294 2. ANALOGS OF ENZYME REACTION COMPONENTS 



plot presented, although the point scatter is marked, the inhibition might 

 be uncompetitive. The inhibition of nitrogen fixation in soybean root nod- 

 ules by O2 is complex, due to both plant and bacterial components of the 

 respiration, but is competitive when the O2 is above 80% (Bergersen, 

 1962). NgO is a somewhat more potent inhibitor of H2 evolution than is 

 Ng, but the most potent is NO, complete and irreversible inhibition being 

 produced by concentrations of 1% or greater (Shug et al., 1956). The hy- 

 drogenase in cell-free extracts of Proteus vulgaris is inhibited 87% by 

 0.002% or 0.00004 m.M NO, and the inhibition at these low concentrations 

 is partially reversible (Krasna and Rittenberg, 1954). The NO is neither 

 oxidized nor reduced by the enzyme. 



Although the configurations, electronic structures, and physical prop- 

 erties of these simple gases must be important in determining the inter- 

 action with nitrogenase and hydrogenase, it is difficult to establish correla- 

 tions. Some structural and physical properties that might relate to the 

 interactions of these molecules are given in Table 2-5. Comparing the effects 

 of H2, N2O, ethane, and the rare gases on Azotobacter nitrogen fixation, 

 Molnar et al. (1948) concluded there is no correlation with the van der 

 Waals constants and doubted if any mechanism could be based on physical 

 properties alone. Wilson and Roberts (1954) postulated that N2O is inhi- 

 bitory because the N — N distance is close to that in Ng', since NgO is linear, 

 the oxygen might neither interfere nor be involved in the binding. If the 

 binding is to metal groups on the enzymes, the degree of interaction would 

 depend more on the types of bond possible and hence on the electronic 

 structures of both the gases and the metal, as it is in the interactions of 

 O2, CO, and NO with hemoglobin and cytochrome oxidase. 



The inhibition of nitrogen fixation by O2 has been explained as a compe- 

 tition between N2 and 0, as terminal acceptors for electrons originating 

 in the oxidation of substrates by various dehydrogenases, nitrogen fixation 



SH, »- XH, 



being considered as a form of respiration (Parker, 1954; Parker and Scutt, 

 1958, 1960). It is likely that the interrelationships between N, and Hg 

 metabolism, and the inhibitions on these systems, must be considered in 

 the light of a hydrogen or electron pool with all the possible pathways for 

 formation and utilization of hydrogen atoms. The scheme below, modified 

 from Gest et al. (1956), may serve as a means of visualizing some of these 

 pathways. Some of the inhibitions observed are due to competition for 



