Amino Acid Synthesis by Plants
Ammonia formation is achieved by plants either by nitrogen fixation or by reduction of nitrate to nitrite. Ammonium (NH4+) is the most reduced form of inorganic combined nitrogen. This ammonium now becomes the major source for the production of amino acids, which are the building blocks of enzymes and proteins.
Amino acids have two important chemical groups:
- amino group (NH2)
- carboxyl group (-COOH)
Ammonium so produced is the major source of amino group. However, the carboxyl group has to be provided by other organic molecule synthesized by the plants. There are two major reactions for amino acid biosynthesis in plants.
Reductive Amination Reaction
In this reaction, ammonia combines with a keto acid. The most important keto acid is the alpha ketoglutaric acid produced during the operation of Krebs cycle. The keto acid then undergoes enzymatic reductive amination to produce an amino acid.
α-ketoglutaric acid + NH3 → Glutamic (amino acid)
Similarly another amino acid called aspartic acid is produced by reductive amination of oxaloacetic acid.
Transamination Reaction
This is another very important reaction for amino acid biosynthesis. The reaction involves transfer of amino group, from already synthesized amino acid, to the keto acid.
α-Ketoglutaric acid + Aspartic acid → Glutamic acid + Oxaloacetic acid
In the above reaction, aspartic acid has transferred its amino group (NH2) to the α-ketoglutaric acid to synthesize glutamic acid and release keto acid. The reaction is catalyzed by enzymes called transaminases. A large number of amino acids are synthesized by this transamination reaction. Amino acids are organic molecules containing nitorgen. The incorporation of amino group, from ammonium, into keto acids represents the major step for synthesis of nitrogenous organic biomolecules.