Glutamate synthase forms glutamate from glutamine and α-oxoglutarate (α-ketoglutaric acid). These enzymes are also active in roots and nodules (N2 fixation). These enzymes assimilate most of the ammonium derived from absorption from dilute solutions, reduction of nitrate, N2 fixation, or photorespiration (18–25). Further discussions of glutamine synthetase, glutamate synthase, and glutamic acid dehydrogenase follow.
Ammonium + glutamate + ATP + reduced ferredoxin → glutamine + oxidized ferredoxin
Glutamic Acid Dehydrogenase
Ammonium + α-oxoglutarate + ATP + reduced pyridine nucleotide (NADH, NADPH) → glutamate + oxidized pyridine nucleotide (NAD+, NADP+)
Another pathway for ammonium assimilation into organic compounds is by glutamic acid dehydrogenase, which is located in the mitochondria (28). Glutamic acid dehydrogenase has a low affinity for ammonium and becomes important in ammonium assimilation at high concentrations of ammonium and at low pH in growth media (15).
Glutamate + α-oxyacid → α-oxoglutarate + α-amino acid
Ammonium that is assimilated into glutamate from mitochondrial or chloroplastic assimilation can be transferred by aminotransferases (transaminases) to an appropriate α-oxyacid (α- ketoacid) to form an α-amino acid. The transfer can also be to other keto-groups on carbon chains to form, for example, λ- or δ-amino acids. The keto acids for the synthesis of amino acids are derived from photosynthesis, glycolysis, and the tricarboxylic acid cycle, among other processes.
Glutamate + ammonium + ATP → glutamine + ADP
Amides are formed by the amidation of carboxyl groups. Amides are nitrogen-rich compounds that can store or transport nitrogen. Common amides are glutamine (5C, 2N) and asparagine (4C, 2N). Glutamine is formed from amidation of glutamic acid (glutamate), and asparagine is formed by amidation of aspartic acid (aspartate). Often, when the external supply of ammonium is high, asparagine, a metabolite unique to plants, will dominate among the amides, as plants respond to conserve carbon in the detoxification of ammonium.
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