Q.Explain GS/GOGAT Pathway.

The GS/GOGAT pathway is the two-enzyme system (Glutamine Synthetase and Glutamate Synthase) by which plants assimilate inorganic ammonia into the organic amino acid glutamate, the entry point for nitrogen into amino acid metabolism.

After nitrate is absorbed and reduced to ammonium (NH4+) inside the plant (or ammonium is directly supplied/fixed by nitrogen-fixing bacteria), this inorganic nitrogen must be incorporated into organic molecules — this is achieved primarily through the GS/GOGAT pathway, which occurs in two enzymatic steps:

Step 1 — Glutamine Synthetase (GS):

Glutamate (glutamic acid) + NH4+ + ATP → Glutamine + ADP + Pi

GS combines a molecule of ammonium directly with glutamate, using the energy of ATP hydrolysis, to form glutamine. This is a high-affinity reaction, efficient even at the low ammonium concentrations typically found in plant tissue.

Step 2 — Glutamate Synthase (GOGAT, Glutamine-oxoglutarate aminotransferase):

Glutamine + alpha-ketoglutarate (2-oxoglutarate) + reducing power (NADH/NADPH or reduced ferredoxin) → 2 Glutamate

GOGAT transfers the amide (–NH2) group from glutamine onto a molecule of alpha-ketoglutarate (a Krebs cycle intermediate), producing two molecules of glutamate.

Net outcome: one of the two glutamate molecules produced is used again as the substrate for GS in Step 1 (regenerating the cycle), while the other represents the NET new glutamate formed from the assimilated ammonium. This glutamate is then used as the amino donor in transamination reactions, transferring its amino group to various keto-acids to synthesise the full range of other amino acids required by the plant.

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