At physiological pH, the ammonia is protonated to form NH4+ (ammonium) ion while most of the plants can assimilate nitrate as well as ammonium ions, the latter is quite toxic to plants and hence cannot accumulate in them.
There are two main ways by which this can take place :
(i) Reductive amination : In these processes, ammonia reacts with α-ketoglutaric acid and The two most important amides - asparagine and glutamine - found in plants are a structural part of proteins. They are formed from two amino acids, namely aspartic acid and glutamic acid, respectively, by addition of another amino group to each. The hydroxyl part of the acid is replaced by another NH2-radicle.
forms glutamic acid as indicated in the equation given below:
α -ketoglutaric acid + NH4+ + NAD
\(\underset{Dehydrogenase}{\stackrel{Glutamate}{\rightarrow}}\) Glutamate + H2O+ NADP
(ii) Transamination: It involves the transfer of amino group from one amino acid to the keto group of a keto acid. Glutamic acid is the main amino acid from which the transfer of NH2, the amino group takes place and other amino acids are formed through transamination. The enzyme transaminase catalyses all such reactions.
For example,
