188 UNITY AND DIVERSITY IN BIOCHEMISTRY 



esters. They isolated fructofuranose — 1,6 — PP (Harden and Young ester) 

 and later other esters have been isolated from the fermentation liquor : 

 fructofuranose — 6 — P (Neuberg ester); glucose — 6 — P (Robinson ester) 

 and glucose — 1 — P (Cori ester). 



In 1905, a new fundamental discovery was announced simultaneously 

 by Buchner and by Harden and Young. When a fermenting juice is dia- 

 lysed, fermentation ceases in the dialysate (termed cozymase) and in the 

 residue (termed apoenzyme). In the "cozymase" or "coenzyme" (the name 

 given initially to the dialysate), we find substances of the nature of our 

 "coenzymes", ATP, DPN, DPT. 



The reducing action of yeast thus became the foremost topic of interest. 



Neuberg carried out another important experiment by adding bisulphite 

 to the fermenting mixture. A precipitation of the bisulphite complex of 

 acetaldehyde occurs and glycerol accumulates (one mole per mole of the 

 bisulphite compound). Neuberg concluded that fermentation takes place 

 by a decarboxylation of pyruvic acid forming acetaldehyde, and that the 

 glycerol contains the hydrogen which, in fermentation, reduces the acetal- 

 dehyde to alcohol. But what was the substance which on hydrogenation 

 yielded glycerol? This could only be a triose. For a long time it was believed 

 to be methylglyoxal, but we now know that it is an equilibrium mixture of 

 phosphoglyceraldehyde and phosphodihydroxyacetone. We know today 

 how these trioses are produced from glucose. Alcoholic fermentation takes 

 place in the following stages : 



1. In the presence of hexokinase, a molecule of ATP reacts with glucose 

 to form G — 6 — P. This reaction is a transphosphorylation having a large 

 negative AF. Equilibrium is as follows : 



ATP + glucose -> ADP + G— 6— P 



1% 99% 



Hence it is an almost irreversible reaction in which the energy of the ATP 

 bond is lost almost completely. 



2. In the presence of phosphoglucoisomerase, G — 6 — P is transformed 

 rapidly into F — 6 — P and the equilibrium of this reversible reaction is : 



G— 6— P ^ F— 6— P 



70% 30% 



3. Phosphofructokinase, in the presence of Mg++, and very specifically, 

 catalyses the transfer of the terminal group of ATP to F — 6 — P with 

 formation F — 1, 6 — PP. Here also, as in 1, we have a transphosphorylase 



