Pyrimidine biosynthesis regulation

Purine and pyrimidine biosynthesis parallel one another mole for mole, suggesting coordinated control of their biosynthesis. Several sites of cross-regulation characterize purine and pyrimidine nucleotide biosynthesis. The PRPP synthase reaction (reaction 1, Figure 34-2), which forms a precursor essential for both processes, is feedback-inhibited by both purine and pyrimidine nucleotides. 

Purine Biosynthesis Is Regulated at Two Levels Pyrimidine Biosynthesis Is Regulated at the Level of Carbamoyl Aspartate Formation Deoxyribonucleotide Synthesis Is Regulated by Both Activators and Inhibitors... 

Pyrimidine Biosynthesis Is Regulated at the Level of Carbamoyl Aspartate Formation... 

How do you explain the observation that pyrimidine biosynthesis in bacteria is regulated at the level of aspartate carbamoyltransferase, whereas most of the regulation in humans is at the level of carbamoyl phosphate synthase ... 

Aspartate transcarbamoylase (aspartate carbamoyltransferase ATCase), a key enzyme in pyrimidine biosynthesis (see Topic FI), provides a good example of allosteric regulation. ATCase catalyzes the formation of N-carbamoylaspar-tate from aspartate and carbamoyl phosphate, and is the committed step in pyrimidine biosynthesis (Fig. 2). The binding of the two substrates aspartate and carbamoyl phosphate is cooperative, as shown by the sigmoidal curve of V0 against substrate concentration (Fig. 3). 

Unlike in purine biosynthesis, the pyrimidine ring is synthesized before it is conjugated to PRPP. The first reaction is the conjugation of carbamoyl phosphate and aspartate to make N-carbamoylaspartate. The carbamoyl phosphate synthetase used in pyrimidine biosynthesis is located in the cytoplasm, in contrast to the carbamoyl phosphate used in urea synthesis, which is made in the mitochondrion. The enzyme that carries out the reaction is aspartate transcarbamoylase, an enzyme that is closely regulated. 

Nucleotide biosynthesis is regulated by feedback inhibition in a manner similar to the regulation of amino acid biosynthesis (Section 24,3). Indeed, aspartate transcarbamoylase, one of the key enzymes for the regulation of pyrimidine biosynthesis in bacteria, was described in detail in Chapter 10. Recall ihaiATCase is inhibited by CTP, the final product ofpyrimidine biosynthesis, and stimulated by ATP. Carbamoyl phosphate synthetase is a site of feedback inhibition in both prokaryotes and eukaryotes. 

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. 

In eukaryotic cells, two separate pools of carbamoyl phosphate are synthesized by different enzymes located at different sites. Carbamoyl phosphate synthetase I (CPS I) is located in the inner membrane of mitochondria in the liver and, to lesser extent, in the kidneys and small intestine. It supplies carbamoyl phosphate for the urea cycle. CPS 1 is specific for ammonia as nitrogen donor and requires N-acetylglutamate as activator. Carbamoyl phosphate synthetase II (CPS II) is present in the cytosol. It supplies carbamoyl phosphate for pyrimidine nucleotide biosynthesis and uses the amido group of glutamine as nitrogen donor. The presence of physically separated CPSs in eukaryotes probably reflects the need for independent regulation of pyrimidine biosynthesis and urea formation, despite the fact that both pathways require carbamoyl phosphate. In prokaryotes, one CPS serves both pathways. 

The precise role of CO2 fixation in the economy of the parasite remains uncertain (see Scheibel, 1988), but the up-regulation of PEPCK in the early stages of sexual development in P. falciparum has been suggested to be important during parasite transition to the mosquito vector (Hayward, 2000). Another possibility is that CO2 fixation supplies aspartate for pyrimidine biosynthesis as well as NADPH (via glutamate dehydrogenase) and NAD (via malic dehydrogenase). 

Pyrimidines play a central role in cellular regulation and metabolism. They are substrates for DNA and RNA biosynthesis, regulators of biosynthesis of some amino acids, and cofactors in the biosynthesis of phospholipids, glycolipids, sugars, and polysaccharides (17B45). Pyrimidine biosynthesis is very complicated and involves formic acid, glutamate, and aspartate as starting materials in a series of enzymatic reactions to eventually form orotic acid. Orotic acid, or uracil-... 

The reaction of carbamoyl phosphate with aspartate to produce W-carbamo-ylaspartate is the committed step in pyrimidine biosynthesis. The compounds involved in reactions up to this point in the pathway can play other roles in metabolism after this point, A -carbamoylaspartate can be used only to produce pyrimidines—thus the term committed step. This reaction is catalyzed by aspartate transcarbamoylase, which we discussed in detail in Ghapter 7 as a prime example of an allosteric enzyme subject to feedback regulation. The next step, the conversion of A-carbamoylaspartate to dihydroorotate, takes place in a reaction that involves an intramolecular dehydration (loss of water) as well as cyclization. This reaction is catalyzed by dihydroorotase. Dihydroorotate is converted to orotate by dihydroorotate dehydrogenase, with the concomitant conversion of NAD to NADH. A pyrimidine nucleotide is now formed by the reaction of orotate with PRPP to give orotidine-5 -monophosphate (OMP), which is a reaction similar to the one that takes place in purine salvage (Section 23.8). Orotate phosphoribosyltransferase catalyzes this reaction. Finally, orotidine-5 -phosphate decarboxylase catalyzes the conversion of OMP to UMP... 

Carrey EA. Key enzymes in the biosynthesis of purines and pyrimidines Their regulation by allosteric efifectors and by phosphoryladon. Biochem Soc Trans 1995 23(4) 899-902. 

C39 Crandall, D. E., Lovatt, C. J. and Tremblay, G. C. Regulation of pyrimidine biosynthesis by purine and pyrimidine nucleosides in slices of rat tissues. Arch. Biochem. Biophys., 188, 194-205 (1978)... 

Question is raised as to whether the allosteric CTP-reacting model of ATCase is a universal mechanism for regulating pyrimidine biosynthesis. In other systems, uridine, rather than cytidine, nucleotides are the most efficient inhibitors, and these usually act noncompetitively [103-105], In other cases, ATCase is not inhibited by any pyrimidine derivative [87,105]. In yeast, as mentioned above [83a], ATCase occurs as a multienzyme complex shared with CP synthase activity. Both activities are inhibited by UTP, and desensitization by heat can occur leading to loss of UTP inhibition and CP synthase activity. 

N. Z5liner and W. Grdbner. Dietary feedback regulation of purine and pyrimidine biosynthesis in man. 

Dietary feedback regulation of purine and pyrimidine biosynthesis in man. 

The biosynthesis of purines and pyrimidines is stringently regulated and coordinated by feedback mechanisms that ensure their production in quantities and at times appropriate to varying physiologic demand. Genetic diseases of purine metabolism include gout, Lesch-Nyhan syndrome, adenosine deaminase deficiency, and purine nucleoside phosphorylase deficiency. By contrast, apart from the orotic acidurias, there are few clinically significant disorders of pyrimidine catabolism. 

Purine Pyrimidine Nucleotide Biosynthesis Are Coordinately Regulated... 

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