The Transitionary Stage from Anaerobiosis to Aerobtosis 



603 



against O2 in aerobosis through the transitionary stage from anaerobiosis to 

 aerobiosis. 



To understand these problems the hydrogenase system from Azotobacter was 

 compared with those from CI. pasteurianum and E. coli. It appears from our 

 studies that in the former practically all of the enzyme system is botmd to the 

 insoluble particulate fraction, being different from those systems in the latter 

 organism. Spectroscopic studies of the particles have disclosed a new haemo- 

 protein (cytochrome 552) with absorption peaks at 415 m/u (y-band), 524 m/x 

 (j8-band), 552 Ta.11 (a-band) in the reduced state, and absorption peaks at 390 mjLt, 

 450 m/i characteristic for flavin in the oxidized state. This spectral change may 

 conceivably be associated with the hydrogenase system itself, but more likely it 

 is due to indirect reduction of a distantly related porphyrin enzyme. Thus the 

 hydrogenase enzyme itself probably is a flavoprotein and may be able to oxidize 

 H2 with O2 through a cytochrome system as intermediate hydrogen carrier. 

 This oxidation of H2 with O2, that is the oxyhydrogen reaction, occurs actively 

 only when the system is boimd to the particles It is very interesting that a partial 

 pressure of over 5% O2 in the atmosphere causes a marked depression in the 

 oxyhydrogen reaction catalysed by the particulate system. This fact shows that 

 the characters owing to which the hydrogenase system is essentially sensitive 

 and tmstable for O2 have been maintained even when it has developed from 

 anaerobiosis to aerobiosis. 



But when the system becomes coupled with the succinic oxidase system in 

 the particulate fraction, it is very stable for the partial pressure of O2 in the 

 atmosphere. From several points of view, it may be inferred that the hydro- 

 genase system has differentiated morphologically from the soluble state to the 

 particle-bound state, and functionally from a simple system to a complex one 

 by coupling with other enzyme systems bound to particles in comphance with 

 the environmental changes, that is, particularly with the appearance of O2 on 

 the Earth. 



In this cormection, in order to understand quaHtative differences between the 

 aerobic and anaerobic N2-fixing bacteria in the interaction of N2 reduction and 

 hydrogenase system the following tentative mechanism can be proposed : 



(Hydrogen 

 donor) 



H,^ 



AH, 



(I) 



N, 



(3) 



-Nitrogenase 



(4) I (2) I (5) 



^ ~^ Xi ^ ~^ X2 ^ Cytochrome a^, a^ 



I (FAD ?) (Cytochrome 



Hydrogenase 552 ?) 



Oo 



In the diagram, Xi and X2 are tentative components, which may take part in 

 electron transport. Particularly Xi is inferred to be also concerned with the 



