NUCLEOPROTEINS, NUCLEIC ACIDS, RELATED SUBSTANCES 



159 



Besides the monophosphate, adenosine forms a diphosphate, ADP, and 

 a triphosphate, ATP. The three derivatives of adenosine play an out- 

 standing role in enzyme chemistry and intermediary metabolism. 



Coenzymes I and II. These compounds are dinucleotides of adenine 

 and the base, nicotinamide. Coenzyme I is also known as diphospho- 

 pyridine nucleotide (DPN) , and coenzyme II as triphosphopyridine nu- 

 cleotide (TPN). The make-up of DPN can be seen from the following 

 designation: 



O 



II 

 Adenine-ribose — — P — OH 



I 



O 

 I 

 Nicotinamide-ribose — — P — OH 



II 

 O 



Note that the two nucleotides are joined through the phosphoric acid 

 molecules, instead of the pentose-phosphoric acid-pentose structure found 

 in nucleic acids. The phosphoric acid groups are linked to carbon-S' 

 of the ribose units. 



TPN is like DPN except that it has a third phosphoric group attached 

 to carbon-2' of the ribose found in the adenine nucleotide. For the struc- 

 tural formulas of DPN and TPN see p. 276. 



Flavin Nucleotides. There is a so-called mononucleotide, riboflavin 

 phosphate, and a dinucleotide of riboflavin phosphate and adenylic acid. 

 The riboflavin phosphate is not a true nucleotide because the ribose part 

 is replaced by the sugar alcohol corresponding to ribose, viz., ribitol. The 

 difference in structure is evident from the formula on p. 278. The flavin 

 nucleotides are coenzymes, and a discussion of their function will be 

 given in the chapter on enzymes. 



5,Q-Dimethylbenzimidazole Riboside. This nucleotide and the corre- 

 sponding nucleoside have been obtained as degradation products of vita- 

 min Bi2. The structural formula is 



OH 

 I 

 O— P=0 



B.zC^((i) 



(1) 





HaC (5) 



<WA 



(4) 



N- 



<3) 



/ 



CH 



0- 



OH 



OH 



-C— C— C— C-CH2OH 

 H H H H 



a') (2') (3') (4') (5') 



Note that the ribose has a furanose, instead of a pyranose, structure. 

 In the formula, the phosphoric acid is attached to carbon 3' of the ribose, 

 but the point of attachment is uncertain. It may be at carbon 2'. 



