NADP-glutamate dehydrogenase isoenzymes of Saccharomyces cerevisiae. Purification, kinetic properties, and physiological roles

In the yeast Saccharomyces cerevisiae, two NADP(+)-dependent glutamate dehydrogenases (NADP-GDHs) encoded by GDH1 and GDH3 catalyze the synthesis of glutamate from ammonium and alpha-ketoglutarate. The GDH2-encoded NAD(+)-dependent glutamate dehydrogenase degrades glutamate producing ammonium and alpha-ketoglutarate. Until very recently, it was considered that only one biosynthetic NADP-GDH was present in S. cerevisiae. This fact hindered understanding the physiological role of each isoenzyme and the mechanisms involved in alpha-ketoglutarate channeling for glutamate biosynthesis. In this study, we purified and characterized the GDH1- and GDH3-encoded NADP-GDHs; they showed different allosteric properties and rates of alpha-ketoglutarate utilization. Analysis of the relative levels of these proteins revealed that the expression of GDH1 and GDH3 is differentially regulated and depends on the nature of the carbon source. Moreover, the physiological study of mutants lacking or overexpressing GDH1 or GDH3 suggested that these genes play nonredundant physiological roles. Our results indicate that the coordinated regulation of GDH1-, GDH3-, and GDH2-encoded enzymes results in glutamate biosynthesis and balanced utilization of alpha-ketoglutarate under fermentative and respiratory conditions. The possible relevance of the duplicated NADP-GDH pathway in the adaptation to facultative metabolism is discussed.