NU'fhaiiisin of Nitrogen Fixation 203 



amino acid is tlu' first product of nitrogen fixation. This is brought 

 out in tlu- iollowiuu; series of reactions: ' 



X2 -* •■' -^ XH.OII 



Hydr<)\.\l:uninc 



C0H12O,, -> HOOCCOCHo COOH 



Oxalacetic acid 



HOOC C(X0H)CH2 COOH -^ HOOC CIKMLjCH. COOll 



Oxiiue Asijartic acid 



HOOC CH(XH2)CH2 COOH + HaC ( O COOH -> 



Aspartic acid Pyruvic acid 



HOOC CO CH2 COOH + CH3 CHIXH.) COOH 



Oxalacetic acid Alanine 



The first step in the reaction has also been presented by Virtanen as 

 follows: 



Xo ^ HX— XH ^ XH. or X.> -> HX--XH -^> XH,OH 



11"' 



OH OH OH 



According to Wilson and his associates, the biochemical nitrogen 

 fixation b\- Azotobacter has much in common with that of the 

 legumes. The following points were recognized: 



1. Hydrogen and carbon monoxide are specific inhibitors for both 

 types of fixation. 



2. Aspartic acid, possibK with an oxime as a precursor, occupies a 

 key position. 



0. MoKbdenum acts as a specific catalyst in Azotobacter and 

 appears to have a similar effect in legumes. 



4. Azotobacter produces a hydrogenase which seems to be con- 

 nected with the nitrogen fixation; this enzyme is not found in nodules 

 or in cultures of the nodule bacteria in vitro. 



5. Nitrogen fixation by Azotobacter has, except in one species, an 

 optimum at /;H 7.0-7.5, and ceases at pW 6.0, which seems to rep- 

 resent the normal reaction of the nodule tissue and therefore pre- 

 sumably also the optimum for the s\ nibiotic process of fixation. 



6. Azotobacter fixes nitrogen only during active cell multiplication, 

 and uses virtually all the fixed nitrogen for cell synthesis. This does 

 not seem to appK' to the legmnes, where the fixation appears more 

 like a kind of respiration process which results in a steady transfer 

 of some 80-90 per cent of the fixed nitrogen from the nodules to the 



