Ribose/phosphate reaction sugars

In the data of Table 14-1 it is seen that when uridine was the sole pyrimidine source, the specific activities of the sugar portions of the RNA pyrimidine nucleotides were the same as those derived from DNA. This means that the pyrimidine deoxyribonucleotides were synthesized exclusively from the same precursors as the pyrimidine ribonucleotides. There was some reduction in the isotope content of the pentose, but none in that of the base this apparently came about through exchange with cellular nonisotopic ribose phosphates by way of the reversible reaction of uridine phosphorylase. 

Pentoses. The names and formulas of the aldopentdses may be found in Section 1. Ribose and ribose phosphates are components of the nucleic acids and the nucleotide coenzymes. In these derivatives the furanose form has been proved to be present, whereas free ribose exists in the pyranose form (formulas in Section 2). Deoxy-ribose (formula in Chapt. VII) is responsible for the name of the deoxyribonucleic acids. The free sugars are in equilibrium with the aldehyde form, which in this case can be demonstrated with fuchsin sulfurous acid. The lack of a hydroxyl group on C-2 is the basis of this reaction (used in Feulgen s nuclear staining technique). 

In fact, it has been found (52) that in unbuffered solution, at room temperature, authentic 2-deoxy ribose 5-phosphate reduces more than 4 molar equivalents of periodate, but. that there is no noticeable slowing down of the reaction rate after the reduction of the first molar equivalent. This may be owing to the fact that only the aldehydo form (76) of 2-deoxy ribose 5-phosphate has a free vicinal diol group as the acyclic 2-deoxy ribitol 5-phosphate reduces one molar equivalent of periodate quite fast (58), it is probable that the time-curve of periodate uptake by the phosphorylated sugar reflects the rate of formation of the aldehyde form from the furanose form. 

Many kinds of organisms and some mammalian organs, notably liver, possess an alternative pathway for the oxidation of hexoses which results in a pentose phosphate and carbon dioxide. This pentose can be used as a precursor of the ribose found in nucleic acids or other sugars containing from three to seven carbon atoms which are needed in smaller amounts. The first and third reactions in the pentose phosphate pathway generate NADPH which is a major source of reducing power in many cells. 

In the presence of molecular oxygen, C-C bond fragmentation occurs (see Sects. II,4b and III,2b). In DNA, this process leads to the release of a 2-deoxytetrodialdose (127) (see Scheme 20) and to the same sugar bound247 to DNA (128) (see Scheme 20). The mechanism of the C-C bond-scission, and the subsequent reactions to give 128, have been discussed for the model compound D-ribose 5-phosphate... 

Measurements of the crude specific activity (mmoles of product synthesized per minute per mg of protein in the supernatant after a 50,000 x g centrifugation) of the two isomerases in E. coli indicated that the conversion of D-ribulose-5-phosphate to D-ri-bose-5-phosphate was approximately 20- to 30-fold greater than the conversion of D-ribulose-5-phosphate to D-arabinose-5-phosphate. This rate of reaction strongly pulls the reaction substrate to D-ribose-5-phosphate, since the isomerase reaction at equilibrium strongly favors the formation of the aldo-sugar over the key intermediate D-ribulose-5-phosphate. 

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