Pyrimidine biosynthesis, de novo

Under normal conditions, many of the enzymes involved in the pathway for pyrimidine biosynthesis de novo may not operate at maximum efficiency but rather exist in an inhibited state. This inhibition is released during the course of regeneration (for example after partial hepatectomy [133] or castration... 

Hammond, D. J., Burchell, J. R., and Pudney, M. (1985). Inhibition of pyrimidine biosynthesis de novo in Plasmodium falciparum by 2-(4-t-butylcyclohexyl)-3-hydroxy-l,4-naphthoquinone in vitro. Mol. Biochem. Parasitol. 14,97-109. 

Pyrimidine biosynthesis, de novo pyrimidine biosynthesis total synthesis of the pyrimidine ring of uracil, thymine, cytosine and their derivatives from carbamoyl phosphate and aspartate in all living cells. The pyrimidine ring of thiamin (vitamin Bj) has a different biosynthetic origin (see below). 

Table 5-III lists the glutamine amide transfer reactions of purine and pyrimidine biosynthesis de novo, and of purine and pyrimidine ribonucleotide interconversion, and several more that occur in other areas of metabolism. All have features in common 1, 3).

Allopurinol and oxipurinol inhibit pyrimidine biosynthesis de novo by establishing a metabolic block which results in orotic aciduria and orotidinuria. This block appears to be due to inhibition of ODC by one or more of at least four ribonucleotide derivatives 1-allopurinol ribonucleotide, 1-oxipurinol ribonucleotide, 7-oxipurinol ribonucleotide and xanthosine-5 -monophosphate. However, to date the interference has not produced any recognized, clinically significant abnormalities. 

Orotic acid, an intermediate of pyrimidine biosynthesis de novo, reacts with PP-ribose-P to form orotidine-5-monophosphate in... 

Nutrition studies in bacteria indicated that carbamoyl aspartate (also known as ureidosuccinate) is an intermediate in pyrimidine biosynthesis. Finally, uridine nucleotides were found as end products of pyrimidine synthesis de novo [73]. The sequence of reactions leading to the synthesis of UMP is designated as the orotate pathway or pyrimidine synthesis de novo in distinction to the salvage pathway. 

The pyrimidine ring moiety of thiamine has recently been shown to be derived in part from an intermediate in the pathway of purine biosynthesis de novo, although many details regarding this synthesis still remain unclear. The proposed scheme is shown in Fig. 3-3. Aspartate may provide the other part of this ring. 

The pathway of pyrimidine ribonucleotide biosynthesis de novo is summarized below. The enzymes involved are listed in Table 11-1. 

Biosynthesis de novo is the initial regulatory level to be considered. The first complete nucleotide to be formed in the purine biosynthetic pathway is inosine-5 -phosphate (IMP), and that in the pyrimidine pathway is uridine-5 -phosphate (UMP). The two pathways are operationally separate and distinct and metabolically related only in that they share some common participants such as glutamine, CO2,... 

Orotidylate decarboxylase is a key enzyme in the pathway of pyrimidine nucleotide biosynthesis de novo (Fig. 2) ... 

At the present time, we just report some experimental results of a study on the mechanism of action of allopurinol (U-hydroxy-pyrazolo (3, -d ) pyrimidine) and thiopurinol k thiopyrazolo (3, d) pyrimidine) on de novo biosynthesis of uric acid. In this present work, we have compared effect of alio and thiopurinol on oxypurine (xanthine and hypoxanthine) urinary excretion with their rate of synthesis of ribonucleotides in vitro by erythrocyte hemolysate in some particular enzymatic deficiencies (hypoxanthine-guanine phosphoribosyltransferase HGPRT, adenine phosphoribosyl-transferase APRT and xanthinuria). 

The major difference between purine and pyrimidine de novo biosynthesis is that the pyrimidine ring is assembled and then added to PRPP (Fig. 20-1). With purines, the purine ring is built directly on the PRPP. 

Atovaquone is a naphthoquinone whose mechanism of action involves inhibition of the mitochondrial electron transport system in the protozoa. Malaria parasites depend on de novo pyrimidine biosynthesis through dihy-droorotate dehydrogenase coupled to electron transport. Plasmodia are unable to salvage and recycle pyrimidines as do mammalian cells. 

The de novo pathways for purine and pyrimidine biosynthesis appear to be nearly identical in all living organisms. Notably, the free bases guanine, adenine, thymine, cytidine, and uracil are not intermediates in these pathways that is, the bases are not synthesized and then attached to ribose, as might be expected. The purine ring structure is built up one or a few atoms at... 

The common pyrimidine ribonucleotides are cytidine 5 -monophosphate (CMP cytidylate) and uridine 5 -monophosphate (UMP uridylate), which contain the pyrimidines cytosine and uracil. De novo pyrimidine nucleotide biosynthesis (Fig. 22-36) proceeds in a somewhat different manner from purine nucleotide synthesis the six-membered pyrimidine ring is made first and then attached to ribose 5-phosphate. Required in this process is carbamoyl phosphate, also an intermediate in the urea cycle (see Fig. 18-10). However, as we noted... 

FIGURE 22-36 De novo synthesis of pyrimidine nucleotides biosynthesis of UTP and CTP via orotidylate. The pyrimidine is constructed from carbamoyl phosphate and aspartate. The ribose 5-phosphate is then added to the completed pyrimidine ring by orotate phosphori-bosyltransferase. The first step in this pathway (not shown here see Fig. 18-11a) is the synthesis of carbamoyl phosphate from C02 and NH), catalyzed in eukaryotes by carbamoyl phosphate synthetase II. 

A review on the potent inhibitors of de novo pyrimidine and purine biosynthesis summarizes the developments in this field <90MI 718-04) and another report is concerned mainly with the synthetic approaches to the various types of inhibitors of folate-dependent enzymes [Pg.729]

UTP is converted to CTP by amination (see Figure 10.9). UMP is the product of de novo pyrimidine biosynthesis process. 

The atoms of the pyrimidine ring are derived from carbamoyl phosphate and aspartate, as shown in Fig. 15-14. The de novo biosynthesis of pyrimidine nucleotides is shown in Fig. 15-15. The first completely formed pyrimidine ring is that of dihydroorotate. Only after oxidation to orotate is the ribose attached to produce orotidylate. The compound 5-phosphoribosyl 1-pyrophosphate (P-Rib-PP) provides the ribose phosphate. L-Glutamine is used as a substrate donating nitrogen atoms at reactions 1 and 9, catalyzed by carbamoyl phosphate synthetase II and CTP synthetase, respectively a second... 

There are two multifunctional proteins in the pathway for de novo biosynthesis of pyrimidine nucleotides. A trifunctional protein, called dihydroorotate synthetase (or CAD, where the letters are the initials of the three enzymatic activities), catalyzes reactions 1, 2 and 3 of the pathway (HCC>5"- CAP— CA-asp—> DHO Fig. 15-15). The enzymatic activities of carbamoyl phosphate synthetase, aspartate transcarbamoylase and dihydroorotase, are contained in discrete globular domains of a single polypeptide chain of 243 kDa, where they are covalently connected by segments of polypeptide chain whch are susceptible to digestion by proteases such as trypsin. A bifunctional enzyme, UMP synthase, catalyzes reactions 5 and 6 of the pyrimidine pathway (orotate— OMP—> UMP Fig. 15-15). Two enzymatic activities, those of orotate phosphoribosyltransferase and OMP decarboxylase, are contained in a single protein of 51.5 kDa which associates as a dimer. 

Dihydroorotate dehydrogenase, the enzyme catalyzing the dehydrogenation of dihydroorotate to orotate (reaction 4 of the pathway Fig. 15-15), is located on the outer side of the inner mitochondrial membrane. This enzyme has FAD as a prosthetic group and in mammals electrons are passed to ubiquinone. The de novo pyrimidine pathway is thus compartmentalized dihydroorotate synthesized by trifunctional DHO synthetase in the cytosol must pass across the outer mitochondrial membrane to be oxidized to orotate, which in turn passes back to the cytosol to be a substrate for bifunctional UMP synthase. Mammalian cells contain two carbamoyl phosphate synthetases the glutamine-dependent enzyme (CPSase II) which is part of CAD, and an ammonia-dependent enzyme (CPSase /) which is found in the mitochondrial matrix, and which is used for urea and arginine biosynthesis. Under certain conditions (e.g., hyperammonemia), carbamoyl phosphate synthesized in the matrix by CPSase I may enter pyrimidine biosynthesis in the cytosol. 

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