COENZYMES 



57 



to be two reactions catalysed by separate enzymes, alcohol dehydrogenase and 

 aldehyde dehydrogenase 



alcohol dehydrogenase 



CH,CHO 



CH,CH,OH 



aldehyde dehydrogenase 



CH3CHO > CH3COOH 



The aldehyde dehydrogenase is a diphosphopyridine-nucleotide-Hnked enzyme 

 (Racker, 1949). The apparent absence of alcohol dehydrogenase previously reported 

 was due to the accidental removal of the coenzyme diaphorase during the purification. 



COENZYMES 



Yeast juice when dialysed no longer ferments glucose and from the dialysate 

 coenzyme I can be isolated. This is a compound of nicotinamide, ribose, pyro- 

 phosphoric acid and adenine. Coenzyme II, isolated from erythrocytes has a similar 

 structure to Coenzyme I, but it contains two phosphate residues. The Coenzymes I 

 and II ow^e their biological function to reversible oxidation-reduction of the pyridine 

 ring. A transient intermediate form in the reduction has been deduced and might be 

 represented as follows (Waters, 1948) : — 



CONH2 ^^^^ONH2 



R 

 Co-enzyme 



Not only is Coenzyme I essential for yeast fermentation as mentioned above, but 

 it is also necessary for the glycolysis of sugars in muscle, etc. The coenzyme is 

 reduced when diphosphoglyceraldehyde is oxidised to diphosphoglyceric acid in the 

 presence of triosephosphate dehydrogenase and reoxidised when pyruvic acid is 

 reduced to lactic acid later on in the glycolytic cycle, or when acetaldehyde is reduced 

 to ethyl alcohol during alcoholic fermentation (as described in the next chapter). 



In the following table (from Ochoa, 1946) are summarised the oxidation-reduction 

 potentials of a number of dehydrogenase systems in which coenzymes I and II 

 co-operate. 



TABLE 10 

 Pyridine Nucleotide Dehydrogenases 



