5-Phosphoribosyl-1 -pyrophosphate purine biosynthesis

The first step of this sequence, which is not unique to de novo purine nucleotide biosynthesis, is the synthesis of 5-phosphoribosylpyrophosphate (PRPP) from ribose-5-phosphate and adenosine triphosphate. Phosphoribosyl-pyrophosphate synthetase, the enzyme that catalyses this reaction [278], is under feedback control by adenosine triphosphate [279]. Cordycepin interferes with thede novo pathway [229, 280, 281), and cordycepin triphosphate inhibits the synthesis of PRPP in extracts from Ehrlich ascites tumour cells [282]. Formycin [283], probably as the triphosphate, 9-0-D-xylofuranosyladenine [157] triphosphate, and decoyinine (LXXlll) [284-286] (p. 89) also inhibit the synthesis of PRPP in tumour cells, and this is held to be the blockade most important to their cytotoxic action. It has been suggested but not established that tubercidin (triphosphate) may also be an inhibitor of this reaction [193]. 

Synthesis of 5 phosphoribosylamine from PRPP and glutamine is catalized by glutamine phosphoribosyl pyrophosphate amidotransferase. This enzyme is inhibited by the purine 5 -nucleotides, AMP, GMP, and IMP—the end-products of the pathway. This is the committed step in purine nucleotide biosynthesis. 

De novo purine biosynthesis, like pyrimidine biosynthesis, requires PRPP, but for purines, PRPP provides the foundation on which the bases are constructed step by step. The initial committed step is the displacement of pyrophosphate by ammonia, rather than by a preassembled base, to produce 5-phosphoribosyl-l-amine, with the amine in the P configuration. 

The answer is c. (Ivlurray, pp 375— /O I. Scrivt i, pp 2513—2570. Sack, pp 121—138. Wilson, pp 287—320.1 Several control sites exist in the path of purine synthesis where feedback inhibition occurs, AMP, GMP, or IMP may inhibit the first step of the pathway, which is the synthesis ol 5-phosphoribosyl-l-pyrophosphate (PRPP). PRPP synthetase is specifically inhibited. All three nucleotides can inhibit glutamine PRPP aminotranslerase, which catalyzes the second step of the. pathway. AMP blocks the conversion ol IMP to adenylosuccinate. GMP inhibits the lormation ol xanthylate Irom IMP Thus, blockage rather than enhancement ol IMP metabolism to AMP and GMP effectively inhibits purine biosynthesis. 

As purines are built on a ribose base (see Fig. 41.2), an activated form of ribose is used to initiate the purine biosynthetic pathway. 5-Phosphoribosyl-l-pyrophosphate (PRPP) is the activated source of the ribose moiety. It is synthesized from ATP and ribose 5 -phosphate (Fig. 41.3), which is produced from glucose through the pentose phosphate pathway (see Chapter 29). The enzyme that catalyzes this reaction, PRPP synthetase, is a regulated enzyme (see section 1I.A.5) however, this step is not the committed step of purine biosynthesis. PRPP has many other uses, which are described as the chapter progresses. 

Lipstein, B., Boer, P. Sperling, O. (1978). Regulation of de novo purine synthesis in chick liver role of phosphoribosyl-pyrophosphate availability and of salvage purine nucleotide biosynthesis. Biochim Biophys. Acta, 521, 45—54. 

An alternative mechanism of SAB action could involve its known effects on de novo purine biosynthesis (1, S) and/or nucleoside transport (5). The combined inhibitory effects of SAB and purine analogues on purine biosynthesis could result in sufficient depletion of intracellular nucleotide pools to result in enhanced cellular cytotoxicity. In addition, these effects would lead to an increased bioavailability of 5-phosphoribosyl-l-pyrophosphate (PRPP), the first enzymic product in the de novo pathway. Increased PRPP levels would enhance the activity of hypoxanthine phosphoribosyl transferase, leading to increased salvage of purine analogues. 

Formation of L-histidine is closely related to purine biosynthesis (D 10.4, Fig. 239). ATP is the actual precursor. It condenses with phosphoribosyl pyrophosphate at position 1 before the purine ring is splitted between the positions 1 and 6. An Amadori rearrangement in the two-prime-ribose unit yields the ribulose derivative l-(5 -phosphoribosyl)-4-carboxamido-5-iV-(N -5 -phospho-ribosyl)-formamidinoimidazole, which reacts with glutamine to D-erythro-imidazole glycerol phosphate (the precursor of histidine) and l-(5 -phospho-ribosyl)-4-carboxamido-5-aminoimidazole, which may be regenerated to ATP (D 10.4). 

The reaction that catalyzes the conversion of ribosyl pyrophosphate to 5 -phosphoribosylamine is likely to be the rate-limiting step in purine biosynthesis. Of course, it is difficult to pinpoint a rate-limiting step in an intact mammal, but in vitro experiments have established a feedback inhibition of glutamine phosphoribosyl pyrophosphate amino transferase by adenylic and guanylic nucleotides (ATP, ADP, GMP, GDP, and IMP). 

Glucagon increases DNA synthesis in regenerating rat liver or even in nonoperated rat liver and an elevated level of plasma glucagon has been documented after hepatectomy. Other investigations have demonstrated that the concentration of 5-phosphoribosyl 1-pyrophosphate(PP-ribose-P), which is an important regulator of purine biosynthesis de novo, and the rate of purine biosynthesis de novo itself are increased by glucagon administered in mice and in isolated rat hepatocytes,... 

Since intracellular 5-phosphoribosyl-1-pyrophosphate (PRPP) concentration limits purine biosynthesis de novo and is also a substrate in the reutilization pathways for purine bases we assessed PRPP levels and PRPP "generation" in erythrocytes after oral administration and during constant rate infusion of varying xylitol doses. 

Even though orotidylic acid or orotidine was implicated in pyrimidine formation, the precise role of orotic acid per se remained to be evaluated. On the other hand, it was posable that orotic acid was a normal intermediate that condensed with a ribose compound to yield orotidine or orotidylic acid during the biosynthetic process. In support of this thesis, it was found that 5-phosphoribosyl-l-pyrophosphate was utilized for nucleotide formation from orotic acid (83). On the other hand, it was equally posable that an aliphatic compound, such as aminofumaric acid (335) or carbamylaspartic acid (339), could have coupled with a ribose compound and formed orotidine or orotidylic acid directly without the existence or participation of orotic acid per se. In this latter instance, orotic acid would not be conadered a true intermediate in pyrimidine biosynthesis but merely an accidental cleavage product of hi ly labile orotidine or orotidylic acid. At this time research in the area of purine biosynthesis indicated that a series of acyclic intermediates attached to ribose 6-phosphate were biosynthetic intermediates and that free purines per se were not (Section II, B.). 

A number of derivatives of mercaptopurine have antimetabolite activity, but cell lines with acquired resistance to 6-MP are also cross-resistant to these derivatives, showing that their mechanism of action is essentially similar to 6-MP. Tumours resistant to 6-MP lose their sensitivity to 6-chloropurine (Fig. 9) at the same time. One possible exception is 6-methylmercaptopurine (6-MeMP, Fig. 9) which still affects tumours which have acquir resistance to 6-MP. Resistance to 6-MP is usually a result of the cell losing the pyrophosphate phosphoribosyl transferase, required to convert 6-MP to TIMP. 6-MeMP probably retains its activity because it is converted to its active ribotides by a different series of enzymes. Its ultimate mechanism of action may in fact be on purine biosynthesis similar to 6-MP. 

Phosphoribosyl-l-pyrophosphate amidotransferase from soybean nodules, however, was clearly located in the proplastid fraction (Boland et al, 1982). Based on the location of PRAT, Boland et al (1982) suggested that the plastid might be the site of purine biosynthesis in the nodule. Subsequently they demonstrated that a plastid fraction isolated on sucrose step gradients incorporated from [ Qglycine into purines in the presence of added PRPP, gluta-... 

Fe/S clusters in regulatory enzymes have been proposed to act as sensors in such a manner that, upon detection of a measurand, the cluster disintegrates and activity stops. Putative examples are NO sensing by the [2Fe-2S] cluster in the terminal enzyme of heme synthesis, ferrochelatase [8], and 02 sensing by the [4Fe-4S] cluster in the regulatory enzyme of purine nucleotide biosynthesis, glutamine 5-phosphoribosyl-l-pyrophosphate amidotransferase [9], This is of course not a catalytic activity, since the cluster is destroyed in the action. 

Phosphoribosyl-l-pyrophosphate (PRPP) is a key intermediate in nucleotide biosynthesis. It is required for de novo synthesis of purine and pyrimidine nucleotides and the salvage pathways, in which purines are converted to their respective nucleotides via transfer of ribose 1-phosphate group from PRPP to the base that is. 

Hypoxanthine is a base found in an intermediate of purine nucleotide biosynthesis. Figure 22.4 summarizes the pathway leading from phosphoribosyl-1-pyrophosphate (PRPP) to the first fully formed purine nucleotide, inosine 5 -monophosphate (IMP), also called inosinic acid. IMP contains as its base, hypoxanthine. 

Phosphoribosyl 1-pyrophosphate. S-phos-phoribosa 1-diphosphate, PftPP an energy-rich sugar phosphate, M, 390.1, formed by transfer of a pyro-phosphoryl residue from ATP to ribose 5-phosphate. PRPP is concerned in various biosynthetic reactions, e.g. biosynthesis of purines, pyrimidines and histidine. 

PP-Ribose-P is the most important ribose phosphate donor for purine metabolism (see Chapters 7, 8) and participates in several important reactions of pyrimidine metabolism (Chapters 11, 12) it also transfers this group to a number of other acceptors (Chapter 5). In the course of studies of purine ribonucleotide biosynthesis, a product of the reaction of ribose-5-P and ATP was isolated and eventually identified as 5-phosphoribosyl 1-pyrophosphate. The pyrophosphate group is in the a-configuration, is quite labile, and almost certainly reacts enzymaticaUy as the magnesium complex. It has been chemically synthesized by Tener and Khorana 33). 

Phosphoribosyl-l-pyrophosphate (PRPP) may be considered a precursor in the de novo sjmthetic reactions of purines, since this ribose derivative was required for the formation of 5-phosphoribosylamine (PRA). PRA was the precursor of nitrogen 9, ribose, and phosphate of the completed purine nucleotide structure (Section II, B, 1). PRPP was also a key substance in the biosynthesis of pyrimidine nucleotides. This compound was formed from ribose 5-phosphate and ATP by a pyrophosphorylation of carbon 1 of ribose 5-phosphate (78-80). This was an unusual kinase reaction in that pyrophosphate was transferred rather than phosphate as was the case with most kinases. The ribose 5-phosphate required for the syntheas of PRPP probably originated from glucose, and was formed either by an oxidative pathway from glucose 6-phosphate via 6-pho hogluconate and ribulose 5-phosphate (81) or anaerobically from fructose 6-pho hate (88). The formation of PRPP is shown in Fig. 4. 

It has been established in earlier investigations >2 that the synthesis of purine nucleotides in mammalian red blood cells (RBC) is governed by the extent of endogenous supply of 5-phosphoribosyl-1-pyrophosphate (PRPP), as an essential intermediary. Consequently, the elucidation of the mechanisms controlling the formation of PRPP within the cell appears to be of crucial importance for the understanding of the overall metabolic regulation of purine nucleotide biosynthesis. 

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