132 THE BIOCHEMISTRY OF B VITAMINS 



systems derived from each of these three cofactors have characteristic, 

 but widely differing, potentials. Because of this difference, one type of 

 cofactor is usually much more suitable for a particular reaction than are 

 the other two. The enzyme systems containing nicotinic acid have the 

 lowest potential, those containing riboflavin intermediate, and those re- 

 lated to porphyrins the highest. These vitamin-containing systems either 

 become linked with one another or with a specific metabolite system, thus 

 establishing an oxidation-reduction reaction. Such reactions are respon- 

 sible for most of the biological oxidations which take place. A more 

 detailed account of individual reactions will be discussed in connection 

 with their specific coenzymes. 



Coenzymes Containing Nicotinic Acid 



The first organic coenzyme to be recognized (1904) was a heat-stable 

 factor which activated preparations of zymase, the complex of enzymes 

 in yeast which catalyzes the alcoholic fermentation of carbohydrates. 14 

 This cofactor was designated "cozymase." Its chemical constitution, how- 

 ever, was not established until much later, after a second coenzyme, 

 coenzyme II (the codehydrogenase of Zwischenferment) , had been dis- 

 covered, isolated, and shown to be a derivative of nicotinamide (1934) , 15 

 (Zwischenferment, an enzyme of historical interest, dehydrogenates glu- 

 coses-phosphate.) A year later nicotinamide was also isolated from 

 cozymase and the chemical relationship of the two coenzymes was thus 

 established. 16 



NH 2 



i 



CH N 



HC C CONH 2 /? C N 



II I HC || I 



HC CH \ C CH 



N N N 



HC 1 HC 1 



HCOH I HCOH 

 HCOH I HCOH 

 HC 1 O O HC 







111 

 H 2 C— O P O P O CH 2 



O- OH 



Diphosphopyridine nucleotide (Cozymase) 



Upon hydrolysis the two coenzymes yield identical products; however, 

 only two mols of phosphoric acid are present in cozymase, while three 



