The Central Role of Glutamate and Aspartate in the Post-translational Control of Respiration and Nitrogen Assimilation in Plant Cells

The assimilation of inorganic N, in the form of NH4+, into glutamine and glutamate via glutamine synthetase (GS) and glutamine oxoglutarate aminotransferase (GOGAT) is a pivotal metabolic process in plants. This is because glutamate and glutamine are the primary amino donors for the biosynthesis of all nitrogenous compounds needed by plants. N assimilation must closely interact with glycolytic and respiratory C metabolism, because GS requires ATP and GOGAT requires C skeletons and reductant in the form of 2-oxoglutarate and reduced ferredoxin or NADH, respectively. Over 50% of net plant C may be committed to N assimilation in some tissues in order to generate sufficient 2-oxoglutarate, ATP and reductant for the GS/GOGAT system. Control of the cytosolic phosphoenolpyruvate (PEP) branch point by the tightly regulated enzymes cytosolic pyruvate kinase (PKc) and phosphoenolpyruvate carboxylase (PEPC) exerts a major influence on the overall rate of plant respiration and N assimilation. Glutamate and aspartate are important allosteric effectors of PEP carboxylase and PKc isoforms in plant tissues that are active in NH4+ assimilation. The coordinate feedback control of PEPC and PKc by glutamate and aspartate provides a direct link between N assimilation via GS/GOGAT and the control of respiratory C metabolism.