This pathway utilizes approximately 15% of the cell’s ATP requirement [1] for

the production of glutamine and its activity is, therefore, strictly regulated at both transcriptional and post-translational levels in order to prevent energy wastage (see Figure 1A). Figure 1 Assimilation of nitrogen by (A) GS and GOGAT; (B) NADP + – dependant-glutamate dehydrogenase (GDH1) and NAD + -dependant glutamate dehydrogenase (GDH2). Under conditions of nitrogen excess, glutamine synthetase activity is reduced via adenylylation by the adenylyltransferase GlnE [3, 4] and under these conditions, the low ammonium affinity glutamate dehydrogenase (GDH) pathway plays a major assimilatory role with a comparatively low associated energy cost [5]. GDH enzymes catalyse the reversible amination of α-ketoglutarate to form glutamate (see Figure 1B) with concomitant reduction Wnt inhibitor of NAD(P)H. They also serve as metabolic branch enzymes

as the GDH enzymes are involved in anapleurotic Kinase Inhibitor Library processes which regulate the flux of intermediates such as α-ketoglutarate between the Krebs cycle and nitrogen metabolism [6]. The GDH enzymes identified in prokaryotes usually function with either NADP+ (EC 1.4.1.4) or NAD+ (EC 1.4.1.2) as co-factors whilst in higher eukaryotes the enzymes have dual co-factor specificity (EC 1.4.1.3). NADP+-specific enzymes are normally involved in the assimilation of nitrogen via amination of α-ketoglutarate [7] and may be transcriptionally

regulated by a variety of growth conditions, including carbon and nitrogen limitation [8–11]. In contrast, NAD+-specific GDH enzymes are thought to be largely involved in glutamate catabolism (deamination) [12–14] and do not appear to be regulated in response to ammonium limitation [15, 16]. GDH enzymes described to date are oligomeric structures and can be grouped into three subgroups according to subunit Urease composition. Many NADP+- and NAD+-GDH enzymes from a number of organisms are hexameric structures made up of subunits that are approximately 50 kDa in size [6]. The second GDH class comprise NAD+-specific GDH enzymes with tetrameric structures whose subunits have a molecular mass of approximately 115 kDa [17]. Recently, a third class of oligomeric NAD+-specific GDH enzymes was defined whose subunits are approximately 180 kDa in size [18–20]. Information regarding nitrogen metabolism and its regulation in the mycobacteria is relatively limited. Glutamine synthetase (encoded by glnA1) has traditionally formed an isolated focal point of study with regard to nitrogen metabolism in the mycobacteria as it has been associated with Mycobacterium tuberculosis virulence and pathogenicity [21, 22]. It has previously been demonstrated that GS from pathogenic mycobacterial species such as M. tuberculosis and M.