THE ALCOHOLIC FERMENTATION PROPER 329 



of H2-DPN. DPN is again available for the oxidation of another 

 triose diphosphate (or triose phosphate) molecule. 



The 1,3-diphosphogly eerie acid subsequently loses one phosphate 

 group, the high-energy phosphate linkage to ADP (converting it to 

 ATP), to become 3-phosphoglyceric acid. 



O 



CO-'P03H2 COOH 



HCOH + ADP ;:± HCOH + ATP (8) 



CH2O— PO3H2 CH2O— PO3H2 



1,3-Diphospho- 3-Phospho- 



glyceric acid glyceric acid 



This intermediate in turn is in equilibrium with 2-phosphoglyceric 

 acid. 



COOH COOH 



Phosphoglucomutase 



HCOH ± HCO— PO3H2 (9) 



■ > CH2O— PO3H2 CH2OH 



•» 3-Phosphoglyceric 2-Phosphoglyceric 



acid acid 



The 2-phosphoglyceric acid then loses one molecule of water to be- 

 come phospho-enol-pyruvate. 



COOH COOH 



Enolase 



HCO— PO3H2 ; ^ CO~P03H2 + H2O (10) 



a • " 



CH2OH CH2 



2-Phosphoglyceric Phosphopyruvic 



acid acid (enol) 



This transformation actually involves more than merely a simple 

 dehydration. In the dehydration of 2-phosphoglyceric acid, a reac- 

 tion catalyzed by the enzyme enolase and freely reversible, a change 

 occurs in which the phosphate linkage, an ordinary ester linkage and, 

 as such, a low-energy linkage on the glycerate side, becomes an 

 energy-rich enol phosphate linkage on the pyruvate side. The total 

 or the overall energy over the whole molecule is equal on both sides 

 of the reaction. However, the dehydration changes the energy distri- 

 bution within the molecule in such a way as to concentrate and make 

 available a much larger portion of energy in the phosphate linkage 

 on the pyruvate side. 



