The assimilation ofNH3 obtained from N2 reduction occurs principally via the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway. The first enzyme catalyzes the ATP-dependent assimilation of ammonia into glutamine using glutamate as a substrate:
The second enzyme catalyzes the reductive transfer of the amide group from glutamine to a-ketoglutarate, forming two molecules of glutamate. The reductants are NAD(P)H or reduced ferredoxin (Fdxred):
Glutamine + a-ketoglutarate + NAD(P)H(or Fdxred) ! 2glutamate + NA(D)P+(or Fdxox)
One glutamate serves to keep the pathway going, whereas the other represents the gain in organic nitrogen.
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