Glutamine synthetase/guanido kinase, catalytic domain <p>The C-terminal catalytic domains of glutamine synthetase and the guanido kinase family (which includes creatine kinase and arginine kinase) share a common structural fold, namely a common core consisting of two beta-alpha-beta2-alpha repeats [<cite idref="PUB00034749"/>].</p><p> Glutamine synthetase (<db_xref db="EC" dbkey="6.3.1.2"/>) (GS) [<cite idref="PUB00006495"/>] plays an essential role in the metabolism of nitrogen by catalysing the condensation of glutamate and ammonia to form glutamine. There seem to be three different classes of GS [<cite idref="PUB00004813"/>, <cite idref="PUB00003408"/>, <cite idref="PUB00003430"/>]. Class I enzymes (GSI) are specific to prokaryotes, and are oligomers of 12 identical subunits; the activity of GSI-type enzyme is controlled by the adenylation of a tyrosine residue. Class II enzymes (GSII) are found in eukaryotes and in bacteria, and are oligomers of 8 identical subunits. Class III enzymes (GSIII) have been found in <taxon tax_id="817">Bacteroides fragilis</taxon> in <taxon tax_id="831">Butyrivibrio fibrisolvens</taxon>, and are oligomers of six identical subunits. While the three classes of GS's are clearly structurally related, the sequence similarities are not so extensive.</p><p> ATP:guanido phosphotransferases are a family of structurally and functionally related enzymes [<cite idref="PUB00002616"/>, <cite idref="PUB00000670"/>] that reversibly catalyse the transfer of phosphate between ATP and various phosphogens. The enzymes belonging to this family include:</p><p> <ul><li>Glycocyamine kinase (<db_xref db="EC" dbkey="2.7.3.1"/>), which catalyses the transfer of phosphate from ATP to guanidoacetate.</li><li>Arginine kinase (<db_xref db="EC" dbkey="2.7.3.3"/>), which catalyses the transfer of phosphate from ATP to arginine.</li><li>Taurocyamine kinase (<db_xref db="EC" dbkey="2.7.3.4"/>), an annelid-specific enzyme that catalyses the transfer of phosphate from ATP to taurocyamine.</li><li>Lombricine kinase (<db_xref db="EC" dbkey="2.7.3.5"/>), an annelid-specific enzyme that catalyses the transfer of phosphate from ATP to lombricine.</li><li>Smc74, a cercaria-specific enzyme from <taxon tax_id="6183">Schistosoma mansoni</taxon> [<cite idref="PUB00002616"/>].</li><li>Creatine kinase (<db_xref db="EC" dbkey="2.7.3.2"/>) (CK) [<cite idref="PUB00000040"/>, <cite idref="PUB00002619"/>], which plays an important role in energy metabolism of vertebrates.</li></ul> </p>