309 Amino Acids and Related Compounds 



to fumaric acid, the former process probably being more preva- 

 lent. 



O NH2 



Either equation ties aspartic acid in with the citric acid cycle. 

 Like glutamic acid, aspartic acid acts as an ammonia carrier 

 through its half amide, asparagine. One role of aspartic acid 

 was seen above in the biosynthesis of arginine. Aspartic acid 

 has been proved a precursor of pyrimidines in certain microor- 

 ganisms. It is also a precursor of threonine and of both a- and 

 /3-alanines. Separate enzymes control the selective decarboxyla- 

 tions to form the alanines. 



a-Alanine (either isomer) can be synthesized, too, from py- 

 ruvic acid by a wide variety of biological systems. Some mi- 

 croorganisms effect this amination directly from ammonia, but 

 the transamination from glutamate is probably more prevalent. 

 Alanine, therefore, is also closely connected with carbohydrate 

 and fat metabolism, and it is used as an energy source by many 

 microbes. Through pyruvate it also may be considered a pre- 

 cursor of glycine, serine, cysteine and of valine, leucine and 



O NH2 



CH3— C— COOH + HOOC— CH—CH2—CH2— COOH ^ CH3— CH— COOH + 



NHo 

 Pyruvic Acid Glutamic Acid a-Alanine 



O 



I 



HOOC— C—CH2—CH2— COOH 

 a-Ketoglutaric Acid 



