140 PROVISION OF energy: fermentation 



MISCELLANEOUS FERMENTATION REACTIONS 

 In tlie preceding sections we have traced the formation and 

 fate of pyruvic acid in various fermentations. It is not 

 possible, however, to derive all fermentation products from 

 pyruvic acid, and we must now consider the production of 

 some of these substances. The point is experimentally tested 

 by comparing the fermentation products of an organism 

 when, first, glucose and, second, pyruvic acid is used as 

 fermentation substrate. Thus the products of the fermenta- 

 tion of glucose by Esch. coli are Ho, COg, ethyl alcohol, formic 

 acid, acetic acid, lactic acid, and succinic acid, but if pyruvic 

 acid is the substrate then no ethyl alcohol is formed and very 

 much less lactic acid. 



Ethyl alcohol formation 



In the alcoholic fermentation of yeast (Saccharomyces 

 cerevisiae), alcohol is derived from pyruvic acid by the action of 

 carboxylase which decarboxylates pyruvic acid to acetaldehyde : 



Carboxylase 

 CH3.CO.COOH > CH3.CHO + CO2, 



and acetaldehyde is then reduced to alcohol by alcohol 

 dehydrogenase working in reverse. The coliform organisms 

 do not, however, possess carboxylase, and the presence of 

 this enzyme in bacteria has yet to be demonstrated. Ethyl 

 alcohol does not arise from pyruvic acid in these organisms. 

 If we return to the fermentation of glucose, as described 

 on p. 128, we find that the formation of pyruvic acid was 

 traced from hexosediphosphate through glyceraldehyde- 

 phosphate and phosphoglyceric acid. The oxidation of 

 glyceraldehy de-phosphate to phosphoglyceric acid forms half of 

 an oxido-reduction reaction, which is completed, for the other 

 half, by the reduction of dihydroxyacetone-phosphate to 

 a-glycerophosphate. If fluoride is added to the fermentation 

 system, then the reaction is stopped at this point and a 

 mixture of phosphoglyceric acid and a-glycerophosphate 

 remain as the products. 



