Phosphoribosyl pyrophosphate synthesis

In many cells, the capacity for de novo synthesis to supply purines and pyrimidines is insufficient, and the salvage pathway is essential for adequate nucleotide synthesis. In patients with Lesch-Nyhan disease, an enzyme for purine salvage (hypoxanthine guanine phosphoribosyl pyrophosphate transferase, HPRT) is absent. People with this genetic deficiency have CNS deterioration, mental retardation, and spastic cerebral palsy associated with compulsive self-mutilation, Cells in the basal ganglia of the brain (fine motor control) normally have very high HPRT activity. These patients also all have hyperuricemia because purines cannot be salvaged. 

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. 

The trinucleotide ZTP also accumulates, not only in bacteria but also in many eukaryotic cells. Bochner and Ames suggested it may be an alarmone signaling a deficit of folate coenzymes in the cell and causing a shutdown of protein synthesis. ZTP is synthesized by an unusual reaction, transfer of a pyrophosphate group from PRPP (phosphoribosyl pyrophosphate). 

Fig. 13.2. Synthesis of IMP. c = Hypoxanthine phosphoribosyl transferase (HPRT) GAR = glycinamide ribonucleotide FGAR = formyl glycinamide ribonucleotide PRPP = phosphoribosyl pyrophosphate AICAR = 5 aminoimidazole-4-carboxamide...

Figure 8.2. Synthesis of NAD from nicotinamide, nicotinic acid, and qninolinic acid. Quinolinate phosphoribosyltransferase, EC 2.4.2.19 nicotinic acid phosphoribosyl-transferase, EC 2.4.2.11 nicotinamide phosphoribosyltransferase, EC 2.4.2.12 nicotinamide deamidase, EC 3.5.1.19 NAD glycohydrolase, EC 3.2.2.S NAD pyrophosphatase, EC 3.6.1.22 ADP-ribosyltransferases, EC 2.4.2.31 and EC 2.4.2.36 and poly(ADP-ribose) polymerase, EC 2.4.2.30. PRPP, phosphoribosyl pyrophosphate.

Two enzyme abnormalities resulting in an overproduction of uric acid have been well described (Fig. 91-1). The first is an increase in the activity of phosphoribosyl pyrophosphate (PRPP) synthetase, which leads to an increased concentration of PRPP. PRPP is a key determinant of purine synthesis and thus uric acid production. The second is a deficiency of hypoxanthine guanine phosphoribosyl transferase (HGPRT). 

Purine salvage pathway The synthesis of purine nucleotides by the condensation of the purine bases with phosphoribosyl pyrophosphate. As the name suggests, it is a way in which purine bases can be recycled back to nucleotides. The purine salvage pathway consists of two enzymes, HGPRT and adenine phosphoribosyltransferase (APRT). 

In nucleotide synthesis, GMP is formed from phosphoribosyl pyrophosphate in the first reaction below catalyzed by a phosphoribosyl transferase ... 

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. 

Sperling, 0., G. Eilam, S. Persky-Brosh, and A. de Vries. 1972. Accelerated erythrocyte S-phosphoribosyl-l-pyrophosphate synthesis. A familial abnormality associated with excessive uric acid production and gout. Biochem. Med. 6 310. 

There is not enough time and space to describe my early work on thiamine pyrophos-phokinase, but I would like to point out that the pyrophosphorylation is a reaction less common than either phosphorylation or adenylylation. So far, only three examples are known. The one first reported by A. Kornberg< > was the synthesis of phosphoribosyl pyrophosphate and the most recent one was the formation of ppGpp from ATP and GDP as demonstrated by Sy and Lipmann.< > I feel it no mere coincidence that I am now working a little on the role of this unusual nucleotide in the stringent control. 

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. 

In detail, the synthesis as studied by Buchanan and Greenberg takes the following route 5-phosphoribosylamine (stemming from phosphoribosyl pyrophosphate and glutamine, as mentioned under pyrimidines) condenses with glycine to form the amide with the aid of ATP the... 

Nucleosidases are enzymes which cleave the bond between the sugar residue and the base. The cleavage of the nucleoside bond does not go via hydrolysis, but through phosphorolysis. Usually orthophosphate is involved, but occasionally pyrophosphate in conjunction with an appropriate enzyme accomplishes the same result, which is equivalent to the reverse of nucleoside synthesis from base and phosphoribosyl pyrophosphate (cf. Section 2). 

Seasonal variations in the metabolic fate of adenine nucleotides prelabelled with [8—1-4C] adenine were examined in leaf disks prepared at 1-month intervals, over the course of 1 year, from the shoots of tea plants (Camellia sinensis L. cv. Yabukita) which were growing under natural field conditions by Fujimori et al.33 Incorporation of radioactivity into nucleic acids and catabolites of purine nucleotides was found throughout the experimental period, but incorporation into theobromine and caffeine was found only in the young leaves harvested from April to June. Methy-lation of xanthosine, 7-methylxanthine, and theobromine was catalyzed by gel-filtered leaf extracts from young shoots (April to June), but the reactions could not be detected in extracts from leaves in which no synthesis of caffeine was observed in vivo. By contrast, the activity of 5-phosphoribosyl-1-pyrophosphate synthetase was still found in leaves harvested in July and August. 

A different, simpler , pathway is involved in the synthesis of pyrimidine nucleotides. A pyrimidine base (orotate), is synthesised first. Then the ribose is added from 5-phosphoribosyl 1-pyrophosphate. The two precursors for the formation of orotate are carbamoylphosphate and aspartate, which form carbamoyl aspartate, catalysed by aspartate carbamoyltransferase. 

Figure 10-1. Overview of purine synthesis. Details of the first two reactions and sources of the atoms of the purine ring in inosine 5 -monophosphate (IMP) are shown. PRPP, 5 -phosphoribosyl-1-pyrophosphate Gin, glutamine Gly, glycine Asp, aspartate THF, tetrahydrofolate.

Figure 10-2. Regulation of purine synthesis by the nucleotides and the intermediate, 5 -phosphoribosyl-1 -pyrophosphate (PRPP). Both feedback and feed-forward mechanisms are utilized in this intricate scheme. IMP, inosine monophosphate.

Attack at the /3 phosphate of ATP displaces AMP and transfers a pyrophosphoiyl (not pyrophosphate) group to the attacking nucleophile (Pig. 13-10b). For example, the formation of 5 -phosphoribosyl-1-pyrophosphate (p. XXX), a key intermediate in nucleotide synthesis, results from attack of an —OH of the ribose on the /3 phosphate. 

A useful way to organize these biosynthetic pathways is to group them into six families corresponding to their metabolic precursors (Table 22-1), and we use this approach to structure the detailed descriptions that follow. In addition to these six precursors, there is a notable intermediate in several pathways of amino acid and nucleotide synthesis—5-phosphoribosyl-l-pyrophosphate (PRPP) ... 

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