128 PROVISION OF energy: fermentation 



In yeast cells pyruvic acid is decarboxylated by the enzyme 

 carboxylase, and the acetaldehyde so formed is reduced to 

 ethyl alcohol by alcohol dehydrogenase acting in reverse in a 

 manner analogous to the lactic dehydrogenase of muscle, 



Carboxylase 



CH3.CO.COOH ^ > CIIaCHO + CO2 



PTT nuf\ _L Reduced ___°1_^_^ CH3.CH2OH + 

 CH3 .CHO + ^^^^^^^ T T^ehydrogenar Coenzyme I 



THE FORMATION OF PYRUVIC ACID FROM GLUCOSE 

 BY BACTERIA 



In the scheme described above for the breakdown of glucose 

 by yeast, the initial stages consist of a phosphorylation of the 

 glucose molecule to form hexosediphosphate. The majority 

 of bacteria are unable to ferment glucose in the absence of 

 phosphate. This can be demonstrated very easily in some 

 cases by centrifuging the organisms out of culture, washing 

 them very thoroughly, and then incubating the washed 

 suspension of organisms with glucose in the presence and 

 absence of phosphate. If the washing has been successful in 

 removing phosphate from the organisms, then fermentation 

 will often not occur in the absence of phosphate, though it 

 proceeds normally in its presence. This constitutes a priori 

 evidence that phosphate is involved in the fermentation 

 processes, but the existence of the enzymes catalysing the 

 intermediate steps involved in the conversion of glucose to 

 hexosediphosphate has yet to be proved in bacteria. It is 

 highly probable that the same reactions occur, as Esch. coli, 

 for example, will ferment fructosediphosphate to the same 

 fermentation products as those obtained from glucose. Also 

 when glucose is being fermented by the cells, there is an uptake 

 of inorganic phosphate from the medium, and if sodium 

 fluoride (which inhibits enolase) is added to the fermentation 

 mixture, phosphoglyceric acid can be isolated as the chief 



