Glutamine purine biosynthesis

One example of a naturally occurring diazirine, duazomycin A (137 Scheme 11.20), has been reported, isolated in 1985 from a Streptomyces species during a screen for herbicidal compounds [196], It was fotind to inhibit de novo starch synthesis and it was suggested that this is due to direct inhibition of protein synthesis. Duazomycin A is structurally related to 6-diazo-5-oxo-L-norleucine (138), also reported as a natural product from Streptomyces [197], which acts as a glutamine antagonist and inhibits purine biosynthesis [198],... 

Since biosynthesis of IMP consumes glycine, glutamine, tetrahydrofolate derivatives, aspartate, and ATP, it is advantageous to regulate purine biosynthesis. The major determinant of the rate of de novo purine nucleotide biosynthesis is the concentration of PRPP, whose pool size depends on its rates of synthesis, utilization, and degradation. The rate of PRPP synthesis depends on the availabihty of ribose 5-phosphate and on the activity of PRPP synthase, an enzyme sensitive to feedback inhibition by AMP, ADP, GMP, and GDP. 

Several reactions of IMP biosynthesis require folate derivatives and glutamine. Consequently, antifolate drugs and glutamine analogs inhibit purine biosynthesis. 

FIGURE 22-20 Biosynthesis of histidine in bacteria and plants. Atoms derived from PRPP and ATP are shaded red and blue, respectively. Two of the histidine nitrogens are derived from glutamine and glutamate (green). Note that the derivative of ATP remaining after step (AICAR) is an intermediate in purine biosynthesis (see Fig. 22-33, step ), so ATP is rapidly regenerated. 

The first reaction in purine biosynthesis is the transfer of the amide from glutamine to PRPP with release of pyrophosphate. The product is phosphoribosylamine (PRA). 

Figure 10.7 Structure of azaserine, a glutamine analog and inhibitor of purine biosynthesis.

The design for pyrimidine synthesis differs somewhat from that of purine biosynthesis in that the sugar is attached to the pyrimidine ring at the end of the pathway. In addition, pyrimidine biosynthesis occurs in part in the cytosol and in part in the mitochondria and involves the participation of two multifunctional enzymes. The pathway is summarized in Figure 10.9. One of the initial reactants is the compound carbamoyl phosphate (carbamoyl phosphoric acid). This compound is also formed in the urea biosynthetic pathway, but this takes place in the mitochondria and requires NH3 (Chapter 20). The cytosolic biosynthesis of carbamoyl phosphate for the purpose of pyrimidine biosynthesis requires glutamine as the nitrogen donor ... 

Inhibiting purine biosynthesis. Amidotransferases are inhibited by the antibiotic azaserine (0-diazoacetyl-L-serine), which is an analog of glutamine. 

During purine biosynthesis, the entire glycine molecule is incorporated into the growing ring structure, glutamine provides N3 and N9, and aspartate provides Nl. 

Both compounds interfere with purine biosynthesis at one of the three stages where glutamine is required. Addition of either of these two inhibitors to cells leads to an accumulation of formylglycineamide ribotide. The inhibition of the enzyme is reversible for a short period but soon becomes irreversible as a result of the inhibitor alkylating the enzyme, probably through a sulphydryl group of a cysteine residue. 

Regulation of de novo purine biosynthesis is essential because it consumes a large amount of energy as well as of glycine, glutamine, N °-formyl FH4, and aspartate. Regulation occurs at the PRPP synthetase reaction, the ami-... 

The answer is c. (Ivlurray, pp 375— /O I. Scrivt i, pp 2513—2570. Sack, pp 121—138. Wilson, pp 287—320.1 Several control sites exist in the path of purine synthesis where feedback inhibition occurs, AMP, GMP, or IMP may inhibit the first step of the pathway, which is the synthesis ol 5-phosphoribosyl-l-pyrophosphate (PRPP). PRPP synthetase is specifically inhibited. All three nucleotides can inhibit glutamine PRPP aminotranslerase, which catalyzes the second step of the. pathway. AMP blocks the conversion ol IMP to adenylosuccinate. GMP inhibits the lormation ol xanthylate Irom IMP Thus, blockage rather than enhancement ol IMP metabolism to AMP and GMP effectively inhibits purine biosynthesis. 

Cellular concentrations of PRPP and glutamine are usually below their for glutamine phosphoribosyl amidotransferase. Thus, any situation which leads to an increase in their concentration can lead to an increase in de novo purine biosynthesis. 

Hyperuricemia in Lotta Topa ne s case arose as a consequence of over-j production of uric acid. Treatment with allopurinol not only inhibits xan-thine oxidase, lowering the formation of uric acid with an increase in the excretion of hypoxanthine and xanthine, but also decreases the overall synthesis of purine nucleotides. Hypoxanthine and xanthine produced by purine degradation are salvaged (i.e., converted to nucleotides) by a process that requires the consumption of PRPP. PRPP is a substrate for the glutamine phosphoribosyl amidotransferase reaction that initiates purine biosynthesis. Because the normal cellular levels of PRPP and glutamine are below the of the enzyme, changes in the level of either substrate can accelerate or reduce the rate of the reaction. Therefore, decreased levels of PRPP cause decreased synthesis of purine nucleotides. 

The drug is believed to be a glutamine antagonist that specifically inhiits purine biosynthesis and thus may exert antitumour activity. 

The conversion of PRPP into phosphoribosylamine by glutamine phosphoryl amidotrans-Jerase is the committed step in purine biosynthesis. Compounds a, b, and d are involved in the regulation. 

The amide group of glutamine is used in purine biosynthesis, where it provides N3 and N9 of the purine ring, and the 2-NH2 group of guanine. 

The ratio of the Michaelis constant of glutamine to inhibitor constants for azaserine and diazo-oxo-norleucine for the two amidotransferases of purine biosynthesis in pigeon liver are given in the accompanying tabulation. 

Another reaction of purine biosynthesis de novo bears a resemblance to these reactions the synthesis of the amide, phosphoribosyl glycineamide. This is also listed in Table 5-IV, together with two closely related reactions, the synthesis of glutamine and of glutathione. 

Because of rapid distribution throughout the amino acid pool of from labeled ammonium salts and individual amino acids, the specific sources of nitrogens other than N-7 could not be determined in whole animals. Following the development of a cell-free system in which purine biosynthesis de novo could be achieved (see below), the rate of this process was shown to be markedly stimulated by supplementation with aspartate and glutamme. Experiments with these amino acids labeled with N indicated that the former contributed N-1, while the amide group of glutamine contributed N-3 andN-9. 

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