Formation of UMP

The pyrimidine base is formed first and then the nucleotide by the addition of ribose 5-phosphate from 

Sources of the pyrimidine ring atoms in de novo biosynthesis. 

In contrast, in de novo purine nucleotide biosynthesis, ribose 5-phosphate is an integral part of the earliest precursor molecule. 

In the biosynthesis of both pyrimidine and urea (or arginine) (Chapter 17), carbamoyl phosphate is the source of carbon and nitrogen atoms. In pyrimidine biosynthesis, carbamoyl phosphate serves as donor of the carbamoyl group to aspartate with the formation of carbamoyl aspartate. In urea synthesis, the carbamoyl moiety of carbamoyl phosphate is transferred to ornithine, giving rise to citrulline. 

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. 

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Answer B. Leflunomide, used in rheumatoid arthritis, inhibits dihydro-orotic acid dehydrogenase >4 formation of UMP ->4- de novo synthesis of ribonucleotides — arrest of lymphocytes in the GI phase. Glucocorticoids do not decrease expression of lipoxygenase, but by preventing arachidonate formation they decrease activity of the pathway. Misoprostol, used in NSAID-induced GI ulcers, activates receptors colchicine decreases microtubular polymerization ketorolac is a potent NSAID but a nonselective inhibitor of cyclooxygenases. 

The reduction in urinary excretion of both compounds following uridine therapy reflects the utilization of uridine for the formation of UMP by the salvage pathway. A similar phenomenon was observed in hereditary orotic aciduria following uridine replacement therapy which bypasses the congenital enzyme defect (Chapter 5). The reversal of 6-azauridine-induced orotic aciduria by hydroxyurea, methotrexate and cyclophosphamide [251] (i.e. by the drugs affecting the synthesis of DNA without any effect on orotic acid synthesis) suggests that the control of pyrimidine synthesis de novo is linked to DNA synthesis. 

S ATP 4- UMP <4> (<4> the enzyme plays a crucial role in the formation of UDP, CDP and dCDP which are required for cellular nucleic acid synthesis... 

S Additional information <2, 4> (<4> formation of a ternary complex, addition of substrates is random [5] <2> ATP-mediated induced-fit of LID in CMPKcoli modulated by CMP leading to a closed conformation of the active site, protected from water [15] <4> the UMP-CMP kinase has a relaxed enantiospecificity for the nucleoside monophosphate acceptor site, but it is restricted to D-nucleotides at the donor site [26]) [5, 15, 26]... 

Biosynthesis of UMP. The parts of the intermediates derived from aspartate are shown in red. Bold type indicates atoms derived from carbamoyl phosphate. In contrast to purine nucleotide synthesis, where ring formation starts on the sugar, in pyrimidine biosynthesis the pyrimidine ring is completed before being attached to the ribose. 

Richard and coworkers7 showed that proton exchange from UMP in the presence of OMPDC, which requires formation of the carbanion that is generated by decarboxylation, occurs more readily when bound to the protein but is too slow to observe in the absence of protein. In the case of that enzyme, there is... 

Figure 9.88 shows the separation of UMP and uridine with and without the enzyme. Plasma and erythrocytes from both normal subjects and those with a variety of pathologies were used as enzyme sources. The formation of uridine during the reaction was observed. 

The pyrimidine ring is assembled first and then linked to ribose phosphate to form a pyrimidine nucleotide. PRPP is the donor of the ribose phosphate moiety. The synthesis of the pyrimidine ring starts with the formation of carbamoylaspartate from carbamoyl phosphate and aspartate, a reaction catalyzed by aspartate transcarbamoylase. Dehydration, cyclization, and oxidation yield orotate, which reacts with PRPP to give orotidylate. Decarboxylation of this pyrimidine nucleotide yields UMP. CTP is then formed by the amination of UTP. 

The sensitivity of this test is increased by increasing the flux in the pyrimidine biosynthetic pathway. The enhanced flux is accomplished by allopurinol, which by way of oxypurinol ribonucleotide inhibits the formation of final product uridine 5 -phosphate (UMP) in the pyrimidine biosynthesis (Chapter 27). 

The biosynthesis of pyrimidine nucleotides may be conveniently considered in two stages the formation of uridine monophosphate (UMP) and the conversion of UMP to other pyrimidine nucleotides. 

The answer is c. (Murray, pp 375-401. Scriver, pp 2513-2570. Sack, pp 121-138. Wilson, pp 287-320.) The steps of pyrimicfine nucleotide biosynthesis are summarized in the figure below. The first step in pyrimidine synthesis is the formation of carbamoyl phosphate. The enzyme catalyzing this step, carbamoyl phosphate synthetase (1), is feedback-inhibited by UMP through allosteric effects on enzyme structure (not by competitive inhibition with its substrates). The enzyme of the second step, aspartate transcarbamoylase, is composed of catalytic and regulatory subunits. The regulatory subunit binds CTP or ATP TTP has no role in the feedback inhibition of pyrimidine synthesis. Decreased rather than increased activity of enzymes 1 and 2 would be produced by allosteric feedback inhibition. 

The initiation reaction for O antigen subunit biosynthesis occurs at the interface of the plasma membrane and the cytosol where the nucleotide sugar precursors are available. The reaction involves the formation of a phosphodiester bond between a membrane-associated polyprenyl phosphate and a cytosolic UDP-sugar with the release of UMP. Depending on the specific microorganism, this reaction is catalyzed by two different... 

The a- and p-L-fucosyl esters of UMP and dTMP have been prepared as anomeric mixtures by treating the nucleotides with orthoester (15) in DMF. In the absence of methylenetetrahydrofolate cofactor, thymidylate synthetase has been observed to catalyse nucleophilic addition reactions at the ethynyl moiety of 5-ethynyl-2 -deoxyuridylate. On the basis of isotope effects and other kinetic studies, it has been suggested that addition of a thiol group of the enzyme at C-6 of the nucleotides results in formation of the allenic intermediate (16), followed by addition of 2-mercaptoethanol, morpholine, or other nucleophiles at the jp-hybridized carbon of the allene, and subsequent elimination of the enzyme. 

Based on the crystal structure of the Bacillus subtilis enzyme in complex with product UMP, Appleby and associates have proposed a novel, electrophilic substitution mechanism for the ODCase reaction (Fig. 3d) [26]. Instead of requiring the formation of a discrete carbanion in the transition state for decarboxylation, this mechanism postulates that Lys-93 protonates... 

Biosynthesis of the nucleoside skeleton was initially considered to be originated by uridine (or UMP, uridine-5-monophosphate), first oxidized to the 5 -aldehyde, which undergoes aldol condensation with phosphoenolpyruvate to give the octofu-ranuloseuronic acid nucleoside. Subsequent elimination of two carbons (C-7 and C-8 ) followed by transamination would result in the formation of the nucleoside skeleton of the polyoxins. Such a pathway was supported by the isolation of octosyl acid (Figure 4.6) [26, 27]. 

PolA is responsible for the activation of UMP for accepting the PEP (phospho-enolpyruvic acid) moiety for the eventual formation of 3 -EUMP (3 -enolpyruvyl uridine-5-monophosphate). Dephosphorylation by PolJ (a putative tyrosine phosphatase) and cyclization of 3 -EUMPby PolH would immediately follow the reaction of PolA to generate an intermediate which is then converted to the aminohexuronic acid moiety with stepwise functions of PolK, PolD, and Poll, respectively. This latter proposal results in a slight modification of the previously proposed pathway for the biosynthesis of the polyoxin nucleoside skeleton, whose biosynthesis was proposed to be initiated by uridine and PEP as starting substrates [17]. 

Cytidylic acid, thymidylic acid, and uridylic acid (UMP) are compounds in which the sugar moiety of each of the related nucleosides described above is phosphorylated at the 5 position.The sugar moiety combined with the uracil base of the uridyHc acid used for RNA formation is D-ribose, whereas the sugar combined with the thymine of thymidylic acid used in DNA formation is D-2-deoxyribose. 2 -Deoxycytidyhc acid is used in RNA synthesis, whereas cytidyHc acid is used in the formation of DNA. 

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