Abstract
Abstract Glutamine synthetase (GS), as the first enzyme in the main pathway of ammonia assimilation in higher plants, occupies the same key position in nitrogen metabolism as ribulose bisphosphate carboxylase (Rubisco) does in carbon metabolism. The assimilation of am rnonia occurs primarily through the glutamate synthase cycle, in which GS (EC 6.3.1.2) acts in conjunction with glutamate synthase (GOGAT; EC 1.4.7.1 and EC 1.4.1.14) to convert ammonium ions and 2-oxoglutarate to glutamate, via glutarnine, at the expense of ATP and reducing power (Miflin and Lea, 1980). The glutarnine and glutamate produced by the cycle can then be used to synthesize all other nitrogen-containing compounds in the plant. Ammonium can arise from a diversity of sources, both primary (such as nitrate or ammonium ions in the soil or nitrogen-fixing bacteria in root nodules) and secondary (such as photorespiration in green leaves, phenylpropanoid metabolism, or de-arnination of amino acids in germinating seeds).
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