330 BIOLOGICAL ACTIVITIES OF YEASTS 



At this point it is believed that the phosphopyruvate yields the 

 energy-rich phosphate linkage to ADP or AMP to form ATP.|I 



COOH COOH 



CO^POsHa + ADP -^ C=0 + ATP (11a) 



CH2 CH3 



2-Pho.sphopyruvic Pyruvic acid 



acid (enol) 



COOH COOH 



2CO~P03H2 + AMP -^ 2C=0 + ATP (116) 



CH2 CH3 



2-Phosphopyruvic Pyruvic acid 



acid (enol) 



Pyruvic acid then yields carbon dioxide, one of the two main end 

 products of the alcoholic fermentation, and acetaldehyde. This re- 

 action, a decarboxylation, is catalyzed by the enzyme carboxylase 

 and its coenzyme, cocarboxylase (diphosphothiarain). 



COOH 



_ „ Carboxylase, 



C=0 



cocarboxylase 



CHs 



Pyruvic Acid 



-^C02 

 Carbon dioxide 



+ 



CHO 

 CHs 



Acetaldehyde 



(12) 



Acetaldehyde serves as an oxidizing agent (hydrogen acceptor) for 

 H2-DPN (converting it to DPN as previously described) and is it- 

 self reduced to ethyl alcohol, thus yielding the other main end product 

 of this fermentation. 



(13) 



To recapitulate the above dissimilation scheme .briefly, a hexose 

 molecule is diphosphorylated. A cleavage occurs with the formation 



II Lardy and Ziegler (/. Biol. Chem., 159, 343-351, 1945) state that they have 

 been able to demonstrate the enzymatic synthesis of phosphopyruvate from 

 pyruvate. Previously the reaction, phosphopyruvate to pyruvate, had been con- 

 sidered irreversible. 



