ENERGY METABOLISM 409 



acid. According to the results of H. J. Koepsell and his 

 colleagues," cell-free extracts of CI. hutylicum transform 

 pyruvic acid into acetyl phosphate, CO2 and hydrogen. Butyl 

 alcohol is produced by the reduction of butyryl-coenzyme A. 



Thus we see that the anaerobic breakdown of sugar by 

 the various butyric acid bacteria is based on the same cata- 

 lytic mechanisms and the same sequence of reactions with 

 which we are familiar in alcoholic fermentation. The 4- and 

 3-carbon compounds characteristic of butyric acid fermenta- 

 tion are formed by the further anaerobic transformation of 

 pyruvic acid, that is to say, by the formation, transfer and 

 condensation of acetyl residues. 



Another well-known example of anaerobic decomposition 

 of carbohydrates is provided by lactic acid fermentation 

 which is brought about by various species of facultative 

 anaerobes, e.g. Lactobacillus spp. and Streptococcus spp. The 

 main product of this fermentation is lactic acid, which is 

 formed by the dissimilation of sugar. In its initial stages this 

 process passes through the same intermediate reactions as 

 ordinary alcoholic fermentation, right up to the formation 

 of pyruvic acid. However, owing to the absence of carboxy- 

 lase, in lactic acid bacteria the pyruvic acid is not transformed 

 into acetaldehyde and cOo, but is reduced directly to lactic 

 acid. 



In some lactic acid-producing bacteria such as Escherischia 

 coli or Strep, faecalis large amounts of acetic acid and 

 ethvl alcohol are formed as well as lactic acid. Here this 

 process can only take place anaerobically, by the anaerobic 

 dehydrogenation and decarboxylation of pyruvic acid. In the 

 course of this acetyl-coenzyme A is formed. At the same 

 time the hydrogen liberated by means of diphosphopyridine 

 nucleotide may be transferred to another molecule of pyruvic 

 acid, reducing it to lactic acid, or may react with one of the 

 two molecules of acetyl-coenzyme A which had been formed, 

 so that, in addition to acetic acid, ethyl alcohol is also formed. 

 Thus we see, here too, the same reactions and catalytic 

 mechanisms as are found in strictly anaerobic butyric acid 

 fermentation.'*' ®^ Unlike the bacteria which carry out this 

 fermentation, however, lactic acid bacteria are facultative 

 anaerobes and the organisation of their metabolism shows 



