Glutamine synthetase (GS) (EC 126.96.36.199) 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
There seem to be three different classes of GS:   
- Class I enzymes (GSI) are specific to prokaryotes, and are oligomers of 12 identical subunits. The Entrez Global Query Cross-Database Search System is a powerful Federated search engine or Web portal that allows users to search many discrete Health sciences The Mendelian Inheritance in Man project is a Database that catalogues all the known Diseases with a genetic component, and—when possible—links them The National Center for Biotechnology Information ( NCBI) is part of the United States National Library of Medicine (NLM a branch of the National Institutes Swiss-Prot is a manually curated Biological database of Protein sequences This article is about the Enzyme Commission codes For the European Commission system for coding chemicals see EC-No. In the fields of Genetics and Evolutionary computation, a locus (plural loci) is a fixed position on a Chromosome such as the position of a Chromosome 1 is the designation for the largest Human Chromosome. This article is about the Enzyme Commission codes For the European Commission system for coding chemicals see EC-No. Enzymes are Biomolecules that catalyze ( ie increase the rates of Chemical reactions Almost all enzymes are Proteins Metabolism is the set of Chemical reactions that occur in living Organisms in order to maintain Life. Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Glutamic acid (abbreviated as Glu or E) is one of the 20 Alpha Amino acids It is not among the human Essential amino acids Its Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor Glutamine (abbreviated as Gln or Q; the abbreviation Glx or Z represents either glutamate or Glutamic acid) is one of the 20 Adenosine-5'-triphosphate ( ATP) is a multifunctional Nucleotide that is most important as a " molecular currency" of intracellular Energy Adenosine diphosphate, abbreviated ADP, is a Nucleotide. It is an Ester of Pyrophosphoric acid with the Nucleoside Adenosine The prokaryotes (proʊˈkærioʊts singular prokaryote /proʊˈkæriət/ are a group of Organisms that lack a Cell nucleus (= karyon or any other In Structural biology, a protein subunit or subunit protein is a single Protein Molecule that assembles (or " coassembles "  The activity of GSI-type enzyme is controlled by the adenylation of a tyrosine residue. Tyrosine (abbreviated as Tyr or Y) or 4-hydroxyphenylalanine, is one of the 20 Amino acids that are used by cells to synthesize The adenylated enzyme is inactive .
- 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). Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex The Rhizobiaceae are a family of Proteobacteria, including many (but not all species of Rhizobia as well as plant parasites like Agrobacterium This article refers to the bacteria Frankia was also one of the names of the Frankish Empire. Streptomycetaceae is a family of Actinobacteria, including the important genus Streptomyces, which was the original source of most Antibiotics GSII are decamer of identical subunits . Plants have two or more isozymes of GSII, one of the isozymes is translocated into the chloroplast. Chloroplasts are Organelles found in Plant cells and eukaryotic Algae that conduct Photosynthesis.
- Class III enzymes (GSIII) has, currently, only been found in Bacteroides fragilis and in Butyrivibrio fibrisolvens. Bacteroides is a Genus of Gram-negative, bacillis bacteria. Bacteroides species are non- Endospore -forming anaerobes It is a hexamer of identical chains. 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.
- ^ Some evolutionary relationships of the primary biological catalysts glutamine synthetase and RuBisCO. 1987; PubMed: 2900091
- ^ Evolution of the glutamine synthetase gene, one of the oldest existing and functioning genes. PubMed is a free search engine for accessing the MEDLINE database of citations and abstracts of biomedical research articles 1993 Apr 1; PubMed: 8096645 Free text
- ^ Glutamine synthetase II in Rhizobium: reexamination of the proposed horizontal transfer of DNA from eukaryotes to prokaryotes. PubMed is a free search engine for accessing the MEDLINE database of citations and abstracts of biomedical research articles 1989 Nov; PubMed: 2575672
- ^ Evolutionary relationships of bacterial and archaeal glutamine synthetase genes. PubMed is a free search engine for accessing the MEDLINE database of citations and abstracts of biomedical research articles 1994 Jun; PubMed: 7916055
- ^ GSI picture
- ^ InterPro:IPR001637 Glutamine synthetase class-I, adenylation site
- ^ Crystal structure of human glutamine synthetase in complex with ADP and phosphate http://www.rcsb.org/pdb/explore.do?structureId=2OJW
External links PubMed is a free search engine for accessing the MEDLINE database of citations and abstracts of biomedical research articles
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