162 



S. S. COHEN 



In this reaction, glucose held in 1, 4a-glycosidic hnkage is transferred to 

 inorganic orthophosphate, thereby preservmg the energy of the organic 

 linkage. The group transfer potential of the C — — P linkages of glucose- 

 1- phosphate is approximately the same as that of the glucosidic hnkages of 

 the polysaccharides; glycogen (glucose) „ + P ^ a-glucose-l-P+ (glucose)„_j, 

 as shown in formula (XX). 



CH,OH 



CH,OH 



-o- 



-o- 



(XX) 



Phosphorylytic cleavages by specific enzymes have been observed for only 

 a few other saccharides, such as sucrose and maltose: 



sucrose -|- H3PO4 ^ 

 maltose -(- H3PO4 



a-glucose-1 -phosphate -|- fructose 

 -jS-glucose-1 -phosphate -\- glucose 



Comparable reactions also exist for the cleavage of N-glycosides, which 

 exist in purine and pyrimidine nucleosides. Separate enzymes are responsible 

 for these activities; indeed, in E. coli, separate pyrimidine nucleoside phos- 

 phorylases exist for the ribosides and deoxyribosides. Of particular signific- 

 ance to our discussion has been the recent discovery of the phosphorylytic 

 cleavage of the nucleotide phosphodiesters of RNA. The equation for this 

 reaction which will be considered below in greater detail can be represented 

 as follows: 



(ribonucieoside-P)„ + P ^ ribonucIeoside-P-P + (ribonuc!eoside-P)„^i 



It is clear then that the phosphoryljrtic and pyrophosphorylytic reactions 

 indicated above are of the greatest importance. They are involved not only 

 in the mobihzation and activation of small molecules so that they may 

 participate in polymer sjmtheses but also in the terminal steps of the polymer 

 biosyntheses themselves. 



