Phosphoribosylpyrophosphate

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... 

During the biosynthetic transformation, chorismate is the point of divergence for the biosynthesis of Phe, Tyr, Trp, and other amino acids containing aromatic groups. For example, the biosynthesis of Trp begins with the conversion of chorismate to anthranilate (Scheme 4(a)). A sequence of amination and aromatization reactions produces anthranilate, which is then condensed with phosphoribosylpyrophosphate. The intermediate is carried through a series of reactions to yield Trp (Scheme 4(b)). 

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]. 

It has been suggested that thioguanine s multistep inhibition, one step of which is the inhibition of phosphoribosylpyrophosphate amidotransferase, results in a profound lowering of the intracellular concentration of guanine nucleoside phosphates and that this depletion causes a marked depression in cellular metabolism that presumably would lead to cell death [91 ]. 

In addition to the analogues listed in Table 2.3, cordycepin [302]. 3 -amino-3 -deoxyadenosine [173], and formycin [303] can inhibit the de novo pathway by blocking the phosphoribosylpyrophosphate amidotransferase. Thus, a number ofpurine analogues—after anabolism to nucleoside phosphates—can act as feedback inhibitors, and this inhibition may be the primary cause of their cytotoxicity. 

Glutamine 5-phosphoribosyl-l-pyrophosphate amidotransferase, 38 310-311 Glutamine phosphoribosylpyrophosphate amidotransferase, ground spin state variability, 38 99-100 Glutathion... 

Step 1, in which phosphoribosylamine is formed, is catalyzed by glutamine phosphoribosylpyrophosphate ami-... 

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

ThiolMP and ThioGMP are feedback inhibitors of phosphoribosylpyrophosphate amido-transferase, which is the first, and rate-limiting step in the synthesis of purine. In addition, these analogs inhibit the de novo biosynthesis of purine and block the conversion of inosinic acid to adenylic acid or guanylic acid. The triphosphate nucleotides are incorporated into DNA, and this results in delayed toxicity after several cell divisions. 

When injected, azathioprine (Imuran) is rapidly converted to 6-mercaptopurine. The half-life of azathioprine after intravenous injection is 10 to 20 min, and that of 6-mercaptopurine is somewhat longer. The cytotoxic activity of these thiopurines is due to the conversion of mercaptopurine to 6-thiouric acid, a noncarcinostatic metabolite. This action is thought to block the excess synthesis of inosinic acid from its precursors, glutamine and phosphoribosylpyrophosphate. In addition, unlike cyclophosphamide, azathioprine is a potent anti-inflammatory substance that can cause a reduction in the number of monocytes and neutrophils at inflammatory sites. Antibody responses are also inhibited by azathioprine. Studies in humans have shown that azathioprine decreases the y-globulin and antibody levels, thus influencing IgG rather than IgM production. This makes azathioprine an effective immunosuppressant in the early phases of immune responses. It is less effective or completely ineffective in altering either the effector phase or already established reactivities. 

Figure 2.3(D). Uricogenesis during alanine catabolism and gluconeogenesis in avian liver. Some abbreviations are as in figure 2.3(C). 1 C refers to one-carbon units MDH, malate dehydrogenase XDH, xanthine dehydrogenase PRPP, phosphoribosylpyrophosphate IMP, inosoine monophosphate ino, inosine hyp, hypoxanthine xan, xanthine.

Glucose was first converted enzymatically into ribose 5 -phosphate from which GMP was subsequently obtained by the action of phosphoribosylpyrophosphate synthetase and guanosine phos-phoribosyl transferase. A two-step phosphorylation to GTP followed by treatment with recombinant GTP-cyclohydrolase I from E. coli gave (51) <90Ml 718-12). This method also allows l4C-labeling in the 6- and 7-position as well as the carbon sidechain. 

Know the various salvage pathways using ribose-1 -phosphate and phosphoribosylpyrophosphate. Know the action of the various kinases. Know the etiology of Lesch-Nyhan syndrome. 

Phosphoribosylpyrophosphate does not react with which compound ... 

The first ATP-dependent synthetase to be subjected to analysis for substitution stereochemistry was phosphoribosylpyrophosphate synthetase [78]. This analysis was novel in that it utilized a coordination exchange-inert Co-ATP complex for this purpose and circular dichroic analysis for relative configurations of substrate and product. The reaction of Equation 16 was catalyzed by this enzyme. 

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. 

Hypoxanthine guanosine phosphoribosyltransferase (HGPRT) catalyzes the formation of IMP and pyrophosphate (PPj) from hypoxanthine (Hyp) and phosphoribosylpyrophosphate (PRibPP) as shown in reaction (1) ... 

The synthesis of phosphoribosylpyrophosphate from ATP and ribose-5-phosphate is catalyzed by phosphoribosylpyrophosphate synthetase. Superactivity of this enzyme may be a cause of overproduction of uric acid, which may result in gout. 

Figure 9.118 HPLC profiles of the reaction mixtures after the enzyme reaction of erythrocyte phosphoribosylpyrophosphate synthetase from a healthy subject. Incubation times were 10 minutes (a) and 30 minutes (fc). (From Sakuura, et al., 1991.)...

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