Nitrogen Metabolism — Core Principles

Core Principles

Nitrogen metabolism is the sum of processes by which organisms acquire, transform, and utilize nitrogen, an essential element for proteins, nucleic acids, and ATP. The core of this is the Nitrogen Cycle, which begins with Nitrogen Fixation, converting atmospheric $N_2$ into ammonia ($NH_3$) by specialized prokaryotes (e.

g., *Rhizobium*, *Azotobacter*) using the oxygen-sensitive nitrogenase enzyme. Ammonia is then oxidized to nitrites and nitrates by nitrifying bacteria (*Nitrosomonas*, *Nitrobacter*) in Nitrification.

Plants absorb nitrates and reduce them back to ammonia via Nitrate Assimilation (involving nitrate and nitrite reductases). The ammonia is then incorporated into organic molecules, primarily amino acids like glutamate and glutamine, through Ammonia Assimilation pathways such as reductive amination and transamination.

Decomposers return organic nitrogen to ammonia via Ammonification. Finally, Denitrification by certain bacteria converts nitrates back to $N_2$ gas, completing the cycle. Key enzymes and bacterial types are crucial for NEET.

Important Differences

vs Reductive Amination vs. Transamination

Aspect	This Topic	Reductive Amination vs. Transamination
Primary Function	Initial incorporation of inorganic ammonia into an organic molecule.	Synthesis of various amino acids from pre-existing amino acids and keto acids.
Substrates	$\alpha$-ketoglutaric acid and inorganic ammonia ($NH_4^+$).	An amino acid (donor) and a keto acid (acceptor).
Product	Glutamate.	A new amino acid and a new keto acid.
Enzyme	Glutamate Dehydrogenase (GDH).	Transaminases (or Aminotransferases).
Coenzyme/Reductant	NADH or NADPH.	Pyridoxal phosphate (Vitamin $B_6$).
Role in Nitrogen Assimilation	Directly incorporates free ammonia, especially when ammonia concentration is high.	Distributes nitrogen from glutamate to synthesize a wide range of other amino acids.

Reductive amination is the initial step for incorporating free inorganic ammonia into an organic compound, specifically forming glutamate from $\alpha$-ketoglutarate, catalyzed by glutamate dehydrogenase. In contrast, transamination is a subsequent, broader process where an amino group from an existing amino acid (often glutamate) is transferred to a different keto acid to synthesize a new amino acid, catalyzed by transaminases. While reductive amination is crucial for the initial capture of ammonia, transamination is vital for diversifying the amino acid pool, allowing the synthesis of nearly all other amino acids required by the organism.