The biosynthesis of pentoses and their 

 incorporation into mononucleotides 



by 



HANS KLENOW 



Universitetets Institutfor Cytofysiologi, Kabenhavn 



The importance of mononucleotides both as building-blocks of the nucleic acids and 

 as constituents of a number of coenzymes for reactions in intermediary metabolism 

 is generally accepted. An understanding of the mechanism by which the mono- 

 nucleotides are formed might therefore be of significance for the explanation of 

 various biological phenomena. I should like to discuss possible pathways by which 

 mononucleotides may be formed, and also to mention the present evidence for the 

 pathways of the biosynthesis of the sugar part of the nucleotides, i.e. the ribose. 



In the last few years considerable knowledge has accumulated about enzyme 

 reactions leading to ribose phosphate formation. These new facts have been obtained 

 mainly from experiments on the oxidative breakdown of carbohydrates. By this term 

 we are accustomed to mean the extremely important oxidative cycle of Krebs. The 

 existence of an alternative pathway of carbohydrate oxidation was, however, indi- 

 cated by work of Warburg and Christian (1937), Lipmann (1936) and Dickens 

 ( J 936). 



Glucose-6-phosphate 



6-Phosphogluconic acid 



Fructose-6- phosphate -j-Tetrose phosphate 



D-glyceraldehyde-3-phosphate 



+ 



Sedoheptulose-7-phosphate 



Figure 1 . The oxidative cycle. 

 67 



Ribulose-5-phosphate 



Ribose-5-phosphate 



