SYNTHESIS OF AMINO-ACIDS 



93 



carbon and energy source. The organism can be maintained 

 indefinitely by serial subculture in such a medium. 



(c) OrGAKISMS exacting TOWARDS CERTAIN AMINO-ACIDS : 



The bacteria belonging to the sub-groups so far discussed 

 are able to synthesise all their amino-acids from a source of 

 inorganic nitrogen and a suitable source of carbon. Analysis 

 of the proteins of bacteria shows that they resemble all other 

 proteins in being composed of some twenty-odd amino-acids, 

 all of which can be synthesised by these organisms from 

 ammonia and a source of carbon such as glucose. 



Aspartic acid can be synthesised from fumaric acid and 

 ammonia by the enzyme aspartase (see p. 162), while glutamic 

 acid can be synthesised from a-ketoglutaric acid and ammonia 

 by reversal of the glutamic dehydrogenase system (p. 47). In 

 mammalian tissues glutamic acid, and to a smaller extent 

 aspartic acid, act as the starting point for the formation of 

 other amino-acids by a process of transamination whereby 

 the amino-group of the dicarboxylic amino-acid is transferred 

 to the a-position of an a-keto-acid: 



COOH 



COOH 



CH, 



CHo 



I 

 CHNR 



R 

 I 



I 

 COOH 



Transaminase 



CH2 R 



I I 



CH2 -f CHNH2 



I I 

 C=0 COOH 



COOH 



COOH 



and the a-ketoglutaric acid so formed is then resynthesised 

 to glutamic acid through the reversal of the glutamic dehydro- 

 genase system. This reaction was originally discovered by 

 Braunstein and Kritzmann, and it has been suggested that any 

 a-keto-acid can enter into the reaction, so that transamination 

 opens up a way for the general synthesis of other amino-acids 

 from glutamic acid and the corresponding keto-acids. Since 



