Phosphoribosylpyrophosphate PRPP

Phosphoribosylpyrophosphate (PRPP) synthetase is one of the very few enzymes which transfer a pyrophosphoryl group from ATP in one step. When the synthesis is carried out in lsO-enriched water, lsO is incorporated into the PRPP, but not into AMP.91 The lsO in the PRPP arises from a pre-exchange between the H2180 and the ribose phosphate, and hence the results confirm that fission of the /5-P—O bond takes place. PRPP and ATP are starting materials in the biosynthesis of histidine, and Ai-(5 -phospho-D-ribosyl)adenosine triphosphate (29) is an intermediate. The... 

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 phosphoribosylpyrophosphate (PRPP). This is an unusual kinase-catalyzed reaction because the group transferred is the pyrophosphate group rather than the phosphate group. 

The application of the HPLC assay method to studies on reaction mechanisms has been limited, and the reader is referred to the work of Sloan (1984). Sloan and his colleagues studied the formation of IMP or GMP (and pyrophosphate) from the substrates phosphoribosylpyrophosphate (PRPP) and either hypoxanthine or guanine. These reactions, catalyzed by hypoxan-thine/guanine phosphoribosyltransferase (GHPRTase), were studied by HPLC after a method was developed to separate all the reactants and products simultaneously. 

Purine synthesis starts with ribose-5-phosphate, which can be formed from the pentosephosphate pathway either via the oxidative arm or, in the reverse fashion, via the nonoxidative arm. Many tissues have the ability to form ribose-5-phosphate and their own purines. The first reaction is with ATP, where pyrophosphate is added to ribose-5-phosphate to produce phosphoribosylpyrophosphate (PRPP). 

FIGURE 23.24 Purine salvage, (a) Adenine is the purine in this example. There are analogous reactions for salvage of guanine and hypoxanthine (see page 697). (b) The formation of phosphoribosylpyrophosphate (PRPP). 

Hypoxanthine-guanine phosphoribosyl transferase (HGPRT), a salvage enzyme of nucleotide metabolism, uses 5 -phosphoribosylpyrophosphate (PRPP) to convert hypox-... 

No evidence for the presence of a known enzyme abnormality causing purine overproduction could be obtained. The erythrocyte activity of hypoxanthine-guanine phos-phoribosyltransferase (HGPRT), of adenine phosphoribosyltransferase (APRT), and of phosphoribosylpyrophosphate (PRPP) synthetase were all in the normal range. Erythrocyte PRPP generation, as well as the acitivity of the pentose phosphate pathway was also normal (Table 1). In addition, the rate of de novo synthesis of purine nucleotides in cultured skin fibroblasts from the patient was found to be normi. 

HUMAN PHOSPHORIBOSYLPYROPHOSPHATE (PRPP) SYNTHETASE REQUIREMENTS FOR SUBUNIT AGGREGATION... 

Phosphoribosylpyrophosphate (PRPP) synthetase (E.C. 2.7.6.1) catalyzes the formation of PRPP from ribose-5-phosphate and ATP in the presence of Mg and inorganic phosphate. The product, PRPP, is a substrate of the first rate limiting step of the de novo synthesis of purine nucleotides and its availability has been shown to regulate this pathway in human tissue (l). A superactive mutant erythrocyte PRPP synthetase with decreased sensitivity to feedback inhibition has recently been found by us in a gouty family (2,3). 

FIGURE 15 Examples o a + proteins, (a) Ubiquitin, (b) Papain consists of one alpha-helix and four strands of antiparallel beta-sheet, (c) ribonuclease A, and (d) N-terminal domain of E. co//glutamine phosphoribosylpyrophosphate (Prpp) amidotransferase that contains a four structural layers appa. 

HGPRT and APRT were assayed in dialyzed hemolysates by a radiochemical method in which labelled hypoxanthine, guanine and adenine are converted by reaction with phosphoribosylpyrophosphate (PRPP) to their respective nucleotides, which then are separated by thin-layer chromatography on microcrystalline cellulose (4). 

The deficiency of hypoxanthine guanine-phosphoribosyltrans-ferase (HGPRT) activity of patients with the Lesch-Nyhan syndrome is accompanied by an accelerated rate of purine biosynthesis de novo. This elevated rate of purine biosynthesis is reflected in fibroblasts cultured from the skin of these patients and is accompanied by elevated levels of phosphoribosylpyrophosphate (PRPP). 

In the last few years, phosphoribosylpyrophosphate (PRPP) has been viewed as a most important intermediate metabolite and studies have indicated that its concentration and rate of synthesis may play a critical role in the regulation of purine biosynthesis da novo. Hershko et al (l) found increased in vitvo formation of PRPP in the erythrocytes of some gouty subjects. They further suggested that the increase in the rate of purine uptake found in these patients was attributable to enhanced PRPP formation. 

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