602 YOSHIHARU ODA 



acid cycle in the stage of aerobiosis through coupling with enzyme systems which 

 condense the active Co derivative with various C compounds, and the cyto- 

 chrome system in insoluble particulate particles. 



On the other hand, the hydrogenase system bound to the particulate fraction 

 became able to utilize various hydrogen acceptors such as O2, NO3" in the oxida- 

 tion of H2 by linkage with the cytochrome system bound to the particles. The 

 soluble hydrogenase system can utilize methylene blue as electron acceptor, but 

 neither Oj nor NO3". It was originally suggested by Yamagata & Nakamura [18] 

 that the reduction of acceptors by H2 requires several carriers and enz>'mes, as 

 indicated by the schematic sequence below. 



H2 > Intermediary Carriers ^ Acceptors 



y Y T 



Hydrogenase X H2 Specific 



Acceptor Enzymes 



Thus, the hydrogenase system has developed to couple with enzyme systems 

 bound to particulate particles, and particularly its linking with the cytochrome 

 system has exploited the important principles of O2 utiHzation, that is, the 

 attaiimient of higher energy efficiency. Furthermore, it acquired the abilities to 

 reduce fumarate, NOa", SO3, and O2 as electron acceptors. In this way the 

 development of special living beings possessing the partictilar systems which 

 assimilate CO2 chemoautotrophically by the energy liberated in the oxidation of 

 H2 with SO3 or O2 has occurred on the Earth. 



But these mechanisms are essentially similar to those of photosynthetic re- 

 action only with the exception of substitution of the oxidation of inorganic 

 substances, for the photochemical reaction. 



Accordingly, it may be infeired that various fermentation, photosynthetic, 

 and respiratory processes were formed on the basis of the process of EMP 

 system and are related to each other. And here, again, it is evident that there 

 are certain types of similarity and a wide array of differences among organisms. 



STAGE OF AEROBIOSIS 



The best example of this type is the non-symbiotic N2-fixing organism, 

 Azotobacter. This strictly aerobic organism contains a powerful hydrogenase 

 system which does not oxidize nor produce H2 during normal metabolism. The 

 system is formed particularly when Azotobacter is grown under conditions where 

 it is restricted to use N2 as the nitrogen source [19]. Cultivation in media con- 

 taining ammonium salts or other fixed nitrogen compounds causes a marked 

 depression of the hydrogenase system content. This observation suggests a close 

 relationship between H2 metabohsm and N2 fixation. A striking apparent con- 

 tradiction is found in the competitive inhibition of N2 fixation by H2 in spite of 

 the fact that the hydrogenase system is present only under No fixing conditions 

 [20]. There is clear-cut competitive inhibition in Azotobacter and symbiotic 

 N-fixing organisms, but none with the anaerobic Clostridia and photosynthetic 

 bacteria [6, 21]. In this connection, it is also important how the hydrogenase 

 system which is inherently unstable to O2 has developed to acquire stability 



