The central reactions of cellular nitrogen metabolism and the related enzymes. (A) and the physiological range of regulator activity and associated feedback mechanisms (B). A) The metabolites are given in blue and the enzymes in green. Abbreviations: alpha-kg, alpha-ketoglutarate; glu, glutamate; gln, glutamine; GS, glutamine synthetase; GOGAT, glutamine alpha-ketoglutarate aminotransferase; GDH, glutamate dehydrogenase; G, glutaminase. B) Glutamine synthetase (GS) is feedback inhibited (FBI-GS) by glutamine and AMP. In the presence of FBI-GS, GlnR has a higher affinity for the binding site, whereas the activity of TnrA is repressed through a physical interaction with FBI-GS.