ENERGY METABOLISM 43 1 



The bacteria which carry out propionic fermentation may 

 serve as a furtlier example. Under anaerobic conditions their 

 metaboHsm is in complete accord with the scheme for alco- 

 holic fermentation ; sugar is broken dow^n to pyruvic acid 

 by means of the same enzymes and with the formation of 

 the same intermediate products as in yeast. The peculiarity 

 of these bacteria is that in them the pyruvic acid, which is 

 formed in the ordinary way, is not decarboxylated but, on 

 the contrary, combines with CO2 and is transformed into 

 oxaloacetic acid, which is first reduced to succinic acid and 

 then decarboxylated to propionic acid according to the 

 scheme : 



2H2 



CH3.CO.COOH + COo-^HOOC.CHo.CO.COOH -> 



HOOC.CHo.CHo.COOH^ CH3.CH0.COOH + CO,. 



All these reactions take place with the help of the mechan- 

 isms with which we have become familiar, in particular 

 coenzyme A and codehydrogenase. In the air, however, 

 these same bacteria can carry out the typical aerobic oxida- 

 tion of various organic acids, among them pyruvic acid.^"^ 

 Accordingly they, unlike obligate anaerobes, are often found 

 to contain such oxidative mechanisms as cytochrome a^"^ and 

 the enzyme catalase.^"^ 



Like alcoholic fermentation, the anaerobic breakdown of 

 sugar is the basis not only of the energy metabolism of 

 facultative and obligate anaerobes ; the same glycolytic 

 mechanisms may also be found in typical aerobic bacteria 

 which, when living under natural conditions, absolutely 

 require molecular oxygen. 



For example, in the strictly obligate aerobe Streptomyces 

 coelicolor, V. W. Cochrane"^ found the following enzymes : 

 phosphofructokinase. aldolase, triosephosphate isomerase, 

 triosephosphate dehydrogenase, phosphopherase, enolase and 

 ethanol dehydrogenase, i.e. the typical catalysts with which 

 we have become familiar in the scheme of alcoholic fermenta- 

 tion. 



The acetic acid bacterium Acetohacter suhoxydans carries 

 out its energy metabolism by the aerobic oxidation of 

 hexoses. According to E. Simon'"'' it transforms hexose di- 



