Glutamate homeostasis in bacteria

Glutamate homeostasis in bacteria

Glutamate is the major amino group donor that delivers most of the nitrogen that is incorporated into biomass in every organism. Therefore, it is not surprising that glutamate is the predominant cellular metabolite. In addition to its role in anabolism, glutamate fulfils other important tasks in the cell (Fig. A). The enzymatic reactions involved in synthesis and degradation of glutamate represent a central metabolic node, linking carbon to nitrogen metabolism (Fig. B). B. subtilis, exclusively relies on the glutamine synthetase and the glutamate synthase (GOGAT) for biosynthesis of glutamate. In contrast to other bacteria, the two glutamate dehydrogenases (GDH) of B. subtilis are strictly devoted to glutamate catabolism. Both GDHs are so-called trigger enzymes, active in glutamate metabolism and controlling gene expression. Depending on the growth condition, the active GDHs form a complex with GltC and convert the transcriptional activator of the GOGAT-encoding genes into a repressor (Fig. C, D). This elegant control mechanism of glutamate biosynthesis allows the bacteria to maintain a high intracellular glutamate level over a wide range of growth conditions. By applying genetic and biochemical approaches we investigate the molecular details of the protein complex that governs glutamate biosynthesis in B. subtilis.