IV INORGANIC N FIXATION AND TRANSFER 83 



brain, heart, pea seeds, and Staphylococcus aureus (Elliott and Gale, 1948; Speck, 

 1949; Levintow and Meister, 1954; Elliott, 1953). The purified enzyme catalyzes 

 the exchange of ■*^P04H3 with ATP and this exchange is greatly increased in the 

 presence of glutamate. The following mechanism for glutamine synthesis has 

 been proposed : 



i) Enzyme + ATP . — > Enzyme-P + ADP 



2) Enzyme-P + glutamate ^ > Enzyme-glutamic + P 



3) Enzyme-glutamic + NH3 t — > glutamine + Enzyme 



All known enzymes which catalyze the synthesis of glutamine also catalyze a 

 glutamyl transfer reaction with hydroxylamine. The converse is also true : 



Glutamine + NH2OH ^ — r NH3 + glutamohydroxamate 



Phosphorus or arsenate, Mn'^'", and trace amounts of ATP are reqviired. The 

 ATP acts catalytically here in contrast to the glutamine synthesis system. 



(j) Asparagine synthesis. The formation of the amide bond of asparagine has 

 been less studied than the synthesis of glutamine. The reaction is, however, of 

 primary importance in plants. It is not unlikely that the mechanism of synthesis 

 is similar to that of glutamine. 



{4) Carbamyl phosphate synthesis. The formation of carbamyl phosphate from 

 NH3 and CO2 has been discussed previously (Table 4, reaction 38, p. 35). 



(5) Cytidine triphosphate formation. An enzyme purified 45 fold from E. coli extracts 

 catalyzes the amination of UTP or UDP (Table 4, reaction 96, p. 36; Lieber- 

 man, 1955). 



(6) Glucosamine-6-phosphate synthesis. The following reactions occur in kidney, liver, 

 brain, lung or intestine (Leloir and Cardini, 1956): 



a) N-acetylglucosamine-6-phosphate -^ fructose-6-phosphate + NH3 + acetate 



b) Glucosamine-6-phosphate ' — t fructose-6-phosphate + NH3 



Catalytic amounts of N-acetylglucosamine-6-phosphate are required for reaction b) . 

 Reaction b) is reversible, thereby providing a mechanism for ammonia fixation. 

 Although fructosylamine-6-phosphate is probably an intermediate, it isomerizes 

 to glucosamine-6-phosphate and the latter substance is isolated. 



(7) Aspartase. The enzyme, aspartase, of many bacteria and plants catalyzes the 

 following reaction: 



Fumaric + NH3 •< — » aspartic 



J. Amino transfer reactions and transaminations 



Many alpha keto acids can substitute for the corresponding essential amino acids 

 in the diet of animals and microorganisms. It is therefore apparent that the facile 



Literature p. 124 



