2 -deoxyribosyl groups

The difficulty in the chemical synthesis of 2 -deoxyribonucleoside lies in the generation of 2-deoxyribosyl groups. The chemical syntheses of 2-deoxyribosyl groups and further to 2 -deoxyribonucleosides involve complex protection and deprotection steps [8-11]. It is likely that the introduction of biochemical reactions with... 

Phosphate ion (PO4 ) Phosphoryl group (PO ) Pyrophosphate ion Pyrophosphoiyl group Ribosyl group 2-Deoxyribosyl group... 

Abbreviations for ribonucleosides and 2 -deoxyribonucleosides are derived from those for the bases plus those for the ribosyl or 2 -deoxyribosyl groups. Thus AR stands for adenosine, UR for uridine, and BR for any ribonucleoside AdR stands for deoxyadenosine, UdR for deoxyuridine, and BdR for any deoxyribonucleoside. When used in abbreviations for nucleotides, N also stands for any ribonucleoside and dN for any deoxyribonucleoside. 

The first step, deoxyribonucleoside synthesis, has been demonstrated in vitro with purified preparations of purine ribonucleoside phosphorylase and thymidine phosphorylase (11-13). However, it is uncertain whether a significant incorporation of base occurs in cells by this route, without an extracellular source of deoxyribosyl groups (see below). The possibility that deoxyribose 1-phosphate may be generated endogenously by way of deoxyriboaldolase is discussed in Section IV. 

Purine nucleoside phosphorylase and thymidine phosphorylase have been shown to catalyze transfer of the deoxyribosyl group from one base to another by reaction sequences that involve the intermediate formation of free deoxyribose 1-phosphate by the following mechanism ... 

By catalyzing both reactions (1) and (2) above, thymidine phosphorylase will transfer the deoxyribosyl group between thymine and uracil (note that cytosine is not a substrate). For example, the following transfer has been demonstrated with thymidine phosphorylase of human leukocytes (17) ... 

Direct Transfer of Deoxyribosyl Groups by Thymidine Phosphorylase... 

Thymidine phosphorylases of both animal celts and E. coli catalyze this direct transfer of deoxyribosyl groups between thymine and uracil. 

Thymidine monophosphate kinase, which has a high specificity for the deoxyribosyl group, is discussed separately below. 

The concept that the deoxyribosyl groups of DNA nucleotides are derived by reduction of ribonucleoside derivatives is based on a wide array of experimental evidence. 

Extracts of certain bacteria requiring 2-deoxyribonucleosides, namely, Lactobacillus helveticus, Lactobacillus delbriickii, and Thermobacterium acidophilus R 26, catalyze the transfer of the 2-deoxy-D-ribosyl group from one purine or pyrimidine to another. " 2-Deoxyribosyl -uracil, thy-... 

The steric course of the above deoxyribosylations might be explained by the formation of the cation (52) which is readily attacked by a nucleophile from the a-position. An attack from the opposite side is hindered by the bulky p-toluyl group at position 5 of the sugar moiety. Moreover, the cation (52) might be stabilized by a transannular participation of the C-5-p-toluyl group with the formation of the... 

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