430 FURTHER EVOLUTION 



supplementary structural features which, to some extent, 

 enable us to understand the mechanism of the transition from 

 primary anaerobiosis to the aerobic way of life. 



As we have shown, the reaction whereby acetyl-coenzyme 

 A is formed from pyruvic acid can take place, not only 

 anaerobically, but also with the participation of free oxygen. 

 The difference in this case is simply that the hydrogen set 

 free by dehydrogenation is not accepted by pyruvic acid or 

 acetaldehyde but is oxidised by oxygen. 



This oxidative process does not occur spontaneously, it 

 requires special catalytic mechanisms which are completely 

 absent from obligate anaerobes because, in them, the reaction 

 can only follow the first path via the transformation of 

 pyruvic acid. 



In facultative anaerobes, by contrast, there have been 

 found, alongside the ordinary glycolytic mechanisms, specific 

 catalysts promoting the oxidative decarboxylation of pyruvic 

 acid. Thus, according to the studies of I. C. Gunsalus and 

 his colleagues,^"" and L. J. Reed and colleagues,^"^ in Esch. 

 coli and L. delbrilckii and other bacteria this reaction is 

 catalysed by a complex compound of the amide of lipoic acid 

 and cocarboxylase which has been called lipothiamide pyro- 

 phosphate. 



When facultative anaerobes are cultivated in the absence 

 of free oxygen these supplementary mechanisms are of no 

 significance. They can easily be excluded from metabolism ; 

 they remain ' unemployed ' but the bacterial cell continues 

 to exist satisfactorily on the basis of the old organisation. On 

 the other hand, in the presence of oxygen, the oxidative 

 .catalysts give a great advantage because they enable the 

 organisms in which they are present to make considerably 

 more rational use of the organic materials at their disposal. 



It is obvious that, under the reducing conditions of the 

 primaeval atmosphere, only mechanisms subserving anaero- 

 bic metabolism could develop, while oxidative catalysts 

 were only formed as supplementary, and sometimes very 

 unimportant, accessories, after a considerable amount of free 

 oxygen had appeared on the surface of the Earth. This 

 is reflected in the organisation of present-day facultative 

 anaerobes. 



