Nitrogen Assimilation and Amino Acid Synthesis

Nitrogen assimilation : In the soil nitrogen is present as nitrates, nitrites and ammonia (NH4+). It is absorbed by green plants and converted into nitrogenous organic compounds like amino acids and DNA; this is nitrogen assimilation. From plants, nitrogen in biomolecules such as amino acids enters the food chain and moves to animals and then, through the death of animals, to decomposers.

Nitrates are first converted to ammonia, but ammonia is highly toxic and is immediately used for conversion into amino acids, which are then transported to other parts of the plant for the synthesis of proteins:

NO3- + 2e- + 2H+ --> NO2- + H2O

NO2- + 8e- + 8H+ --> NH4+ + 2H2O

Amino Acid synthesis : Amino acids are the building blocks of proteins. They are synthesised through:

Reductive amination : Ammonia reacts with alpha-ketoglutaric acid to form glutamic acid (glutamate).

alpha-Ketoglutaric acid + NH4+ + NADPH2 --(glutamate dehydrogenase)--> Glutamate + H2O + NADP

Transamination : The amino group (-CHNH2) of one amino acid is transferred to the keto position (-CO) of another carboxylic acid.

Glutamic acid + oxaloacetic acid --(glutamate-aspartate aminotransferase)--> alpha-Ketoglutaric acid + Aspartic acid

Amides : Ammonia may be absorbed by an amino acid to produce amides; the process is called amidation. The amides are amino acids having two amino groups: the extra amino group is attached to the acidic group (-COOH) in the presence of ATP.

Glutamic acid + NH4+ + ATP --> Glutamine + ADP …