PRPP amidotransferase

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)... 

Purines are synthesized de novo beginning with PRPP as shown in Figure 1-18-4. The most important enzyme is PRPP amidotransferase, which catalyzes the first and rate-limiting reaction of the pathway. It is inhibited by the three purine nucleotide end products AMP, GMP, and IMP. 

The drugs aUopurinol (used for gout) and 6-mercaptopurine (antineopiastic) also inhibit PRPP amidotransferase, These drugs are purine analogs that must be converted to their respective nucleotides by HPRT within cells. Also note that ... 

AHopurinol inhibits xanthine oxidase and also can reduce purine synthesis by inhibiting PRPP amidotransferase (provided HPRT is active). Hyperuricemia and gout often accompany the following conditions ... 

Three major feedback mechanisms cooperate in regulating the overall rate of de novo purine nucleotide synthesis and the relative rates of formation of the two end products, adenylate and guanylate (Fig. 22-35). The first mechanism is exerted on the first reaction that is unique to purine synthesis—transfer of an amino group to PRPP to form 5-phosphoribosylamine. This reaction is catalyzed by the allosteric enzyme glutamine-PRPP amidotransferase, which is inhibited by the end products IMP, AMP, and GMP. AMP and GMP act synergisti-cally in this concerted inhibition. Thus, whenever either AMP or GMP accumulates to excess, the first step in its biosynthesis from PRPP is partially inhibited. 

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. 

PRPP amidotransferase is inactive as a dimer. Dimer formation is promoted by IMP, GMP, and ADP, monomer formation by PRPP. 

Regulation of enzyme activity Glutamine PRPP amidotransferase [4Fe S]... 

Thymidylate synthase Glutamine-PRPP amidotransferase Topoisomerase II... 

Pyrimidine biosynthesis in E. coli is regulated by the feedback inhibition of aspartate transcarbamoylase, the enzyme that catalyzes the committed step. CTP inhibits and ATP stimulates this enzyme. The feedback inhibition of glutamine-PRPP amidotransferase by purine nucleotides is important in regulating their biosynthesis. 

The committed, regulated step in the pathway catalyzed by PRPP amidotransferase. Note glutamine provides the nitrogen to initiate purine synthesis. 

The PRPP amidotransferase enzyme exists as an active monomer and an inactive polymer (see "Introduction to Metabolism" Lecture). IMP, GMP and AMP all inactivate the enzyme causing a shift towards the polymerized inactive form. PRPP causes a shift towards the active monomeric form. 

Glutamine PRPP amidotransferase (Fig. 25-15) and a penicillin acylase have similar active sites and overall... 

Thioguanine is similar to 6-mercaptopurine in its action. The most active form is 6-thio-GMP, which inhibits guanylate kinase and, at higher concentrations, IMP dehydrogenase. Thio-IMP and thio-GMP also inhibit PRPP amidotransferase. 

The substrate for this reaction, ct-D-ribose-5-phosphate, is a product of the pentose phosphate pathway.) Figure 14.24 illustrates the initial phase in the pathway by which PRPP is converted to inosine monophosphate (inosinate), the first purine nucleotide. The process begins with the displacement of the pyrophosphate group of PRPP by the amide nitrogen of glutamine in a reaction catalyzed by glutamine PRPP amidotransferase. This reaction is the committed step in purine synthesis. The product formed is 5-phospho-/3-D-ribosylamine. 

In the next step, which is the first step uniquely related to purine synthesis, the amide nitrogen from glutamine is added to the PRPP to form 5-phosphoribosylamine, catalyzed by PRPP amidotransferase. This step can be inhibited by azaserine, an antimetabolite of glutamine. Glycine is then added, forming an amide bond. This re-... 

The control of purine synthesis is with an early stepformation of phosphoribosylamine by PRPP amidotransferase. This enzyme is partially inhibited by either AMP or GMP and strongly inhibited by AMP and GMP together. 

Enzymes that act on PRPP include Phosphoribosyltransferases (salvage synthesis and de novo synthesis of pyrimidines), PRPP amidotransferase... 

The reaction pathway is regulated allosterically by AMP, ADP, GMP, and GDP, which all inhibit PRPP amidotransferase, the first enzyme in the pathway. In E. coli, synthesis of the genes for the pathway is controlled by the purR repressor protein, which is activated by binding hypoxanthine or guanine. 

PRPP amidotransferase - Defects in PRPP amidotransferase may render it insensitive to feedback inhibition by purine nucleotides, too, leading to the overproduction of purine nucleotides, excessive uric acid synthesis, and gout. 

PRPP amidotransferase is an enzyme that catalyzes the reaction below ... 

PRPP amidotransferase is regulated allosterically by AMP, ADP, GMP, and GDP, which all inhibit the reaction. 

Phosphoribosylamine is an intermediate in de novo purine biosynthesis. It is produced in the reaction that follows, catalyzed by PRPP amidotransferase ... 

See also De Novo Biosynthesis of Purine Nucleotides, PRPP Amidotransferase, Figure 22.4... 

Decreased flux through this reaction raises the steady-state level of PRPP, thereby increasing the activity of PRPP amidotransferase, which catalyzes the initial step in purine synthesis. Increased activity may make the amidotransferase resistant to feedback inhibition by AMP and GMP, the end products of the purine biosynthetic pathway. 

Fig, 1 Outline of purine metabolism. (1) PRPP amidotransferase (2) hypox Anithine-guanine ribosyltransferase (3) PRPP synthetase (4) adenine phosphoribosyltransferse (5) adenosine de nase (6) purine nucleoside phosphorylase (7) 5 -nucleotidase (8) xanthine oxidase. 

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