General metabolic reactions of amino acids ( Deamination and Transamination ) for the liberation of ammonia

Deamination : The removal of amino group (-NH2) from the amino acids as NH3 is called deamination. Deamination results in the liberation of ammonia for urea synthesis. Deamination may be either oxidative or non-oxidative. 

                               

(1) Oxidative deamination : Oxidative deamination is the liberation of free ammonia from the amino group (-NH2) of amino acids coupled with oxidation. This takes place mostly in liver and kidney. The purpose of oxidative deamination is to provide NH3 for urea synthesis and α- keto acids for a variety of reactions including energy generation. 

It is catalysed by one of three following  enzymes :

                                           

                                                   1. L-Amino acid oxidases

                                                   2. D-Amino acid oxidases 

                                                   3. Glutamate dehydrogenase

Deamination by L- Amino acid oxidase

  

Deamination by D-Amino acid oxidase

Deamination by glutamate dehydrogenase

          

                                                                

(2) Non-oxidative deamination : Non –oxidation deamination is the liberation of free ammonia from the amino acids without undergoing oxidation.

Transamination :

The transfer of an amino (-NH3) group from an amino acid to a keto acid is known as transamination. This process involves the interconversion of a pair of amino acids and a pair of keto acids, catalysed by a group of enzymes called transaminases of aminotransferase

Salient feature of transamination : 

(1) All transaminases require pysidoxal phosphate (PLP), a coenzymes derived from vitamin B6.

(2)Specific transaminases exist for each pair of amino and keto acids. However, only two namely, aspartate transminase and alanine transaminase make a signtificant contribution for transamination.

(3) There is no free NH3 liberated only the transfer of amino group.

(4) Transamination is reversible. 

(5) Transamination is very important for the redistribution of amino groups and production of non-essential amino acids as per the requirement of the cell.

(6) Transamination diverts the excess amino acids towards energy generation.

(7) The amino acids undergo transamination to finally concentrate nitrogen in glutamate. Glutamate is the only amino acid that undergoes oxidative deamination to a significant extent to liberate free NH3 for urea synthesis.

(8) All amino acids extent except lysine, threonine, praline and hydeoxyproline, participate in transmination.

(9) Transamination is not restricted to α-amino groups only. For instance, δ-amino group of ornothine is transaminated.

(10) Serum transminases are important for diagnostic and prognostic purposes.