Conversion of Nucleoside Monophosphates to Diphosphates and Triphosphates

The triphosphates of nucleosides and deoxynucleo-sides are substrates for RNA polymerases and DNA 

The diphosphates are converted to the triphosphates by the ubiquitous enzyme nucleoside diphosphate kinase. Remarkably, the lack of base or sugar specificity applies to the phosphate acceptor and the phosphate donor. Hence, the general reaction is 

Recall that conversion of ADP to ATP occurs mostly by mitochondrial oxidative phosphorylation coupled to electron transport (Chapter 14). 

Conversion of ribonucleotides to the deoxy forms occurs exclusively at the diphosphate level. Ribonucleo-side diphosphate reductase (ribonucleotide reductase) catalyzes the reaction. This enzyme is found in all species and tissues. The immediate source of reducing equivalents is the enzyme (E) itself in which two sulfhydryl groups are oxidized to a disulfide. The general reaction is 

In this reaction, the 2 -hydroxyl group of ribose is replaced by a hydrogen, with retention of configuration at the 3 -carbon atom  

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