IV INORGANIC N FIXATION AND TRANSFER 85 



liver 

 5) Ornithine + glyoxylate ' ^ glutamic-v-semialdehyde + glycine 



Glutamine and asparagine also are amino donors in transamination reactions. 

 The keto analogues of over thirty amino acids are capable of reacting with glu- 

 tamine. The keto analogues of isoleucine, valine, lysine and ornithine are ex- 

 ceptions and do not react. In these reactions, the alpha nitrogen of glutamine or 

 asparagine is transferred, not the amide nitrogen. This has been demonstrated in 

 experiments with glutamine-amide '^N (Meister, 1955; Fig. 39). 



Several apoenzymes catalyze transaminase reactions. The pig heart apoenzyme 

 for the glutamine-oxalacetic reaction has been separated from the apoenzyme for 

 the glutamic-pyruvate reaction. Different transaminase apoenzymes are present 

 in E. coli and in Neurospora (Fincham and Boulter, 1956). From the former, three 

 classes of transaminases have been separated: i) enzyme fraction A catalyzes 

 reactions between a-ketoglutarate and aspartate, phenylalanine, tyrosine, or 

 tryptophane, 2) enzyme fraction B catalyzes the reaction of a-ketoglutarate with 

 isoleucine, valine, and leucine (Meister, 1955), 3) enzyme fraction C is specific for 

 valine and pyruvate or a-ketobutyrate. Table 6 summarizes the principal trans- 

 amination reactions which are of importance in amino acid synthesis. 



4. The transfer of the amide group of glutamine 



The amide group of glutamine^ is utilized in a number of extremely important 

 biosynthetic reactions : 



(j) Phosphoribosylamine [PRA). Liver and yeast enzymes catalyze the synthesis of 

 the purine intermediate, 5-phosphoribosylamine (Goldthwait et al., 1955): 



Glutamine + PRPP -^ PRA + PP + glutamic 



(2) N-formylglycineamidine ribotide {FGAA4). Glutamine-nitrogen is also required 

 in the synthesis of FOAM from formylglycineamide ribotide (FGAR) (Levenberg 

 et al., 1956). 



ATP 



FGAR + glutamine > FGAM + glutamic 



pigeon 

 liver 



(j) Guanylic acid synthesis. The conversion of xanthosinic acid to guanylic acid 

 is catalyzed by enzymes from bone marrow cells ( Abrams and Bentley, 1 955a) and 

 pigeon liver (Lagerkvist, 1955). 



Mg- 

 Xanthosinic acid + glutamine ^ guanylic + glutamic 



(4) Glucosamine-6-phosphate. Kidney and liver enzymes catalyze the formation of 

 glucosamine-6-phosphate from fructose-6-phosphate and NH3. A second mechan- 

 ism of glucosamine-6-phosphate synthesis which has been observed in JVeurospora, in- 

 volves the transfer of the amide nitrogen from glutamine (Leloirand Cardini, 1953). 



Hexose-6-phosphate + glutamine — * glucosamine-6-phosphate + glutamic 



^ See Addendum, Note 4, p. 123. 



Literature p. I2f 



