Ribose 5-phosphate, pentose

D-Ribose Nucleic acids. Structural elements of nucleic acids and coenzymes, eg, ATP, NAD, NADP, flavo-proteins. Ribose phosphates are intermediates in pentose phosphate pathway. ... 

THE PENTOSE PHOSPHATE PATHWAY GENERATES NADPH RIBOSE PHOSPHATE (Figure 20-1)... 

Those nucleosides found in the nucleic acids DNA and RNA involve the joining of ribose of deoxyribose to a purine or a pyrimidine base. One such nucleoside is adenosine, in which a nitrogen of adenine is linked to carbon 1 of the pentose, ribose. In this form it is a component of RNA but as a phosphory-lated derivative of adenosine (e.g. ATP), which is a high energy compound, it fulfils an important role in metabolism. The dinucleotides NAD and NADP are two cofactors necessary for many enzymic transformations and these also contain /V-glycosides of ribose phosphate. Other important nucleosides are found... 

The routes involved in the formation of the various furan sulphides and disulphides involve the interaction of hydrogen sulphide with dicarbonyls, furanones and furfurals. Possible pathways are shown in Scheme 12.8. Furanthiols have been found in heated model systems containing hydrogen sulphide or cysteine with pentoses [56-58]. 2-Methyl-3-furanthiol has also been found as a major product in the reaction of 4-hydroxy-5-methyl-3(2H)-furanone with hydrogen sulphide or cysteine [21, 59]. This furanone is formed in the Maillard reaction of pentoses alternatively it has been suggested that it may be produced by the dephosphorylation and dehydration of ribose phosphate, and that this may be a route to its formation in cooked meat [21, 60]. 

PRPP is synthesized from ribose 5-phosphate derived from the pentose phosphate pathway (see Fig. 14-21), in a reaction catalyzed by ribose phosphate pyro-phosphokinase ... 

PRPP is an "activated pentose" that participates in the synthesis of purines and pyrimidines, and in the salvage of purine bases (see p. 294). Synthesis of PRPP from ATP and ribose 5-phosphate is catalyzed by PRPP synthetase (ribose phosphate pyrophosphokinase, Figure 22.6). This enzyme is activated by inorganic phosphate (Pi) and inhibited by purine nucleotides (end-product inhibition). [Note The sugar moiety of PRPP is ribose, and therefore ribonucleotides are the end products of de novo purine synthesis. When deoxy-ribonucleotides are required for DNA synthesis, the ribose sugar moiety is reduced (see p. 295).]... 

Activated ribose phosphate. Write a balanced equation for the synthesis of PRPP from glucose through the oxidative branch of the pentose phosphate pathway. 

Pentoses are of prime importance for contemporary organisms as structural components of nucleic acids. The existence of the primordial RNA world, in which RNA is suggested to act as a catalyst as well as an informational macromolecule, assumes a large prebiotic source of ribose. Alternatively, the possible existence of pre-RNA molecules with backbones different from ribose phosphate has been considered [3]. 

Amongst the ribose phosphates (12) the 5 isomer is utilised in forming the all-important nucleotides (Section 10.4). It is also an intermediate in the Calvin cycle (Figure 11.19) and in the pentose phosphate pathway (11.69). 

Ribose phosphates phosphorylated derivatives of ribose. Ribose is phosphorylated in position 5 by the action of ribokinase (EC 2.7.1.15) and ATP ribose 5-phosphate is also produced in the Pentose phosphate cycle (see), and in the Calvin c cle (see) of photosynthesis. Phosphoribomutase cat yses the interconversion of ribose 5-phospbate and ribose 1-phosphate, and the cosubstrate of this reaction is ribose l,5-f>isphosphate. 5-Phosphoribosyl 1-pyrophos-phate donates a ribose 5-phosphate moiety in the de novo biosynthesis of purine and pyrimidine nucleotides (see Purine biosynthesis. Pyrimidine biosynthesis), in the Salvage pathway (see) of purine and pyrimidine utilization, in the biosynthesis of L-Histi-dine (see) and L-Tryptophan (see) and in the conversion of nicotinic acid into nicotinic acid ribotide (see Pyridine nucleotide cycle). Ribose 1-phosphate can also take part in nucleotide synthesis (see Salvage pathway). 

The pathways by which ribose phosphates are formed will be considered in two stages the initial formation of pentose phosphates in general, and the formation of PP-ribose-P and ribose-l-P. 

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. 

Having defined the oxidative steps for hexosemonophosphate metabolism in yeast, we asked whether these reactions could also be demonstrated in animal tissues. E. J. Seeg-miller, who joined my group in 1950, used our coupled system to study the oxidation of 6-phosphogluconate with rat liver extracts. Not only did he identify ribulose and ribose phosphates as products of the reaction, but he also observed that with continued incubation the pentose phosphates disappeared, and in their place hexosemonophosphate accumu-lated.f > This was the first indication that we were dealing with a cyclic process, in which pentose phosphate formed by the oxidation of hexosemonophosphate could be converted... 

Recent studies on xylose metabolism in cell-free extracts of Laclobaeillua pentosus tend to confirm this mechanism, at least until the stage of phosphorylation of xylulose. The accumulated pentose phosphate, however, included 70 to 80% of ribose phosphate and a ketose fraction rich in ribulose-5-phosphate. n-xylulose-5-phosphate was not detected but may have been a transitory intermediate. The mechanism of formation of compounds of the ribose series from xylose is not clear at present. (Lampen, J. 0., J. Biol. Chem. 204, 999 (1953).)... 

The origin of ribose-5-phosphate from ribulose- phosphate has been discussed, as well as the degradation of these pentose phosphates to smaller fragments. The problem of forming ribose phosphate arises in at least two other conditions (1) the utilization of ribose added as a nutrient for microbial growth, and (2) the generation of ribose from nucleosides, either directly by the hydrolytic cleavage of a nucleoside, as in... 

RNA is synthesized in the latter case, and since the DNA synthesis is stimulated about fourfold, there appears to be a shunt of ribose phosphate synthesis to desoxyribose phosphate synthesis in virus infection. Since nucleic acid P accounts for about 80% of the phosphorus of the cell, it is evident that this redistribution of pentose P involves a major redistribution of phosphorus metabolism. 

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). 

This enzyme interconverts ribulose-5-P and ribose-5-P via an enediol intermediate (Figure 23.30). The reaction (and mechanism) is quite similar to the phosphoglucoisomerase reaction of glycolysis, which interconverts glucose-6-P and fructose-6-P. The ribose-5-P produced in this reaction is utilized in the biosynthesis of coenzymes (including N/ DH, N/ DPH, F/ D, and Big), nucleotides, and nucleic acids (DNA and RNA). The net reaction for the first four steps of the pentose phosphate pathway is... 

Even when the latter choice has been made, however, the cell must still be cognizant of the relative needs for ribose-5-phosphate and N/VDPH (as well as ATP). Depending on these relative needs, the reactions of glycolysis and the pentose phosphate pathway can be combined in novel ways to emphasize the synthesis of needed metabolites. There are four principal possibilities. 

BOTH RIBOSE-5-P AND NADPH ARE NEEDED BY THE CELL In this case, the first four reactions of the pentose phosphate pathway predominate (Figure 23.37). N/VDPH is produced by the oxidative reactions of the pathway, and ribose-5-P is the principal product of carbon metabolism. As stated earlier, the net reaction for these processes is... 

MORE RIBOSE-5-P THAN NADPH IS NEEDED BY THE CELL Synthesis of ribose-5-P can be accomplished without production of N/VDPH if the oxidative steps of the pentose phosphate pathway are bypassed. The key to this route is the extrac-... 

FIGURE 23.37 Wlien biosynthetic demands dictate, the first four reactions of the pentose phosphate pathway predominate and the principal products are ribose-5-P and NADPH. 

MORE NADPH THAN RmOSE-5-P IS NEEDED BY THE CELL Large amounts of N/VDPH can be supplied for biosynthesis without concomitant production of ribose-5-P, if ribose-5-P produced in the pentose phosphate pathway is recycled to produce glycolytic intermediates. As shown in Figure 23.39, this alternative involves a complex interplay between the transketolase and transaldolase reac-... 

FIGURE 23.38 The oxidative steps of the pentose phosphate pathway can be bypassed if the primary need is for ribose-5-P. 

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