PRIMING REACTIONS 



215 



amino acid and the glutamic acid formed would be deaminated by the 

 specific L-glutaminase. In addition this mechanism accounts for the rapid 

 synthesis of amino acids from ammonia and a-keto acids. 



NH2 — CH-COOH a-ketoglutarate 



R 



0=C— COOH 



L- glut a mate 



NH3-h 2H 



glutamic 

 dehydrogenase 



B. The Complete Degradation of various Amino Acids 



[a] Glutamic Acid 



As we have just seen, L-glutamic acid is not deaminated by the action of 

 the L-amino acid oxidase of animal tissues and bacteria. But, in the presence 

 of a specific enzyme, glutamic dehydrogenase, it undergoes oxidative 

 deamination in the presence of either DPN or TPN. This reversible re- 

 action gives a-iminoglutaric acid. 



COOH COOH 



CH. 



CH, 



CH, 



+ DPN+ 



CH2 



4- DPNH + H+ 



NH2— CH— COOH NH=C— COOH 



L — glutamic acid a — iminoglutaric acid 



The a-iminoglutaric acid is hydrolysed spontaneously to a-ketoglutaric 

 acid and ammonia. The a-ketoglutaric acid can enter the tricarboxylic acid 

 cycle. 



(b) Aspartic Acid 



Aspartic acid, by transamination, gives oxaloacetic acid which then also 

 enters the tricarboxylic acid cycle. 



(c) Histidine 



Histidine, besides its connection with the metabolism of pentoses, 

 nucleotides and certain other amino acids, follows a path leading to 

 glutamic acid. In certain bacteria, the enzyme system forms one mole of 

 glutamic acid, one mole of formic acid, and two moles of ammonia from one 

 mole of histidine, whilst in other bacteria one mole of glutamic acid, one 



