BIOSYNTHESIS OF AMINO ACIDS 267 



Transamination. In organisms generally, the amino group of glu- 

 tamic acid can be transferred to various keto acids, yielding «-ketoglu- 

 taric acid and a new a-amino acid, thus: 



HOOC— CH>— CH 2 — CH(NHo)- COOH + R— CO— COOH 



L-Glutnmic acid a-Keto acid 



— HOOC— CHo-CHo- CO— COOH 



a-Ketoglutaric acid 



+ R— CH(NH 2 )COOH (9) 



a-amino acid 



Investigation of these enzymes in filamentous fungi has only be- 

 gun, with the identification in Neurospora crassa of the transami- 

 nase reactions in which aspartate and alanine participate as the 

 R— CH(NH 2 )COOH of Equation 9 (151). Mutants which lack glu- 

 tamic dehydrogenase exhibit a requirement for any of several amino 

 acids; the fact that so many different amino acids support growth 

 argues for an extensive transaminase system in A r . crassa (151, 152, 

 154). 

 Aspergillus fumigatus homogenates carry out the reaction (445): 



H 2 N— (CH 2 ) 3 — COOH + HOOC— (CH 2 ) 2 — CO— COOH — 



•y-Aminobutyric acid a-Ketoglutaric acid 



HOOC— (CH 2 ) 2 — CH(NHo)— COOH (10) 



L-Glutamic acid 



This represents the transfer of an w -amino group, and is similar to the 

 formation of glutamate from ornithine and a-keto-glutarate by extracts 

 of Neurospora crassa (153, 155). Endomycopsis vernalis also forms 

 glutamic acid by the reaction of Equation 10 (290). As in other organ- 

 isms, transaminations in N. crassa are activated by pyridoxal phos- 

 phate (151). 



A kynurenine transaminase, catalyzing the transfer of the a-amino 

 group of kynurenine or 3-hydroxykynurenine to keto acids, has been 

 partially purified from extracts of Neurospora crassa; again, pyridoxal 

 phosphate is required (274). 



Asparagine and glutamine participate also in transaminations; the 

 amino nitrogen is transferred to any of several keto acids (355). This 

 reaction has not been explored in the fungi. 



Biosynthesis of Amino Acid Carbon Skeletons. Perhaps in no field 

 of biochemistry have the techniques of biochemical genetics been more 

 fruitfully employed than in that of amino acid synthesis; Neurospora 

 crassa has been especially useful. In recent years, the application of 

 isotype and enzymatic methods has provided additional valuable in- 



