ENERGY METABOLISM 425 



with another high-energy phosphate residue from another 

 molecule of ATP, the reaction being catalysed by the enzyme 

 phosphohexokinase. 



These preparatory reactions, involving the expenditure of 

 two high-energy bonds on each glucose molecule, lead to 

 the formation of fructose-i:6-diphosphate. This is followed 

 by a reaction catalysed by aldolase, the disruption of the 

 six-membered carbon chain into two trioses (glyceraldehyde- 

 3-phosphate and dihydroxyacetone phosphate) which can 

 undergo a mutual transformation, catalysed by the enzyme 

 phosphotriose isomerase. 



In the process of fermentation glyceraldehyde-3-phosphate 

 undergoes a further transformation, being continually re- 

 placed at the expense of dihydroxyacetone phosphate. The 

 molecule of glyceraldehyde-3-phosphate undergoes dehydro- 

 genation while simultaneously combining with a phosphate 

 residue (derived from mineral phosphate) and a new high- 

 energy bond is thus formed in which is stored the energy 

 liberated by the removal of hydrogen from the glyceralde- 

 hyde-3-phosphate. The hydrogen thus liberated combines 

 with coenzyme I (DPN) Avhich constitutes the active group 

 of the enzyme triosephosphate dehydrogenase which catalyses 

 this reaction. This hydrogen can be used further for a 

 number of reducing transformations in the living cell ; in 

 particular, in alcoholic fermentation, it reduces acetaldehyde 

 to ethyl alcohol. 



The i:3-diphosphoglyceric acid formed from the glycer- 

 aldehyde-3-phosphate gives its high-energy phosphate residue 

 to ADP. This reaction thus brings about the regeneration of 

 one of the two molecules of ATP which had earlier been 

 used for the phosphorylation of glucose. It is catalysed by 

 phosphopherase (phosphoglyceric phosphokinase). 



The 3-phosphoglyceric acid formed from i:3-diphospho- 

 glyceric acid is converted, by the action of phosphoglycero- 

 mutase, into 2-phosphoglyceric acid which is transformed, 

 with the help of enolase, into phosphoenolpyruvic acid. In 

 this reaction 2-phosphoglyceric acid gives up water, which 

 leads to a rearrangement of the internal energy of the mole- 

 cule and the formation of a second high-energy bond at the 



