Estimation of Glutamine Synthetase (GS), Glutamate Synthase (GOGAT), and Glucose Dehydrogenase (GDH)

Abstract

Plants utilize nitrate, ammonium, and dinitrogen (N₂) molecules as external nitrogen sources. N₂ is reduced to NH₃ during the rhizobium-legume symbiosis. Ammonia a primary N₂ fixation product in bacteroids of a symbiotic form of rhizobia in nodules is rapidly excreted to the plant cytosol and assimilated into glutamine via the GS/GOGAT system. Ammonium is the final form of inorganic nitrogen prior to the synthesis of organic nitrogen compounds. Ammonium is assimilated into the Gln amide group, which is then transferred to the position of 2-oxoglutarate, yielding two molecules of Glu by the concerted reaction of Gln synthetase (GS; EC 6.1.1.3) and Glu synthase (ferredoxin [Fd]-GOGAT; EC 1.4.7.1; NADHGOGAT; EC 1.4.1.14). Nitrogen is then incorporated into Asp, Ala, Asn, and other amides and amino acids. Gln-dependent Asn synthetase (AS; EC 6.3.5.4) provides Asn, which serves as a nitrogen carrier together with Gln and Glu. Ammonium might be directly incorporated into Glu by amination of 2-oxoglutarate via mitochondrial Glu dehydrogenase (NADH-GDH; EC 1.4.1.2) and subsequently into Gln by cytosolic GS1 under particular physiological conditions.