52 NITROGEN METABOLISM 



nodular tissues [45]. Nitrogen-fixation by Azotobacter, 

 Nostoc and leguminous plants is inhibited in an apparently 

 non-competitive manner by carbon monoxide in concentra- 

 tions which cause little, if any, inhibition of respiration [48, 

 16]. Carbon monoxide is an isostere of Ng , and it may 

 therefore replace Ng on the surface of the enzyme concerned 



FIG. 4.1. — Nitrogen fixation by Azotobacter as a function of the 

 partial pressure of nitrogen (pNo) in the presence of different 

 partial pressures of hydrogen (6), helium ( X ), or argon (A), 

 or a partial vacuum (D)"- partial pressure of oxygen in all 

 experiments, 0*2 atm. [50] 



with the primary fixation reaction. Alternatively the fixation 

 mechanism may involve a metal ion, perhaps combined 

 in the prosthetic group of an enzyme, whose activity is lost 

 on combination with CO. In this connection it may be of 

 significance that the hydrogenase of Az. vinelandii is also 

 inhibited by CO [44]. 



Role of ammonia in N^-jixatioyi 



The idea has arisen that there is a key substance in 



