The Transitionary Stage from Anaerobiosis to Aerohiosis 



601 



strongly suggest the participation of C2 derivatives in the decomposition of 

 formate to CO2 and H2. On the basis of the available facts, Barkulis & Gest [17] 

 have proposed two alternative mechanisms for the decomposition of formate, 

 both employing C2 derivatives as electron carriers (Scheme 2). 



Scheme 2 



Possible mechanisms of formate decomposition to H2 and CO2 

 {from Barkulis & Gest [17]) 



Diacetyl pyruvate 

 or 

 acetyl phosphate 



Formate + 



+ COp 



Scheme A = Reactions 1,2.3 

 Scheme B = Reactions Ia,3 



According to scheme A (reaction i, 2, 3), formate and an acetyl derivative 

 are condensed to produce a C3 compound, which is then decarboxylated yielding 

 CO2 and an acetaldehyde complex. The acetaldehyde compound is oxidized by 

 transfer of electrons to the hydrogenase system, thereby leading to formation 

 of H2 and regeneration of acetyl X. This scheme implies that pyruvate decompo- 

 sition to CO2 and H2 by E. coli, and CI. butyricum may occur through the same 

 pathway, that is, acetaldehyde-X ^ acetyl-X. Scheme B postulates a direct 

 dehydrogenation (decarboxylation) of formate with acetyl-X serving as the 

 electron acceptor (reaction i a). Regeneration of acetyl-X and production of H2 

 then occurs through reaction 3, as in scheme A. In short, the system in soluble 

 cell-free preparation with hydrogenase system that degrades formate to CO2 

 and H2 is a primitive cyclic reaction where acetaldehyde-X ^ acetyl-X is the basic 

 catalytic mechanism, not metabolized and persisting as a more or less permanent 

 part of the system. 



In connection with reaction 3, in which acetaldehyde-X is oxidized to acetyl-X, 

 the transfer of electrons to the hydrogenase system may not be obligatory. Thus, 

 other oxidants such as methylene blue, or cytochrome system, could substitute 

 for the hydrogenase pathway. Here, the system has exploited the appearance of 

 a respiratory system such as the C4-dicarboxylic acid system or the tricarboxylic 



