Deoxyribonucleoside diphosphates

REDUCTION OF RIBONUCLEOSIDE DIPHOSPHATES FORMS DEOXYRIBONUCLEOSIDE DIPHOSPHATES... 

Figure 34-5. Reduction of ribonucleoside diphosphates to 2 -deoxyribonucleoside diphosphates.

In the preceding sections the conversion of purines and purine nucleosides to purine nucleoside monophosphates has been discussed. The monophosphates of adenosine and guanosine must be converted to their di- and triphosphates for polymerization to RNA, for reduction to 2 -deoxyribonucleoside diphosphates, and for the many other reactions in which they take part. Adenosine triphosphate is produced by oxidative phosphorylation and by transfer of phosphate from 1,3-diphosphoglycerate and phosphopyruvate to adenosine diphosphate. A series of transphosphorylations distributes phosphate from adenosine triphosphate to all of the other nucleotides. Two classes of enzymes, termed nucleoside mono-phosphokinases and nucleoside diphosphokinases, catalyse the formation of the nucleoside di- and triphosphates by the transfer of the terminal phosphoryl group from adenosine triphosphate. Muscle adenylate kinase (myokinase)... 

Ribonucleotide reductase (diphosphate) [EC 1.17.4.1], also known as ribonucleoside-diphosphate reductase, catalyzes the reaction of a 2 -deoxyribonucleoside diphosphate with oxidized thioredoxin and water to produce a ribonucleoside diphosphate and reduced thioredoxin. This system requires the presence of iron ions and ATP. Ribonucleotide reductase (triphosphate) [EC... 

DEOXYRIBONUCLEASES 2 -Deoxyribonucleoside diphosphate, RIBONUCLEOTIDE REDUCTASE 2-DEOXYRIBOSE-5-PHOSPHATE ALDOLASE DEOXYTHYMIDINE KINASE DEP,... 

Nucleoside diphosphate kinase is relatively nonspecific enzyme that transfers a phosphoryl group from a variety of nucleoside triphosphates to nucleoside diphosphates. Its biological function is presumably to use ATP to phosphoryl-ate the various ribo- and deoxyribonucleoside diphosphates to form the triphosphate derivatives need by the cell. 

Inhibition of Conversion of Ribonucleoside Diphosphate to Deoxyribonucleoside Diphosphate... 

NDPK uses energy of ATP to make triphosphates of ribonucleoside diphosphates and deoxyribonucleoside diphosphates in both de novo and salvage biosynthesis. 

Synthesis of deoxythymidine nucleotides occurs differently from that of the other dNTPs, which are derived directly from a ribonucleotide reductase-catalyzed conversion of ribonucleoside diphosphates to deoxyribonucleoside diphosphates (Figure 22.12). The terms thymidine and deoxythymidine (or dTTP and TIP) generally refer to the deoxyribonucleotide, because the ribonucleotide is not a normal metabolite. In the rare instances where the ribonucleotide of thymidine occurs, it is usually designated with an r preceding it, as in rTTP. 

Ribonucleoside diphosphates are reduced to 2 -deoxyribonucleoside diphosphates in all organisms (Figure 23.31a) NADPH is the reducing agent. 

The actual process, which is catalyzed by ribonucleotide reductase, is more complex than the preceding equation would indicate and involves some intermediate electron carriers. The ribonucleotide reductase system from E. coli has been extensively studied, and its mode of action gives some clues to the nature of the process. Two other proteins are required, thioredoxin and thioredoxin reductase. Thioredoxin contains a disulfide (S—S) group in its oxidized form and two sulfhydryl (—SH) groups in its reduced form. NADPH reduces thioredoxin in a reaction catalyzed by thioredoxin reductase. The reduced thioredoxin in turn reduces a ribonucleoside diphosphate (NDP) to a deoxyribonucleoside diphosphate (dNDP), shown in Figure 23.31b, and this reaction is actually catalyzed by ribonucleotide reductase. Note that this reaction produces dADP, dGDP, dCDP, and dUDP. The first three are phosphorylated to... 

FIGURE 23.31 Conversion of ribonucleoside diphosphates to deoxyribonucleoside diphosphates, (a) The (—S—S—) / (—SH HS—) oxidation-reduction cycle involving ribonucleotide reductase, thioredoxin, thioredoxin reductase, and NADPH. (b) The structures of NDP and dNDP. 

What is conversion of ribonucleotides to deoxyri-bonucleotides Deoxyribonucleotides for DNA synthesis are produced by the reduction of ribonucleoside diphosphates to deoxyribonucleoside diphosphates. 

Hydroxyurea inhibits ribonucleotide reductase. By sequestering ferric ions, hydroxyurea destabilizes the organic bee radical in the R2 subunit of the enzyme. The inhibition of enzyme activity leads to a depletion of deoxyribonucleoside diphosphates, which are normally converted to deoxyribonucleoside triphosphates, the substrates for DNA synthesis. 

A number of conjugated deoxyribonucleoside diphosphates are known, although those knoum are less numerous than similar compounds of the ribose series. For example, dCDP-choline and dCDP-ethanolamine have been found in sea urchin eggs and in animal tissues, but their significance is unknown. Various thymidine diphosphate sugars (see Table l-I) have been isolated from bacterial cells these compounds ser e as coenzymes in the synthesis of polysaccharides in Salmonella typhimurium. 

Ribonucleotide Reductase. The ribonucleotide reductases catalyze the reduction of ribonucleoside-diphosphates (or triphosphates) to the corresponding 2 -deoxyribonucleoside-diphosphates (or triphosphates), processes of preeminent importance for the biosynthesis of DNA (see Table 2, entry 4) (65,86). A variety of metal-containing cofactors have been discovered in the ribonucleotide reductases investigated to date (eg, a binuclear iron center in the mammalian and in the E. coli ribonucleoside diphosphate reductase) and the oxidation of two protein thiols to a disulfide unit is indicated as the direct source of the two reduction equivalents. The reductase from Lactobacillus leichmanii employs coenzyme B12 as cofactor in its (normal) base-on form and acts on purine- or pyrimidine-based ribonucleoside-triphosphates. Its crystal structure reveals not only the arrangement of the bound corrinoid cofactor, but also how the enzyme is... 

Both the 2 -chloro- and the 2 -azido-2 -deoxyribonucleoside diphosphates are bound to the substrate binding site of the B1-B2 complex as L. Thelander, J. Biol. Chem. 249, 4858 (1974). 

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