Phosphoribosyl pyrophosphate 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. 

During the conversion of anthranilate to tryptophan, two additional carbon atoms must be incorporated to form the indole ring. These are derived from phosphoribosyl pyrophosphate (PRPP) which is formed from ribose 5-phosphate by transfer of a pyro-phospho group from ATP.60 61 The - OH group on the anomeric carbon of the ribose phosphate displaces AMP by attack on Pp of ATP (Eq. 25-5). In many organisms the enzyme that catalyzes this reaction is fused to subunit II of anthranilate synthase.62 PRPP is also the donor of phosphoribosyl groups for biosynthesis of histidine (Fig. 25-13) and of nucleotides (Figs. 

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. 

Pyrimidine biosynthesis commences with a reaction between carbamyl phosphate and aspartic acid to give carbamyl aspartic acid which then nndergoes ring closure and oxidation to orotic acid. A reaction then occurs between orotic acid and 5-phosphoribosyl pyrophosphate to give orotidine-5-phosphate which on decarboxylation yields uridine-5-phosphate (UMP). By means of two successive reactions with ATP, UMP can then be converted into UTP and this by reaction with ammonia can give rise to cytidine triphosphate, CTP (11.126). 

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). 

Studies on the biosynthesis of histidine in Salmonella by Ames and Hartman [214] established the simultaneous repression of several enzymes. In the series of reactions (at least 10 different steps) involved in the biosynthesis of the histidine molecule, the 5-carbon chain of phosphoribosyl pyrophosphate is converted to the 5-carbon chain of histidine. In Sal-... 

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). 

Anthranilic acid is the precursor to the amino acid tryptophan via the attachment of phosphoribosyl pyrophosphate to the amine group to form A-(5 -phosphoribosyl)-anthranilate (PRA) (Fig. 14.5) [11]. The second step of the tryptophan biosynthesis is the PRA isomerization to l-(o-carboxyphe-nylamino)-l-desoxyribuoso-5-phosphate (CdRP) by PRA isomerase. Further elimination of hydroxyl group and decarboxylation forms indole-3-glycerol phosphate (InGP) by InGP synthase. Tryptophan synthase catalyzes the final two... 

The Biosynthesis of the Pyrimidine Ring begins with aspartic acid and carbamyl phosphate. The latter is an energy-rich compound which reacts with the former to give carbamylaspartic acid. Ring closure consumes ATP and is in principle an acid amide formation (peptide synthesis). The intermediate dihydro-orotic acid is dehydrogenated to orotic acid, probably by action of a flavoprotein. Orotic acid is the key precursor of pyrimidine nucleotides. It reacts with phosphoribosyl pyrophosphate. The removal of pyrophosphate yields the nucleotide of orotic acid, whose enzymic decarboxylation produces uridine 5 -phosphate. Phosphorylation with ATP yields uridine pyrophosphate and, finally, uridine triphosphate. Beside the above pathway, there is the further possibility of converting free uracil and ribose 1-phosphate to the nucleoside and from there with ATP to the nucleotide. 

Besides the 3-phosphate and the o-phosphate of ribose, there is also the 5-phos-phate-l-pyrophosphate (phosphoribosyl pyrophosphate), an intermediate in the biosynthesis of nucleosides (cf. Chapt. VII-2). 

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. 

The atoms of the pyrimidine ring are derived from carbamoyl phosphate and aspartate, as shown in Fig. 15-14. The de novo biosynthesis of pyrimidine nucleotides is shown in Fig. 15-15. The first completely formed pyrimidine ring is that of dihydroorotate. Only after oxidation to orotate is the ribose attached to produce orotidylate. The compound 5-phosphoribosyl 1-pyrophosphate (P-Rib-PP) provides the ribose phosphate. L-Glutamine is used as a substrate donating nitrogen atoms at reactions 1 and 9, catalyzed by carbamoyl phosphate synthetase II and CTP synthetase, respectively a second... 

Fig. 8.16 Biosynthesis of L-Trp. Compounds ANT, anthrani-late CDRP, l-(o-carboxyphenylamino)-l-deoxyribulose-5-phosphate I3GP, indole-3-glycerolphosphate IND, indole PRAA, phosphoribosyl anthranilate PRPP, 5-phosphoribosyl-a-pyrophosphate.

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. 

In December 1988, the company introduced a new strain of B. amyloliquefaciens (strain V), which had been genetically modified to increase the synthesis of 5-phosphoribosyl-l-pyrophosphate, an intermediate in the biosynthesis of tryptophan (see Figure 1). After fermentation, tryptophan was extracted from the broth and purified using a series of filtration, crystallization, and separation processes. The purification procedures included contact with powdered activated carbon and then granulated activated carbon. The amount of powdered activated carbon in each batch was usually... 

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. 

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. 

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. 

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