Distance between the substrate and regulatory reduced coenzyme binding sites of bovine liver glutamate dehydrogenase by resonance energy transfer

Bypassing isophthalate inhibition by modulating glutamate dehydrogenase (GDH): purification and kinetic characterization of NADP-GDHs from isophthalate-degrading Pseudomonas aeruginosa strain PP4 and Acinetobacter lwoffii strain ISP4

Pseudomonas aeruginosa strain PP4 and Acinetobacter lwoffii strain ISP4 metabolize isophthalate as a sole source of carbon and energy. Isophthalate is known to be a competitive inhibitor of glutamate dehydrogenase (GDH), which is involved in C and N metabolism. Strain PP4 showed carbon source-dependent modulation of NADP-GDH; GDH(I) was produced when cells were grown on isophthalate, while GDH(II) was produced when cells were grown on glucose. Strain ISP4 produced a single form of NADP-GDH, GDH(P), when it was grown on either isophthalate or rich medium (2YT). All of the forms of GDH were purified to homogeneity and characterized. GDH(I) and GDH(II) were found to be homotetramers, while GDH(P) was found to be a homohexamer. GDH(II) was more sensitive to inhibition by isophthalate (2.5- and 5.5-fold more sensitive for amination and deamination reactions, respectively) than GDH(I). Differences in the N-terminal sequences and electrophoretic mobilities in an activity-staining gel confirmed the presence of two forms of GDH, GDH(I) and GDH(II), in strain PP4. In strain ISP4, irrespective of the carbon source, the GDH(P) produced showed similar levels of inhibition with isophthalate. However, the specific activity of GDH(P) from isophthalate-grown cells was 2.5- to 3-fold higher than that of GDH(P) from 2YT-grown cells. Identical N-terminal sequences and electrophoretic mobilities in the activity-staining gel suggested the presence of a single form of GDH(P) in strain ISP4. These results demonstrate the ability of organisms to modulate GDH either by producing an entirely different form or by increasing the level of the enzyme, thus enabling strains to utilize isophthalate more efficiently as a sole source of carbon and energy.

Tissue specificity of mitochondrial glutamate pathways and the control of metabolic homeostasis

Glutamate is implicated in numerous metabolic and signalling functions that vary according to specific tissues. Glutamate metabolism is tightly controlled by activities of mitochondrial enzymes and transmembrane carriers, in particular glutamate dehydrogenase and mitochondrial glutamate carriers that have been identified in recent years. It is remarkable that, although glutamate-specific enzymes and transporters share similar properties in most tissues, their regulation varies greatly according to particular organs in order to achieve tissue specific functions. This is illustrated in this review when comparing glutamate handling in liver, brain, and pancreatic beta-cells. We describe the main cellular glutamate pathways and their specific functions in different tissues, ultimately contributing to the control of metabolic homeostasis at the organism level.