Ribonucleosides 5’-phosphates

A route has been developed for the synthesis of ribonucleoside 2 -phosphates 179, which could be incorporated into oligonucleotides. The t-butyl esters could be removed under mild acidic conditions without cleavage of internucleotidic links. 

Another dinuclear Mn enzyme with quite a different functionality is MnRNR. Cell replication and deoxyribonucleic acid (DNA) formation require the reduction of the 2 hydroxyl group of a ribonucleoside phosphate [13a]. 

Lupinus luteus (29,30). Cytokinins also occur naturally at the ribonucleoside and ribonucleoside phosphate levels (31,32,33) and exogenously applied cytokinins have been shown to undergo metabolism to glucosyl derivatives (34,35,36,37,38). 

Inosinic acid (hypoxanthine ribonucleoside phosphate) is the end product of purine biosynthesis. It then gives rise to GMP and AMP. 

Purine and pyrimidine bases + D-riboses —> D-fS-ribonucleosides + phosphate —> D-f-ribonucleotides —> RNA —> Modified RNA... 

Adenylic acids o and b were each converted by trifluoroacetic anhydride into the same 2 3 -cyclic phosphate (13, R = adenin-9-yl) by Brown, Magrath, and Todd. These authors also applied this method to the synthesis of 2 3 -cyclic phosphates (13) of cytidine, uridine, and guanosine. Hydrolysis of these cyclic phosphates gave the corresponding a and b nucleotide mixtures. Markham and Smith found that, during hydrolysis of ribonucleic acid with pancreatic ribonuclease, the 2 3 -cyclic phosphates of the pyrimidine nucleosides are formed as intermediates leading to the ribonucleoside phosphates b. The also showed that 2 3 -cyclic phosphates (13) are formed by very mild, alkaline hydrolysis of ribonucleic acid. The discoveries of these a and b isomers of mononucleotides from... 

Nucleoside 5 phosphates show no tendency to isomerise, and the 2 and 3 isomers are stable under alkaline conditions. Cyclic 2, 3 ribonucleoside phosphates can be prepared from either the 2 or 3 isomers using a dehydrating agent such as dicyclohexyl dicarbodiimide. The latter reagent can be used to prepare cyclic 3, 5 nucleoside phosphates from nucleoside 5 phosphates. 

This facile distribution of phosphate among the high-energy phosphates of the four ribonucleoside phosphate families may be explained by these processes ... 

The deoxyribonucleotides are derived primarily, if not entirely, by reduction of ribonucleoside phosphates each of the several known ribonucleotide reductases (see Chapter 16) accepts as substrates phosphate esters of all four ribonucleosides, adenosine, guanosine, cytidine, and uridine. The ultimate fate of the uracil-containing reduction product, dUDP (or dUTP, in some microorganisms) is conversion to thymidine phosphates by way of deoxyuridylate and the thymidylate s mthetase reaction. 

Azhayev et al. (95) describes the synthesis of phosphoramidate analogs of ribonucleoside phosphates giving values for 5+ compounds using ready for use Silufol UV2S4 silica gel sheets and two solvents ... 

Synthetic capped mRNAs are useful tools to study all of the processes mentioned previously. To create capped mRNAs, DNA templates are transcribed with either a bacterial (Contreras etal., 1982) or bacteriophage (Konarska et al., 1984 Yisraeli and Melton, 1989) RNA polymerase in the presence of all four ribonucleoside triphosphates and a synthetic cap dinucleotide, m7Gp3G. The polymerase initiates transcription with a nucleophilic attack by the 3 -OH of the Guo moiety ofm7Gp3G on the a-phosphate... 

Hydrolysis of ribonucleic acids by snake venom was found to yield inorganic phosphate, nucleosides, and pyrimidine ribonucleoside diphosphates.197 These diphosphates were shown by their behavior toward various enzymes to be mixtures of 2,5- and 3,5-diphosphates, and it therefore seems likely that they were formed through intermediate, cyclic phosphates. Thus, although this evidence confirms the existence of 2(or 3) — 5 linkages, it does not distinguish between the 2- and 3-positions. 

Selective reaction at the ci.s-2,3-diol grouping of unprotected D-ribonucleosides has occasionally been observed. Treatment of D-ribonucleosides with tris(tetramethylammonium) trimetaphosphate in M sodium hydroxide for 4 days at room temperature led to a mixture of nucleoside 2 - and 3 -phosphates in yields of >70% no 5 -phosphate was detected.213 Reaction of ethyl (trichloromethyl)phos-phonate with nucleosides in N,N-dimethylformamide containing triethylamine, followed by basic hydrolysis of the reaction product, yielded 2 (3 )-phosphates in variable yields.214 The participation of the cis-diol grouping in the reaction was suggested by the failure of thymidine or 2, 3 -0-isopropylideneuridine to undergo reaction. 

A nucleoside consists of a purine or pyrimidine base linked to a pentose, either D-ribose to form a ribonucleo-side or 2-deoxy-D-ribose to form a deoxyribonucleoside. Three major purine bases and their corresponding ribo-nucleosides are adenine/adenosine, guanine/guanosine and hypoxanthine/inosine. The three major pyrimidines and their corresponding ribonucleosides are cytosine/ cytodine, uracil/uradine and thymine/thymidine. A nucleotide such as ATP (Fig. 17-1) is a phosphate or polyphosphate ester of a nucleoside. 

The deoxyribonucleotides, except for deoxythymidine nucleotide, are formed from the ribonucleotides by the action of an enzyme complex, which comprises two enzymes, ribonucleoside diphosphate reductase and thioredoxin reductase (Figure 20.11). The removal of a hydroxyl group in the ribose part of the molecule is a reduction reaction, which requires NADPH. This is generated in the pentose phosphate pathway. (Note, this pathway is important in proliferating cells not only for generation... 

Scheme 9.5 Multi-step enzymatic process for 2 -deoxyribo-nucleoside production from glucose, acetaldehyde and a nucleobase through glycolysis, reverse reactions of 2 -deoxy-ribonucleoside degradation and ATP regeneration by the yeast glycolytic pathway recycling the phosphate generated by nucleoside phosphorylase.

The reaction catalyzed by polynucleotide phosphorylase differs fundamentally from the polymerase activities discussed so far in that it is not template-dependent. The enzyme uses the 5 -diphosphates of ribonucleosides as substrates and cannot act on the homologous 5 -triphos-phates or on deoxyribonucleoside 5 -diphosphates. The RNA polymer formed by polynucleotide phosphorylase contains the usual 3, 5 -phosphodiester linkages, which can be hydrolyzed by ribonuclease. The reaction is readily reversible and can be pushed in the direction of breakdown of the polyribonucleotide by increasing the phosphate concentration. The probable function of this enzyme in the cell is the degradation of mRNAs to nucleoside diphosphates. 

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