at the bottom of the active site.<ref name="pmid8099447"/><ref name="pmid7904828"/> The presence of ADP causes a conformational shift in GS that stabilizes the γ-glutamyl phosphate moiety.  Ammonium binds strongly to GS only if the acyl-phosphate intermediate is present. Ammonium, rather than ammonia, binds to GS because the binding site is polar and exposed to solvent.<ref name="pmid7904828"/>  In the second step, deprotonation of ammonium allows ammonia to attack the intermediate from its nearby site to form glutamine.<ref name="pmid235885">{{cite journal |vauthors=Hunt JB, Smyrniotis PZ, Ginsburg A, Stadtman ER | title = Metal ion requirement by glutamine synthetase of Escherichia coli in catalysis of gamma-glutamyl transfer | journal = Arch Biochem Biophys | volume = 166 | issue = 1 | pages = 102–24 |date=Jan 1975 | pmid = 235885 | pmc = | doi = 10.1016/0003-9861(75)90370-7 | url = | issn = }}</ref> Phosphate leaves through the top of the active site, while glutamine leave through the bottom (between two rings).{{cite web |url= http://www.rcsb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb30_1.html |title= Glutamine Synthetase |author= Goodsell, DS |date= June 2002 |work= |publisher= RCSB Protein Data Bank |accessdate=8 May 2010}}<ref name="pmid7904828"/>
ATP binds first to the top of the active site near a cation binding site, while glutamate binds near the second cation binding site at the bottom of the active site.<ref name="pmid8099447"/><ref name="pmid7904828"/> The presence of ADP causes a conformational shift in GS that stabilizes the γ-glutamyl phosphate moiety.  Ammonium binds strongly to GS only if the acyl-phosphate intermediate is present. Ammonium, rather than ammonia, binds to GS because the binding site is polar and exposed to solvent.<ref name="pmid7904828"/>  In the second step, deprotonation of ammonium allows ammonia to attack the intermediate from its nearby site to form glutamine.<ref name="pmid235885">{{cite journal |vauthors=Hunt JB, Smyrniotis PZ, Ginsburg A, Stadtman ER | title = Metal ion requirement by glutamine synthetase of Escherichia coli in catalysis of gamma-glutamyl transfer | journal = Arch Biochem Biophys | volume = 166 | issue = 1 | pages = 102–24 |date=Jan 1975 | pmid = 235885 | pmc = | doi = 10.1016/0003-9861(75)90370-7 | url = | issn = }}</ref> Phosphate leaves through the top of the active site, while glutamine leave through the bottom (between two rings).{{cite web |url= http://www.rcsb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb30_1.html |title= Glutamine Synthetase |author= Goodsell, DS |date= June 2002 |work= |publisher= RCSB Protein Data Bank |accessdate=8 May 2010}}<ref name="pmid7904828"/>


[[File:30-GlutamineSynthetase-1fpy-twoviews.tif|thumb|right|Two views of Glutamine Synthetase PDB ID: 1FPY]]
[[File:30-GlutamineSynthetase-1fpy-twoviews.tif|thumb|right|Two views of glutamine synthetase PDB ID: 1FPY]]


== Biological Function ==
== Biological function ==


GS is present predominantly in the brain, kidneys, and liver.<ref name="pmid8563633"/><ref name="pmid18005987"/> GS in the brain participates in the metabolic regulation of glutamate, the detoxification of brain ammonia, the assimilation of ammonia, recyclization of [[neurotransmitters]], and termination of neurotransmitter signals.<ref name="pmid8563633"/><ref name="pmid12020613">{{cite journal |vauthors=Suarez I, Bodega G, Fernandez B | title = Glutamine synthetase in brain: effect of ammonia | journal = Neurochem Int. | volume = 41 | issue = 2–3 | pages = 123–42 |date=Aug–Sep 2002 | pmid = 12020613 | pmc = | doi = 10.1016/S0197-0186(02)00033-5| url = | issn = }}</ref>  GS, in the brain, is found primarily in [[astrocytes]].<ref>[http://www.neurologyindia.com/article.asp?issn=0028-3886;year=2013;volume=61;issue=4;spage=383;epage=388;aulast=Venkatesh Venkatesh K, Srikanth L, Vengamma B, Chandrasekhar C, Sanjeevkumar A, Mouleshwara Prasad BC, Sarma PV. In vitro differentiation of cultured human CD34+ cells into astrocytes. Neurol India 2013;61:383-8]</ref> Astrocytes protect neurons against excitotoxicity by taking up excess ammonia and glutamate.<ref name="pmid12020613"/> In hyperammonemic environments (high levels of ammonia), astroglial swelling occurs.<ref name="pmid12020613"/><ref name="pmid9053810">{{cite journal  |vauthors=Willard-Mack CL, Koehler RC, Hirata T, etal |title=Inhibition of glutamine synthetase reduces ammonia-induced astrocyte swelling in rat |journal=Neuroscience |volume=71 |issue=2 |pages=589–99 |date=March 1996 |pmid=9053810 |doi=10.1016/0306-4522(95)00462-9 |url=}}</ref><ref name="pmid15708485">{{cite journal |vauthors=Tanigami H, Rebel A, Martin LJ, Chen TY, Brusilow SW, Traystman RJ, Koehler RC | title = Effect of glutamine synthetase inhibition on astrocyte swelling and altered astroglial protein expression during hyperammonemia in rats | journal = Neuroscience | volume = 131 | issue = 2 | pages = 437–49 | year = 2005 | month = | pmid = 15708485 | pmc = 1819407 | doi = 10.1016/j.neuroscience.2004.10.045| url = | issn = }}</ref> Different perspectives have approached the problem of astroglial swelling. One study shows that morphological changes occur that increase GS expression in glutamatergic areas or other adaptations that alleviates high levels of glutamate and ammonia.<ref name="pmid12020613"/> Another perspective is that astrocyte swelling is due to glutamine accumulation. To prevent increased levels of cortical glutamate and cortical water content, a study has been conducted to prevent GS activity in rats by the use of MSO.<ref name="pmid9053810"/>
GS is present predominantly in the brain, kidneys, and liver.<ref name="pmid8563633"/><ref name="pmid18005987"/> GS in the brain participates in the metabolic regulation of glutamate, the detoxification of brain ammonia, the assimilation of ammonia, recyclization of [[neurotransmitters]], and termination of neurotransmitter signals.<ref name="pmid8563633"/><ref name="pmid12020613">{{cite journal |vauthors=Suarez I, Bodega G, Fernandez B | title = Glutamine synthetase in brain: effect of ammonia | journal = Neurochem. Int. | volume = 41 | issue = 2–3 | pages = 123–42 |date=Aug–Sep 2002 | pmid = 12020613 | pmc = | doi = 10.1016/S0197-0186(02)00033-5| url = | issn = }}</ref>  GS, in the brain, is found primarily in [[astrocytes]].<ref>[http://www.neurologyindia.com/article.asp?issn=0028-3886;year=2013;volume=61;issue=4;spage=383;epage=388;aulast=Venkatesh Venkatesh K, Srikanth L, Vengamma B, Chandrasekhar C, Sanjeevkumar A, Mouleshwara Prasad BC, Sarma PV. In vitro differentiation of cultured human CD34+ cells into astrocytes. Neurol India 2013;61:383-8]</ref> Astrocytes protect neurons against excitotoxicity by taking up excess ammonia and glutamate.<ref name="pmid12020613"/> In hyperammonemic environments (high levels of ammonia), astroglial swelling occurs.<ref name="pmid12020613"/><ref name="pmid9053810">{{cite journal  |vauthors=Willard-Mack CL, Koehler RC, Hirata T, etal |title=Inhibition of glutamine synthetase reduces ammonia-induced astrocyte swelling in rat |journal=Neuroscience |volume=71 |issue=2 |pages=589–99 |date=March 1996 |pmid=9053810 |doi=10.1016/0306-4522(95)00462-9 |url=}}</ref><ref name="pmid15708485">{{cite journal |vauthors=Tanigami H, Rebel A, Martin LJ, Chen TY, Brusilow SW, Traystman RJ, Koehler RC | title = Effect of glutamine synthetase inhibition on astrocyte swelling and altered astroglial protein expression during hyperammonemia in rats | journal = Neuroscience | volume = 131 | issue = 2 | pages = 437–49 | year = 2005 | month = | pmid = 15708485 | pmc = 1819407 | doi = 10.1016/j.neuroscience.2004.10.045| url = | issn = }}</ref> Different perspectives have approached the problem of astroglial swelling. One study shows that morphological changes occur that increase GS expression in glutamatergic areas or other adaptations that alleviates high levels of glutamate and ammonia.<ref name="pmid12020613"/> Another perspective is that astrocyte swelling is due to glutamine accumulation. To prevent increased levels of cortical glutamate and cortical water content, a study has been conducted to prevent GS activity in rats by the use of MSO.<ref name="pmid9053810"/>


== Classes ==
== Classes ==
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* Class II enzymes (GSII) are found in [[eukaryotes]] and in bacteria belonging to the [[Rhizobiaceae]], [[Frankiaceae]], and [[Streptomycetaceae]] families (these bacteria have also a class-I GS). GSII are [[decamer]] of identical subunits.<ref name="pmid18005987"/>{{PDB|2OJW}}.
* Class II enzymes (GSII) are found in [[eukaryotes]] and in bacteria belonging to the [[Rhizobiaceae]], [[Frankiaceae]], and [[Streptomycetaceae]] families (these bacteria have also a class-I GS). GSII are [[decamer]] of identical subunits.<ref name="pmid18005987"/>{{PDB|2OJW}}.


Plants have two or more isozymes of GSII, one of the isozymes is translocated into the [[chloroplast]]. Another form is [[cytosol]]ic. The cytosolic GS gene translation is regulated by its [[Five prime untranslated region|5' untranslated region]] (UTR), while its [[Three prime untranslated region|3' UTR]] plays role in transcript turnover.<ref>{{Cite journal|last=Ortega|first=Jose Luis|last2=Wilson|first2=Olivia L.|last3=Sengupta-Gopalan|first3=Champa|date=2012-12-01|title=The 5' untranslated region of the soybean cytosolic glutamine synthetase β(1) gene contains prokaryotic translation initiation signals and acts as a translational enhancer in plants|journal=Molecular genetics and genomics: MGG|volume=287|issue=11–12|pages=881–893|doi=10.1007/s00438-012-0724-6|issn=1617-4623|pmc=3881598|pmid=23080263}}</ref>
Plants have two or more isozymes of GSII, one of the isozymes is translocated into the [[chloroplast]]. Another form is [[cytosol]]ic. The cytosolic GS gene translation is regulated by its [[Five prime untranslated region|5' untranslated region]] (UTR), while its [[Three prime untranslated region|3' UTR]] plays role in transcript turnover.<ref>{{cite journal | vauthors = Ortega JL, Wilson OL, Sengupta-Gopalan C | title = The 5' untranslated region of the soybean cytosolic glutamine synthetase β(1) gene contains prokaryotic translation initiation signals and acts as a translational enhancer in plants | journal = Molecular Genetics and Genomics | volume = 287 | issue = 11–12 | pages = 881–93 | date = December 2012 | pmid = 23080263 | pmc = 3881598 | doi = 10.1007/s00438-012-0724-6 }}</ref>


* Class III enzymes (GSIII) have, currently, only been found in ''[[Bacteroides fragilis]]'' and in ''[[Butyrivibrio fibrisolvens]]''. It is a double-ringed dodecamer of identical chains.<ref name="pmid16879836">{{cite journal |vauthors=van Rooyen JM, Abratt VR, Sewell BT | title = Three-dimensional structure of a type III glutamine synthetase by single-particle reconstruction
* Class III enzymes (GSIII) have, currently, only been found in ''[[Bacteroides fragilis]]'' and in ''[[Butyrivibrio fibrisolvens]]''. It is a double-ringed dodecamer of identical chains.<ref name="pmid16879836">{{cite journal |vauthors=van Rooyen JM, Abratt VR, Sewell BT | title = Three-dimensional structure of a type III glutamine synthetase by single-particle reconstruction
  | journal = J. Mol. Biol. | volume = 361 | issue = 4 | pages = 796–810 |date=August 2006 | pmid = 16879836 | doi=10.1016/j.jmb.2006.06.026  | url = | issn = }}</ref> It is much larger (about 700 amino acids) than the GSI (450 to 470 amino acids) or GSII (350 to 420 amino acids) enzymes.
  | journal = J. Mol. Biol. | volume = 361 | issue = 4 | pages = 796–810 |date=August 2006 | pmid = 16879836 | doi=10.1016/j.jmb.2006.06.026  | url = | issn = }}</ref> It is much larger (about 700 amino acids) than the GSI (450 to 470 amino acids) or GSII (350 to 420 amino acids) enzymes.


While the three classes of GS's are clearly structurally related, the sequence similarities are not so extensive.
While the three classes of GSs are clearly structurally related, the sequence similarities are not so extensive.


== Regulation & Inhibition ==
== Regulation and inhibition ==
Regulation of GS only occurs in prokaryotes.<ref name=":0" /> GS is subject to reversible covalent modification. Tyr<sup>397</sup> of all 12 subunits can undergo [[adenylylation]] or deadenylylation by adenylyl transferase (AT), a bifunctional regulatory enzyme.<ref name=":0">{{Cite book|title=Biochemistry (6th Edition)|last=Garrett, Grisham|first=|publisher=Cengage Learning|year=2017|isbn=978-1-305-57720-6|location=United States of America|pages=886–889}}</ref> Adenylylation is a [[post-translational modification]] <nowiki/>involving the covalent attachment of [[Adenosine monophosphate|AMP]] to a protein side chain. Each adenylylation requires an [[Adenosine triphosphate|ATP]] and complete inhibition of GS requires 12 ATP. Deadenylylation by AT involves phosphorolytic removal of the Tyr-linked adenylyl groups as [[Adenosine diphosphate|ADP]]. AT activity is influenced by the regulatory protein that is associated with it: P<sub>II</sub>, a 44-kD [[Protein trimer|trimer]].<ref name=":0" /> P<sub>II</sub> also undergoes post-translational modification by [[(protein-PII) uridylyltransferase|uridylyl transferase]], thus P<sub>II</sub> has two forms. The state of P<sub>II</sub> dictates the activity of adenylyl transferase. If P<sub>II</sub> is not uridylylated, then it will take on the P<sub>IIA</sub> form. The AT:P<sub>IIA</sub> complex will deactivate GS by adenylylation. If P<sub>II</sub> is uridylylated, then it will take on the P<sub>IID</sub> form. The AT:P<sub>IID</sub> complex will activate GS by deadenylylation.<ref name=":0" /> The AT:P<sub>IIA</sub> and AT:P<sub>IID</sub> complexes are [[Allosteric regulation|allosterically regulated]] in a reciprocal fashion by [[Alpha-Ketoglutaric acid|α-ketoglutarate]] (α-KG) and [[glutamine]] (Gln). Gln will activate AT:P<sub>IIA</sub> activity and inhibits AT:P<sub>IID</sub>, leading to adenylylation and subsequent deactivation of GS. Furthermore, Gln favors the conversion of P<sub>IID</sub> to P<sub>IIA</sub>. The effects of α-KG on the complexes are opposite.<ref name=":0" /> In the majority of gram-negative bacteria, GS can be modified by adenylylation (some cyanobacteria and green algae or exceptions).<ref>{{cite journal |vauthors=Ivanovsky RN, Khatipov EA | title = Evidence of covalent modification of glutamine synthetase in the purple sulfur bacterium | journal = FEMS Microbiology Letters | doi=10.1111/j.1574-6968.1994.tb07153.x | year = 1994 | volume = 122 |issue=1–2| pages = 115–119 }}</ref>
Regulation of GS only occurs in prokaryotes.<ref name=":0" /> GS is subject to reversible covalent modification. Tyr<sup>397</sup> of all 12 subunits can undergo [[adenylylation]] or deadenylylation by adenylyl transferase (AT), a bifunctional regulatory enzyme.<ref name=":0">{{Cite book|title=Biochemistry (6th Edition)|last=Garrett, Grisham|first=|publisher=Cengage Learning|year=2017|isbn=978-1-305-57720-6|location=United States of America|pages=886–889}}</ref> Adenylylation is a [[post-translational modification]] <nowiki/>involving the covalent attachment of [[Adenosine monophosphate|AMP]] to a protein side chain. Each adenylylation requires an [[Adenosine triphosphate|ATP]] and complete inhibition of GS requires 12 ATP. Deadenylylation by AT involves phosphorolytic removal of the Tyr-linked adenylyl groups as [[Adenosine diphosphate|ADP]]. AT activity is influenced by the regulatory protein that is associated with it: P<sub>II</sub>, a 44-kD [[Protein trimer|trimer]].<ref name=":0" /> P<sub>II</sub> also undergoes post-translational modification by [[(protein-PII) uridylyltransferase|uridylyl transferase]], thus P<sub>II</sub> has two forms. The state of P<sub>II</sub> dictates the activity of adenylyl transferase. If P<sub>II</sub> is not uridylylated, then it will take on the P<sub>IIA</sub> form. The AT:P<sub>IIA</sub> complex will deactivate GS by adenylylation. If P<sub>II</sub> is uridylylated, then it will take on the P<sub>IID</sub> form. The AT:P<sub>IID</sub> complex will activate GS by deadenylylation.<ref name=":0" /> The AT:P<sub>IIA</sub> and AT:P<sub>IID</sub> complexes are [[Allosteric regulation|allosterically regulated]] in a reciprocal fashion by [[Alpha-Ketoglutaric acid|α-ketoglutarate]] (α-KG) and [[glutamine]] (Gln). Gln will activate AT:P<sub>IIA</sub> activity and inhibits AT:P<sub>IID</sub>, leading to adenylylation and subsequent deactivation of GS. Furthermore, Gln favors the conversion of P<sub>IID</sub> to P<sub>IIA</sub>. The effects of α-KG on the complexes are opposite.<ref name=":0" /> In the majority of gram-negative bacteria, GS can be modified by adenylylation (some cyanobacteria and green algae or exceptions).<ref>{{cite journal |vauthors=Ivanovsky RN, Khatipov EA | title = Evidence of covalent modification of glutamine synthetase in the purple sulfur bacterium | journal = FEMS Microbiology Letters | doi=10.1111/j.1574-6968.1994.tb07153.x | year = 1994 | volume = 122 |issue=1–2| pages = 115–119 }}</ref>
[[File:Regulation of glutamine synthetase through covalent modification.jpg|alt=|left|thumb|Glutamine Synthetase activity influenced by its regulatory protein, which is designated P<sub>II</sub>.]]
[[File:Regulation of glutamine synthetase through covalent modification.jpg|alt=|left|thumb|Glutamine Synthetase activity influenced by its regulatory protein, which is designated P<sub>II</sub>.]]
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Inhibitors:
Inhibitors:
* Methionine Sulfoximine (MSO): MSO is an inhibitor that binds to the glutamate site. Bound to GS, MSO is phosphorylated by ATP  that results in an irreversible, non-covalent inhibition of GS. The S-isomer configuration is more inhibitory.<ref name="pmid10708854"/> Glutamate entry is blocked into the active site by a stabilization of the flexible loop in the active site by MSO.<ref name="pmid7904828"/>
* [[Methionine sulfoximine]] (MSO): MSO is an inhibitor that binds to the glutamate site. Bound to GS, MSO is phosphorylated by ATP  that results in an irreversible, non-covalent inhibition of GS. The S-isomer configuration is more inhibitory.<ref name="pmid10708854"/> Glutamate entry is blocked into the active site by a stabilization of the flexible loop in the active site by MSO.<ref name="pmid7904828"/>
* [[Phosphinothricin]]<ref name="pmid11329256"/>(PPT, Glufosinate): Phosphinothricin is an inhibitor that binds to the glutamate site. Glufosinate is used as an herbicide. Glufosinate treated plants die due to a buildup of ammonia and a cessation of photosynthesis.<ref name="pmid18005987"/>
* [[Phosphinothricin]]<ref name="pmid11329256"/>(PPT, Glufosinate): Phosphinothricin is an inhibitor that binds to the glutamate site. Glufosinate is used as an herbicide. Glufosinate treated plants die due to a buildup of ammonia and a cessation of photosynthesis.<ref name="pmid18005987"/>
* Many synthetic inhibitors are available today.<ref name="pmid10708854"/>
* Many synthetic inhibitors are available today.<ref name="pmid10708854"/>
Research on ''[[Escherichia coli|E. coli]]'' revealed that GS is regulated through gene expression. The gene that encodes the GS subunit is designated ''[[GlnA RNA motif|GlnA]]''. Transcription of GlnA is dependent on ''NR<sub>I</sub>'' (a specific [[Enhancer (genetics)|transcriptional enhancer]]). Active transcription occurs if NR<sub>I</sub> is in its phosphorylated form, designated ''NR<sub>I</sub>-P''. Phosphorylation of NR<sub>I</sub> is catalyzed by NR<sub>II</sub>, a protein [[kinase]]. If NR<sub>II</sub> is complexed with P<sub>IIA</sub> then it will function as a [[phosphatase]] and NR<sub>I</sub>-P is converted back to NR<sub>I</sub>. In this case, transcription of GlnA ceases.<ref name=":0" />
Research on ''[[Escherichia coli|E. coli]]'' revealed that GS is regulated through gene expression. The gene that encodes the GS subunit is designated ''glnA''. Transcription of ''glnA'' is dependent on ''NR<sub>I</sub>'' (a specific [[Enhancer (genetics)|transcriptional enhancer]]). Active transcription occurs if NR<sub>I</sub> is in its phosphorylated form, designated ''NR<sub>I</sub>-P''. Phosphorylation of NR<sub>I</sub> is catalyzed by NR<sub>II</sub>, a protein [[kinase]]. If NR<sub>II</sub> is complexed with P<sub>IIA</sub> then it will function as a [[phosphatase]] and NR<sub>I</sub>-P is converted back to NR<sub>I</sub>. In this case, transcription of ''glnA'' ceases.<ref name=":0" />
 
GS is subject to completely different regulatory mechanisms in [[cyanobacteria]]<ref>{{cite journal |last1=Bolay |first1=Paul |last2=Muro-Pastor |first2=M. |last3=Florencio |first3=Francisco |last4=Klähn |first4=Stephan |title=The Distinctive Regulation of Cyanobacterial Glutamine Synthetase |journal=Life |date=27 October 2018 |volume=8 |issue=4 |pages=52 |doi=10.3390/life8040052}}</ref>. Instead of  the common NtrC-NtrB two component system<ref>{{cite journal | vauthors = Merrick MJ, Edwards RA | title = Nitrogen control in bacteria | journal = Microbiological Reviews | volume = 59 | issue = 4 | pages = 604–22 | date = December 1995 | pmid = 8531888 }}</ref><ref>{{cite journal | vauthors = Fisher R, Tuli R, Haselkorn R | title = A cloned cyanobacterial gene for glutamine synthetase functions in Escherichia coli, but the enzyme is not adenylylated | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 78 | issue = 6 | pages = 3393–7 | date = June 1981 | pmid = 6115380 }}</ref>, cyanobacteria harbour the transcriptional regulator NtcA which is restricted to this clade and controls expression of GS and a multitude of genes involved in [[Nitrogen|Nitrogen]] metabolism <ref>{{cite journal | vauthors = Vega-Palas MA, Flores E, Herrero A | title = NtcA, a global nitrogen regulator from the cyanobacterium Synechococcus that belongs to the Crp family of bacterial regulators | journal = Molecular Microbiology | volume = 6 | issue = 13 | pages = 1853–9 | date = July 1992 | pmid = 1630321 }}</ref><ref>{{cite journal | vauthors = Reyes JC, Muro-Pastor MI, Florencio FJ | title = Transcription of glutamine synthetase genes (glnA and glnN) from the cyanobacterium Synechocystis sp. strain PCC 6803 is differently regulated in response to nitrogen availability | journal = Journal of Bacteriology | volume = 179 | issue = 8 | pages = 2678–89 | date = April 1997 | pmid = 9098067 }}</ref>. Moreover, GS in [[Cyanobacteria|Cyanobacteria]] is not covalently modified to raise sensitivity for feedback inhibition<ref>{{cite journal | vauthors = Fisher R, Tuli R, Haselkorn R | title = A cloned cyanobacterial gene for glutamine synthetase functions in Escherichia coli, but the enzyme is not adenylylated | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 78 | issue = 6 | pages = 3393–7 | date = June 1981 | pmid = 6115380 }}</ref>. Instead, GS in [[Cyanobacteria|Cyanobacteria]] is inhibited by small proteins, termed GS inactivating factors (IFs) whose transcription is negatively regulated by NtcA. <ref>{{cite journal | vauthors = García-Domínguez M, Reyes JC, Florencio FJ | title = Glutamine synthetase inactivation by protein-protein interaction | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 96 | issue = 13 | pages = 7161–6 | date = June 1999 | pmid = 10377385 }}</ref><ref>{{cite journal | vauthors = García-Domínguez M, Reyes JC, Florencio FJ | title = NtcA represses transcription of gifA and gifB, genes that encode inhibitors of glutamine synthetase type I from Synechocystis sp. PCC 6803 | journal = Molecular Microbiology | volume = 35 | issue = 5 | pages = 1192–201 | date = March 2000 | pmid = 10712699 }}</ref> These inactivating factors are furthermore regulated by different [[Non-coding RNAs|Non-coding RNAs]]: The [[Small RNA|sRNA]] [[NsiR4 small RNA|NsiR4]] interacts with the [[Five prime untranslated region|5'UTR]] of the mRNA of the GS inactivating factor IF7 (<i>gifA</i> mRNA) and reduces its expression. [[NsiR4 small RNA|NsiR4]] expression is under positive control of the nitrogen control transcription factor NtcA<ref>{{cite journal | vauthors = Klähn S, Schaal C, Georg J, Baumgartner D, Knippen G, Hagemann M, Muro-Pastor AM, Hess WR | title = The sRNA NsiR4 is involved in nitrogen assimilation control in cyanobacteria by targeting glutamine synthetase inactivating factor IF7 | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 112 | issue = 45 | pages = E6243-52 | date = November 2015 | pmid = 26494284 | doi = 10.1073/pnas.1508412112 }}</ref>. In addition, expression of the GS inactivating factor IF17 is controlled by a [[GlnA RNA motif|glutamine-binding riboswitch]].<ref>{{cite journal | vauthors = Klähn S, Bolay P, Wright PR, Atilho RM, Brewer KI, Hagemann M, Breaker RR, Hess WR | title = A glutamine riboswitch is a key element for the regulation of glutamine synthetase in cyanobacteria | journal = Nucleic Acids Research | volume =  | issue =  | date = August 2018 | pmid = 30085248 | doi = 10.1093/nar/gky709 }}</ref>


== References ==
== References ==

Latest revision as of 12:21, 13 November 2018

glutamate—ammonia ligase
File:MN MN ADP PPQ.png
Active site between two monomers of glutamine synthetase from Salmonella typhimurium. Cation binding sites are yellow and orange; ADP is pink; phosphinothricin is blue.[1]
Identifiers
EC number6.3.1.2
CAS number9023-70-5
Databases
IntEnzIntEnz view
BRENDABRENDA entry
ExPASyNiceZyme view
KEGGKEGG entry
MetaCycmetabolic pathway
PRIAMprofile
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / QuickGO
Glutamine synthetase,
beta-Grasp domain
Identifiers
SymbolGln-synt_N
PfamPF03951
InterProIPR008147
PROSITEPDOC00162
SCOP2gls
SUPERFAMILY2gls
Glutamine synthetase,
catalytic domain
File:PDB 2gls EBI.jpg
12-subunit enzyme glutamine synthetase from Salmonella typhimurium.[2]
Identifiers
SymbolGln-synt_C
PfamPF00120
Pfam clanCL0286
InterProIPR008146
PROSITEPDOC00162
SCOP2gls
SUPERFAMILY2gls
glutamate-ammonia ligase (glutamine synthetase)
Identifiers
SymbolGLUL
Alt. symbolsGLNS
Entrez2752
HUGO4341
OMIM138290
PDB2qc8
RefSeqNM_002065
UniProtP15104
Other data
EC number6.3.1.2
LocusChr. 1 q31

Glutamine synthetase (GS) (EC 6.3.1.2)[3] is an enzyme that plays an essential role in the metabolism of nitrogen by catalyzing the condensation of glutamate and ammonia to form glutamine:

Glutamate + ATP + NH3 → Glutamine + ADP + phosphate

Glutamine Synthetase reaction.
Glutamine Synthetase Catalyzed Reaction.

Glutamine Synthetase uses ammonia produced by nitrate reduction, amino acid degradation, and photorespiration.[4] The amide group of glutamate is a nitrogen source for the synthesis of glutamine pathway metabolites.[5]

Other reactions may take place via GS. Competition between ammonium ion and water, their binding affinities, and the concentration of ammonium ion, influences glutamine synthesis and glutamine hydrolysis. Glutamine is formed if an ammonium ion attacks the acyl-phosphate intermediate, while glutamate is remade if water attacks the intermediate.[6][7] Ammonium ion binds more strongly than water to GS due to electrostatic forces between a cation and a negatively charged pocket.[4] Another possible reaction is upon NH2OH binding to GS, rather than NH4+, yields γ-glutamylhydroxamate.[6][7]

Structure

GS Dodecamer
Glutamine synthetase – 12 subunits.[1]

Glutamine Synthetase can be composed of 8, 10, or 12 identical subunits separated into two face-to-face rings.[6][8][9][10] Bacterial GS are dodecamers with 12 active sites between each monomer.[6] Each active site creates a ‘tunnel’ which is the site of three distinct substrate binding sites: nucleotide, ammonium ion, and amino acid.[4][6][10][11] ATP binds to the top of the bifunnel that opens to the external surface of GS.[4] Glutamate binds at the bottom of the active site.[7] The middle of the bifunnel contains two sites in which divalent cations bind (Mn+2 or Mg+2). One cation binding site is involved in phosphoryl transfer of ATP to glutamate, while the second stabilizes active GS and helps with the binding of glutamate.[6]

Hydrogen bonding and hydrophobic interactions hold the two rings of GS together. Each subunit possesses a C-terminus and an N-terminus in its sequence. The C-terminus (helical thong) stabilizes the GS structure by inserting into the hydrophobic region of the subunit across in the other ring. The N-terminus is exposed to the solvent. In addition, the central channel is formed via six four-stranded β-sheets composed of anti-parallel loops from the twelve subunits.[6]

Mechanism

GS catalyzes the ATP-dependent condensation of glutamate with ammonia to yield glutamine.[4] The hydrolysis of ATP drives[8] the first step of a two-part, concerted mechanism.[4][6] ATP phosphorylates glutamate to form ADP and an acyl-phosphate intermediate, γ-glutamyl phosphate, which reacts with ammonia, forming glutamine and inorganic phosphate. ADP and Pi do not dissociate until ammonia binds and glutamine is released.[6]

ATP binds first to the top of the active site near a cation binding site, while glutamate binds near the second cation binding site at the bottom of the active site.[5][7] The presence of ADP causes a conformational shift in GS that stabilizes the γ-glutamyl phosphate moiety. Ammonium binds strongly to GS only if the acyl-phosphate intermediate is present. Ammonium, rather than ammonia, binds to GS because the binding site is polar and exposed to solvent.[7] In the second step, deprotonation of ammonium allows ammonia to attack the intermediate from its nearby site to form glutamine.[12] Phosphate leaves through the top of the active site, while glutamine leave through the bottom (between two rings).Goodsell, DS (June 2002). "Glutamine Synthetase". RCSB Protein Data Bank. Retrieved 8 May 2010.[7]

File:30-GlutamineSynthetase-1fpy-twoviews.tif
Two views of glutamine synthetase PDB ID: 1FPY

Biological function

GS is present predominantly in the brain, kidneys, and liver.[4][10] GS in the brain participates in the metabolic regulation of glutamate, the detoxification of brain ammonia, the assimilation of ammonia, recyclization of neurotransmitters, and termination of neurotransmitter signals.[4][13] GS, in the brain, is found primarily in astrocytes.[14] Astrocytes protect neurons against excitotoxicity by taking up excess ammonia and glutamate.[13] In hyperammonemic environments (high levels of ammonia), astroglial swelling occurs.[13][15][16] Different perspectives have approached the problem of astroglial swelling. One study shows that morphological changes occur that increase GS expression in glutamatergic areas or other adaptations that alleviates high levels of glutamate and ammonia.[13] Another perspective is that astrocyte swelling is due to glutamine accumulation. To prevent increased levels of cortical glutamate and cortical water content, a study has been conducted to prevent GS activity in rats by the use of MSO.[15]

Classes

There seem to be three different classes of GS:[17][18][19]

Plants have two or more isozymes of GSII, one of the isozymes is translocated into the chloroplast. Another form is cytosolic. The cytosolic GS gene translation is regulated by its 5' untranslated region (UTR), while its 3' UTR plays role in transcript turnover.[22]

  • Class III enzymes (GSIII) have, currently, only been found in Bacteroides fragilis and in Butyrivibrio fibrisolvens. It is a double-ringed dodecamer of identical chains.[23] It is much larger (about 700 amino acids) than the GSI (450 to 470 amino acids) or GSII (350 to 420 amino acids) enzymes.

While the three classes of GSs are clearly structurally related, the sequence similarities are not so extensive.

Regulation and inhibition

Regulation of GS only occurs in prokaryotes.[24] GS is subject to reversible covalent modification. Tyr397 of all 12 subunits can undergo adenylylation or deadenylylation by adenylyl transferase (AT), a bifunctional regulatory enzyme.[24] Adenylylation is a post-translational modification involving the covalent attachment of AMP to a protein side chain. Each adenylylation requires an ATP and complete inhibition of GS requires 12 ATP. Deadenylylation by AT involves phosphorolytic removal of the Tyr-linked adenylyl groups as ADP. AT activity is influenced by the regulatory protein that is associated with it: PII, a 44-kD trimer.[24] PII also undergoes post-translational modification by uridylyl transferase, thus PII has two forms. The state of PII dictates the activity of adenylyl transferase. If PII is not uridylylated, then it will take on the PIIA form. The AT:PIIA complex will deactivate GS by adenylylation. If PII is uridylylated, then it will take on the PIID form. The AT:PIID complex will activate GS by deadenylylation.[24] The AT:PIIA and AT:PIID complexes are allosterically regulated in a reciprocal fashion by α-ketoglutarate (α-KG) and glutamine (Gln). Gln will activate AT:PIIA activity and inhibits AT:PIID, leading to adenylylation and subsequent deactivation of GS. Furthermore, Gln favors the conversion of PIID to PIIA. The effects of α-KG on the complexes are opposite.[24] In the majority of gram-negative bacteria, GS can be modified by adenylylation (some cyanobacteria and green algae or exceptions).[25]

File:Regulation of glutamine synthetase through covalent modification.jpg
Glutamine Synthetase activity influenced by its regulatory protein, which is designated PII.

Inhibition of GS has largely focused on amino site ligands.[6] Other inhibitors are the result of glutamine metabolism: tryptophan, histidine, carbamoyl phosphate, glucosamine-6-phosphate, cytidine triphosphate (CTP), and adenosine monophosphate (AMP).[5][8][26] Other inhibitors/regulators are glycine and alanine. Alanine, glycine, and serine bind to the glutamate substrate site. GDP, AMP, ADP bind to the ATP site.[6] L-serine, L-alanine, and glycine bind to the site for L-glutamate in unadenylated GS. The four amino acids bind to the site by their common atoms, “the main chain” of amino acids.[5] Glutamate is another product of glutamine metabolism; however, glutamate is a substrate for GS inhibiting it to act as a regulator to GS.2 Each inhibitor can reduce the activity of the enzyme; once all final glutamine metabolites are bound to GS, the activity of GS is almost completely inhibited.[8] Many inhibitory input signals allows for fine tuning of GS by reflecting nitrogen levels in the organism.

Feedback regulation distinguishes the difference between two eukaryotic types of GS: brain and non-brain tissues. Non-brain GS responds to end-product feedback inhibition, while brain GS does not.[6] High concentrations of glutamine-dependent metabolites should inhibit GS activity, while low concentrations should activate GS activity.[6]

MSO.
Methionine Sulfoximine acting as an inhibitor to the glutamate binding site.

Inhibitors:

  • Methionine sulfoximine (MSO): MSO is an inhibitor that binds to the glutamate site. Bound to GS, MSO is phosphorylated by ATP that results in an irreversible, non-covalent inhibition of GS. The S-isomer configuration is more inhibitory.[6] Glutamate entry is blocked into the active site by a stabilization of the flexible loop in the active site by MSO.[7]
  • Phosphinothricin[1](PPT, Glufosinate): Phosphinothricin is an inhibitor that binds to the glutamate site. Glufosinate is used as an herbicide. Glufosinate treated plants die due to a buildup of ammonia and a cessation of photosynthesis.[10]
  • Many synthetic inhibitors are available today.[6]

Research on E. coli revealed that GS is regulated through gene expression. The gene that encodes the GS subunit is designated glnA. Transcription of glnA is dependent on NRI (a specific transcriptional enhancer). Active transcription occurs if NRI is in its phosphorylated form, designated NRI-P. Phosphorylation of NRI is catalyzed by NRII, a protein kinase. If NRII is complexed with PIIA then it will function as a phosphatase and NRI-P is converted back to NRI. In this case, transcription of glnA ceases.[24]

GS is subject to completely different regulatory mechanisms in cyanobacteria[27]. Instead of the common NtrC-NtrB two component system[28][29], cyanobacteria harbour the transcriptional regulator NtcA which is restricted to this clade and controls expression of GS and a multitude of genes involved in Nitrogen metabolism [30][31]. Moreover, GS in Cyanobacteria is not covalently modified to raise sensitivity for feedback inhibition[32]. Instead, GS in Cyanobacteria is inhibited by small proteins, termed GS inactivating factors (IFs) whose transcription is negatively regulated by NtcA. [33][34] These inactivating factors are furthermore regulated by different Non-coding RNAs: The sRNA NsiR4 interacts with the 5'UTR of the mRNA of the GS inactivating factor IF7 (gifA mRNA) and reduces its expression. NsiR4 expression is under positive control of the nitrogen control transcription factor NtcA[35]. In addition, expression of the GS inactivating factor IF17 is controlled by a glutamine-binding riboswitch.[36]

References

  1. ↑ 1.0 1.1 1.2 PDB: 1FPY​; Gill HS, Eisenberg D (February 2001). "The crystal structure of phosphinothricin in the active site of glutamine synthetase illuminates the mechanism of enzymatic inhibition". Biochemistry. 40 (7): 1903–12. doi:10.1021/bi002438h. PMID 11329256.
  2. ↑ PDB: 2GLS​; Yamashita MM, Almassy RJ, Janson CA, Cascio D, Eisenberg D (October 1989). "Refined atomic model of glutamine synthetase at 3.5 A resolution". J. Biol. Chem. 264 (30): 17681–90. PMID 2572586.
  3. ↑ Eisenberg D, Almassy RJ, Janson CA, Chapman MS, Suh SW, Cascio D, Smith WW (1987). "Some evolutionary relationships of the primary biological catalysts glutamine synthetase and RuBisCO". Cold Spring Harb. Symp. Quant. Biol. 52: 483–90. doi:10.1101/sqb.1987.052.01.055. PMID 2900091.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Liaw SH, Kuo I, Eisenberg D (Nov 1995). "Discovery of the ammonium substrate site on glutamine synthetase, a third cation binding site". Protein Sci. 4 (11): 2358–65. doi:10.1002/pro.5560041114. PMC 2143006. PMID 8563633.
  5. ↑ 5.0 5.1 5.2 5.3 Liaw SH, Pan C, Eisenberg D (Jun 1993). "Feedback inhibition of fully unadenylylated glutamine synthetase from Salmonella typhimurium by glycine, alanine, and serine". Proc. Natl. Acad. Sci. USA. 90 (11): 4996–5000. doi:10.1073/pnas.90.11.4996. PMC 46640. PMID 8099447.
  6. ↑ 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 6.11 6.12 6.13 6.14 Eisenberg D, Gill HS, Pfluegl GM, Rotstein SH (Mar 2000). "Structure-function relationships of glutamine synthetases". Biochim Biophys Acta. 1477 (1–2): 122–45. doi:10.1016/S0167-4838(99)00270-8. PMID 10708854.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 Liaw SH, Eisenberg D (Jan 1994). "Structural model for the reaction mechanism of glutamine synthetase, based on five crystal structures of enzyme-substrate complexes". Biochemistry. 33 (3): 675–81. doi:10.1021/bi00169a007. PMID 7904828.
  8. ↑ 8.0 8.1 8.2 8.3 Stryer L, Berg JM, Tymoczko JL (2007). Biochemistry (6th ed.). San Francisco: W.H. Freeman. pp. 679–706. ISBN 0-7167-8724-5.
  9. ↑ Goodsell DS (June 2002). "Glutamine Synthetase". Molecule of the month. RCSB Protein Data Bank. Retrieved 2010-05-08.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 Krajewski WW, Collins R, Holmberg-Schiavone L, Jones TA, Karlberg T, Mowbray SL (Jan 2008). "Crystal structures of mammalian glutamine synthetases illustrate substrate-induced conformational changes and provide opportunities for drug and herbicide design". J Mol Biol. 375 (1): 317–28. doi:10.1016/j.jmb.2007.10.029. PMID 18005987.
  11. ↑ Ginsburg A, Yeh J, Hennig SB, Denton MD (Feb 1970). "Some effects of adenylylation on the biosynthetic properties of the glutamine synthetase from Escherichia coli". Biochemistry. 9 (3): 633–49. doi:10.1021/bi00805a025. PMID 4906326.
  12. ↑ Hunt JB, Smyrniotis PZ, Ginsburg A, Stadtman ER (Jan 1975). "Metal ion requirement by glutamine synthetase of Escherichia coli in catalysis of gamma-glutamyl transfer". Arch Biochem Biophys. 166 (1): 102–24. doi:10.1016/0003-9861(75)90370-7. PMID 235885.
  13. ↑ 13.0 13.1 13.2 13.3 Suarez I, Bodega G, Fernandez B (Aug–Sep 2002). "Glutamine synthetase in brain: effect of ammonia". Neurochem. Int. 41 (2–3): 123–42. doi:10.1016/S0197-0186(02)00033-5. PMID 12020613.
  14. ↑ Venkatesh K, Srikanth L, Vengamma B, Chandrasekhar C, Sanjeevkumar A, Mouleshwara Prasad BC, Sarma PV. In vitro differentiation of cultured human CD34+ cells into astrocytes. Neurol India 2013;61:383-8
  15. ↑ 15.0 15.1 Willard-Mack CL, Koehler RC, Hirata T, et al. (March 1996). "Inhibition of glutamine synthetase reduces ammonia-induced astrocyte swelling in rat". Neuroscience. 71 (2): 589–99. doi:10.1016/0306-4522(95)00462-9. PMID 9053810.
  16. ↑ Tanigami H, Rebel A, Martin LJ, Chen TY, Brusilow SW, Traystman RJ, Koehler RC (2005). "Effect of glutamine synthetase inhibition on astrocyte swelling and altered astroglial protein expression during hyperammonemia in rats". Neuroscience. 131 (2): 437–49. doi:10.1016/j.neuroscience.2004.10.045. PMC 1819407. PMID 15708485.
  17. ↑ Kumada Y, Benson DR, Hillemann D, Hosted TJ, Rochefort DA, Thompson CJ, Wohlleben W, Tateno Y (April 1993). "Evolution of the glutamine synthetase gene, one of the oldest existing and functioning genes". Proc. Natl. Acad. Sci. U.S.A. 90 (7): 3009–13. doi:10.1073/pnas.90.7.3009. PMC 46226. PMID 8096645.
  18. ↑ Shatters RG, Kahn ML (November 1989). "Glutamine synthetase II in Rhizobium: reexamination of the proposed horizontal transfer of DNA from eukaryotes to prokaryotes". J. Mol. Evol. 29 (5): 422–8. doi:10.1007/BF02602912. PMID 2575672.
  19. ↑ Brown JR, Masuchi Y, Robb FT, Doolittle WF (June 1994). "Evolutionary relationships of bacterial and archaeal glutamine synthetase genes". J. Mol. Evol. 38 (6): 566–76. doi:10.1007/BF00175876. PMID 7916055.
  20. ↑ "GSI structure". Archived from the original on 2008-12-17. Retrieved 2009-03-31.
  21. ↑ InterPro:IPR001637 Glutamine synthetase class-I, adenylation site
  22. ↑ Ortega JL, Wilson OL, Sengupta-Gopalan C (December 2012). "The 5' untranslated region of the soybean cytosolic glutamine synthetase β(1) gene contains prokaryotic translation initiation signals and acts as a translational enhancer in plants". Molecular Genetics and Genomics. 287 (11–12): 881–93. doi:10.1007/s00438-012-0724-6. PMC 3881598. PMID 23080263.
  23. ↑ van Rooyen JM, Abratt VR, Sewell BT (August 2006). "Three-dimensional structure of a type III glutamine synthetase by single-particle reconstruction". J. Mol. Biol. 361 (4): 796–810. doi:10.1016/j.jmb.2006.06.026. PMID 16879836.
  24. ↑ 24.0 24.1 24.2 24.3 24.4 24.5 Garrett, Grisham (2017). Biochemistry (6th Edition). United States of America: Cengage Learning. pp. 886–889. ISBN 978-1-305-57720-6.
  25. ↑ Ivanovsky RN, Khatipov EA (1994). "Evidence of covalent modification of glutamine synthetase in the purple sulfur bacterium". FEMS Microbiology Letters. 122 (1–2): 115–119. doi:10.1111/j.1574-6968.1994.tb07153.x.
  26. ↑ Krishnan IS, Singhal RK, Dua RD (Apr 1986). "Purification and characterization of glutamine synthetase from Clostridium pasteurianum". Biochemistry. 25 (7): 1589–99. doi:10.1021/bi00355a021. PMID 2871863.
  27. ↑ Bolay, Paul; Muro-Pastor, M.; Florencio, Francisco; Klähn, Stephan (27 October 2018). "The Distinctive Regulation of Cyanobacterial Glutamine Synthetase". Life. 8 (4): 52. doi:10.3390/life8040052.
  28. ↑ Merrick MJ, Edwards RA (December 1995). "Nitrogen control in bacteria". Microbiological Reviews. 59 (4): 604–22. PMID 8531888.
  29. ↑ Fisher R, Tuli R, Haselkorn R (June 1981). "A cloned cyanobacterial gene for glutamine synthetase functions in Escherichia coli, but the enzyme is not adenylylated". Proceedings of the National Academy of Sciences of the United States of America. 78 (6): 3393–7. PMID 6115380.
  30. ↑ Vega-Palas MA, Flores E, Herrero A (July 1992). "NtcA, a global nitrogen regulator from the cyanobacterium Synechococcus that belongs to the Crp family of bacterial regulators". Molecular Microbiology. 6 (13): 1853–9. PMID 1630321.
  31. ↑ Reyes JC, Muro-Pastor MI, Florencio FJ (April 1997). "Transcription of glutamine synthetase genes (glnA and glnN) from the cyanobacterium Synechocystis sp. strain PCC 6803 is differently regulated in response to nitrogen availability". Journal of Bacteriology. 179 (8): 2678–89. PMID 9098067.
  32. ↑ Fisher R, Tuli R, Haselkorn R (June 1981). "A cloned cyanobacterial gene for glutamine synthetase functions in Escherichia coli, but the enzyme is not adenylylated". Proceedings of the National Academy of Sciences of the United States of America. 78 (6): 3393–7. PMID 6115380.
  33. ↑ García-Domínguez M, Reyes JC, Florencio FJ (June 1999). "Glutamine synthetase inactivation by protein-protein interaction". Proceedings of the National Academy of Sciences of the United States of America. 96 (13): 7161–6. PMID 10377385.
  34. ↑ García-Domínguez M, Reyes JC, Florencio FJ (March 2000). "NtcA represses transcription of gifA and gifB, genes that encode inhibitors of glutamine synthetase type I from Synechocystis sp. PCC 6803". Molecular Microbiology. 35 (5): 1192–201. PMID 10712699.
  35. ↑ Klähn S, Schaal C, Georg J, Baumgartner D, Knippen G, Hagemann M, Muro-Pastor AM, Hess WR (November 2015). "The sRNA NsiR4 is involved in nitrogen assimilation control in cyanobacteria by targeting glutamine synthetase inactivating factor IF7". Proceedings of the National Academy of Sciences of the United States of America. 112 (45): E6243–52. doi:10.1073/pnas.1508412112. PMID 26494284.
  36. ↑ Klähn S, Bolay P, Wright PR, Atilho RM, Brewer KI, Hagemann M, Breaker RR, Hess WR (August 2018). "A glutamine riboswitch is a key element for the regulation of glutamine synthetase in cyanobacteria". Nucleic Acids Research. doi:10.1093/nar/gky709. PMID 30085248.

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