Purine nucleotide biosynthesis

The pathways for thiamine biosynthesis have been elucidated only partiy. Thiamine pyrophosphate is made universally from the precursors 4-amino-5-hydroxymethyl-2-methylpytimidinepyrophosphate [841-01-0] (47) and 4-methyl-5-(2-hydroxyethyl)thiazolephosphate [3269-79-2] (48), but there appear to be different pathways ia the eadier steps. In bacteria, the early steps of the pyrimidine biosynthesis are same as those of purine nucleotide biosynthesis, 5-Aminoimidazole ribotide [41535-66-4] (AIR) (49) appears to be the sole and last common iatermediate ultimately the elements are suppHed by glycine, formate, and ribose. AIR is rearranged in a complex manner to the pyrimidine by an as-yet undetermined mechanism. In yeasts, the pathway to the pyrimidine is less well understood and maybe different (74—83) (Fig. 9). 

Inosine monophosphate dehydrogenase (EVDPDH) is a key enzyme of purine nucleotide biosynthesis. Purine synthesis in lymphocytes exclusively depends on the de novo synthesis, whereas other cells can generate purines via the so-called salvage pathway. Therefore, IMPDH inhibitors preferentially suppress DNA synthesis in activated lymphocytes. 

Inosine monophosphate dehydrogenase (IMPDH) is the key enzyme of purine nucleotide biosynthesis. Proliferation of activated lymphocytes dq ends on rapid de novo production of purine nucleotides for DNA synthesis. 

Three processes contribute to purine nucleotide biosynthesis. These are, in order of decreasing importance (1) synthesis from amphibofic intermediates... 

Multifunctional Catalysts Participate in Purine Nucleotide Biosynthesis... 

Since biosynthesis of IMP consumes glycine, glutamine, tetrahydrofolate derivatives, aspartate, and ATP, it is advantageous to regulate purine biosynthesis. The major determinant of the rate of de novo purine nucleotide biosynthesis is the concentration of PRPP, whose pool size depends on its rates of synthesis, utilization, and degradation. The rate of PRPP synthesis depends on the availabihty of ribose 5-phosphate and on the activity of PRPP synthase, an enzyme sensitive to feedback inhibition by AMP, ADP, GMP, and GDP. 

Liver, the major site of purine nucleotide biosynthesis, provides purines and purine nucleosides for salvage and utilization by tissues incapable of their biosynthesis. For example, human brain has a low level of PRPP amidotransferase (reaction 2, Figure 34-2) and hence depends in part on exogenous purines. Erythrocytes and polymorphonuclear leukocytes cannot synthesize 5-phosphoribosylamine (strucmre III, Figure 34-2)... 

Hepatic purine nucleotide biosynthesis is stringently regulated by the pool size of PRPP and by feedback inhibition of PRPP-glutamyl amidotransferase by AMP and GMP. 

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]. 

Glutamine also supplies an amino function to start off purine nucleotide biosynthesis. This complex little reaction is again an Sn2 reaction on PRPP, but only an amino group from the amide of glutamine is transferred. The product of the enzymic reaction is thus 5-phosphoribosylamine. 

As the first committed step in the biosynthesis of AMP from IMP, AMPSase plays a central role in de novo purine nucleotide biosynthesis. A 6-phosphoryl-IMP intermediate appears to be formed during catalysis, and kinetic studies of E. coli AMPSase demonstrated that the substrates bind to the enzyme active sites randomly. With mammalian AMPSase, aspartate exhibits preferred binding to the E GTPTMP complex rather than to the free enzyme. Other kinetic data support the inference that Mg-aspartate complex formation occurs within the adenylosuccinate synthetase active site and that such a... 

Purine Nucleotide Biosynthesis Is Regulated by Feedback Inhibition... 

The next nine steps in purine nucleotide biosynthesis leading to the synthesis of IMP (whose base is hypoxanthine) are illustrated in Figure 22.7. This pathway requires four ATP molecules as an energy source. Two steps in the pathway require N10-formyltetrahydrofolate. 

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. 

The biosynthesis of histidine. The 5-aminoimidazole-4-carboxamide ribotide formed during the course of histidine biosynthesis is also an intermediate in purine nucleotide biosynthesis. Therefore it can be readily regenerated to an ATP, thus replenishing the ATP consumed in the first step in the histidine biosynthetic pathway (see fig. 23.13). 

The biosynthetic pathway to pyrimidine nucleotides is simpler than that for purine nucleotides, reflecting the simpler structure of the base. In contrast to the biosynthetic pathway for purine nucleotides, in the pyrimidine pathway the pyrimidine ring is constructed before ribose-5-phosphate is incorporated into the nucleotide. The first pyrimidine mononucleotide to be synthesized is orotidine-5 -monophosphate (OMP), and from this compound, pathways lead to nucleotides of uracil, cytosine, and thymine. OMP thus occupies a central role in pyrimidine nucleotide biosynthesis, somewhat analogous to the position of IMP in purine nucleotide biosynthesis. Like IMP, OMP is found only in low concentrations in cells and is not a constituent of RNA. 

Mercaptopurine (table 23.3) and related thiopurines are potent inhibitors of purine nucleotide biosynthesis, but they are inactive until they are converted to the correspond-... 

Acivicin is a potent inhibitor of several steps in purine nucleotide biosynthesis that utilize glutamine. The enzymes it inhibits are glutamine PRPP amidotransferase (step 1, fig. 23.10), phosphoribosyl-A-formylglycinamidine synthase (step 4, fig. 23.10), and GMP synthase (see fig. 23.11). In pyrimidine nucleotide biosynthesis the enzymes inhibited are carbamoyl synthase (step 1, fig. 23.13) and CTP synthase (see fig. 23.14). Acivicin is under trial for the treatment of some forms of cancer. 

Many lines of evidence indicate that the first committed step in de novo purine nucleotide biosynthesis, production of 5-phosphoribosylamine by glutamine PRPP amidotransfer-ase, is rate-limiting for the entire sequence. Consequently, regulation of this enzyme is probably the most important factor in control of purine synthesis de novo (fig. 23.24). The enzyme is inhibited by purine-5 -nucleotides, but the most inhibitory nucleotides vary with the source of the enzyme. Inhibition constants (A, ) are usually in the range 10-3-10-5 M. The maximum effect of this end-product inhibition is produced by certain combinations of nucleotides (e.g., AMP and GMP) in optimum concentrations and ratios, indicating two kinds of inhibitor binding sites. This is an example of a concerted feedback inhibition. 

H,6H-Purine, l,7-dimethyl-6-oxo-UV spectra, 5, 517 7H,8H-Purine, 8-oxo-UV spectra, 5, 517 Purinecarboxamides reactions, 5, 550, 551 Purinecarboxylic adds reactions, 5, 550 Purine-6-carboxylic acids synthesis, 5, 593 Purine-8-carboxylic acids synthesis, 5, 593 Purine nucleotides biosynthesis, 1, 87-88 Purine[9,8-a]quinolines, 5, 566 Purines, 5, 499-605 aldehydes reactions, 5, 549 synthesis, 5, 593 alkylation, 5, 505, 528-538 amination, 5, 541-542 anions... 

Nonsecretory myeloma fusion partners with defective purine nucleotide biosynthesis pathways do now exist for a number of species, including humans, and so hybridoma production by cell fusion using polyethylene glycol (PEG) is now a possibility. 

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. 

IMP is the key intermediate of purine nucleotide biosynthesis. IMP can react along two pathways that yield either GMP or AMP. Oxidation of the 2 position makes xanthine monophosphate, which is transamidated to GMP. Alternatively, the a-amino group of aspartate can replace the ring oxygen of IMP to make AMP. (Note again how this reaction is similar to the synthesis of arginine fromcitrulline.)... 

Aimi,J., Qiu, H., Williams, J., Zalkin, H., and Dixon, J. E. (1990). De novo purine nucleotide biosynthesis cloning of human and avian cDNAs encoding the trifunctional glycinam-ide ribonucleotide synthetase-aminoimidazole ribonucleotide synthetase-glycinamide ribonucleotide transformylase by functional complementation in E. colt. Nucleic Acids Res., 18, 6665-6672. 

B lymphocytes will be eliminated during continuous culture because these cells have a short life span in culture. Commercially available myeloma cells for hybridoma production have mutations in one of the enzymes of the salvage pathway of purine nucleotide biosynthesis. Hybridoma cells are cultured in medium that forces the cells to utilize the salvage pathway for nucleotide synthesis. The mutated myeloma cells or hybridization products of two myeloma cells will die in this selection medium since they are incapable of nucleotide synthesis under these propagation conditions. However, myeloma cells that have fused to the B lymphocytes derived from the spleen of the immunized animal will have an intact salvage pathway and will survive in the selection medium. Thus, only the B lymphocytes-myeloma hybridomas will survive prolonged culture in the selection medium. 

Purine nucleotide biosynthesis Chapter Buchanan and Hartman 59MI40900,59M140901... 

The committed step in purine nucleotide biosynthesis is the conversion of PRPP into phosphoribosylamine by glutamine phosphoribosyl amidotransferase. This important enzyme is feedback-inhibited by many purine ribonucleotides. It is noteworthy that AMP and GMP, the final products of the pathway, are synergistic in inhibiting the amidotransferase. 

V.M. Levdikov, V.V. Barynin, Al. Grebenko, W.R. Melik-Adamyan, V.S. Lamzin, and K.S. Wilson. 1998. The structure of SAICAR synthase An enzyme in the de novo pathway of purine nucleotide biosynthesis Structure 6 363-376. (PubMed)... 

Depletes the supply of PRPP needed for the first step of de novo purine nucleotide biosynthesis... 

The hyperuricemia in Lesch-Nyhan patients is explained, at least in part, on the basis of intracellular accumulation of PRPP leading to increased purine nucleotide biosynthesis de novo and increased production of uric acid. Such patients do not usually develop gouty arthritis early in life but do exhibit uric acid crystalluria and stone formation. 

PRPP. In contrast, in de novo purine nucleotide biosynthesis, ribose 5-phosphate is an integral part of the earliest precursor molecule. 

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