Glutamine-PRPP amidotransferase

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. 

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. 

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

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. 

We have already seen that in mammals glutamine PRPP amidotransferase, an enzyme which is at the gate of the purine biosynthetic pathway, is feedback inhibited by the terminal products of the reaction, AMP, IMP, and GMP. This regulatory mechanism prevents the formation of excessive amounts of purines. Such feedback inhibition is much less marked with the bird enzyme. 

The possibility that two enzyme activities could catalyze PRA synthesis from PRPP was explored. The elution profile of a partially purified enzyme preparation from a G-lOO Sephadex column is illustrated on Figure 4. It can be seen that two enzyme activities catalyze PRA synthesis from PRPP, and that each enzyme activity utilizes a different nitrogenous substrate. Peak A, corresponding to an approximate molecular weight of 95,000 daltons, represents maximal glutamine PRPP amidotransferase activity, while peak B (approximate molecular weight of 67,000 daltons) corresponds to maximal PRPP aminotransferase activity. The source of the enzyme preparation of this experiment was Burkitt lymphoma cells maintained in permanent culture. 

In cell free preparations of HGPRT lymphocytes it was found that two enzyme activities catalyzed PRA synthesis from PRPP glutamine PRPP amidotransferase uses PRPP and glutamine as substrates, and ammonia PRPP aminotransferase uses ammonia and PRPP for PRA synthesis. 

PRPP is a substrate for the enzyme glutamine-PRPP amidotransferase catalyzing the first rate-limiting step of purine nucleotide synthesis de novo (l). Evidence has been obtained that PRPP is an important regulator of this pathway (2-4). A possible role of increased PRPP availability in the enhancement of de novo purine synthesis in primary metabolic gout, a purine overproduction disease, has been suggested by the demonstration of an increased PRPP turnover in gouty subjects (5) and of an increased rate of PRPP formation in erythrocytes and cultured fibroblasts from such patients (6-8). 

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