Uracil nucleotides monophosphate

The salvage activities of T. foetus and T. vaginalis also differ (22,77). Unlike T. foetus, the level of uracil PRTase activity is very low. Uracil is converted into uridine by a uridine phosphorylase uridine is then phosphorylated by a uridine phosphotransferase to UMP (Fig. 6.15). Cytidine and thymidine also are converted into their nucleotide monophosphates by phosphotransferase activities. There is no detectable pyrimidine nucleoside kinase activity and the only significant interconversion among salvaged pyrimidines is catalyzed by cytidine deaminase to form uridine. 

Posttranslational modification of preformed polynucleotides can generate additional bases such as pseudouridine, in which D-ribose is linked to C-5 of uracil by a carbon-to-carbon bond rather than by a P-N-glycosidic bond. The nucleotide pseudouridylic acid T arises by rearrangement of UMP of a preformed tRNA. Similarly, methylation by S-adenosylmethionine of a UMP of preformed tRNA forms TMP (thymidine monophosphate), which contains ribose rather than de-oxyribose. 

Nucleotides can be linked together into oligonucleotides through a phosphate bridge at the 5 position of one ribose unit and the 3 position of another. The purine bases, adenine and guanine, have two heterocyclic rings, while the pyrimidines cytosine, thymine, and uracil have one. The structure of adenosine monophosphate is shown in Figure 11. 

Table XIX contains stability constants for complexes of Ca2+ and of several other M2+ ions with a selection of phosphonate and nucleotide ligands (681,687-695). There is considerably more published information, especially on ATP (and, to a lesser extent, ADP and AMP) complexes at various pHs, ionic strengths, and temperatures (229,696,697), and on phosphonates (688) and bisphosphonates (688,698). The metal-ion binding properties of cytidine have been considered in detail in relation to stability constant determinations for its Ca2+ complex and complexes of seven other M2+ cations (232), and for ternary M21 -cytidine-amino acid and -oxalate complexes (699). Stability constant data for Ca2+ complexes of the nucleosides cytidine and uridine, the nucleoside bases adenine, cytosine, uracil, and thymine, and the 5 -monophosphates of adenosine, cytidine, thymidine, and uridine, have been listed along with values for analogous complexes of a wide range of other metal ions (700). Unfortunately comparisons are sometimes precluded by significant differences in experimental conditions.

Nitrogenous base plus sugar moiety are called nucleosides. Ribonucleic acids (RNA) resemble DNA in that nucleoside monophosphates are joined through phosphodiester bonds. RNAs differ in that the sugars are p-D-ribose units and the pyrimidine uracil is found in place of thymine. Molecular structures and nomenclature for nitrogenous bases, nucleosides, and nucleotides are delineated in Table 2.2. 

If the terminal pyrophosphate is removed from a molecule of ATP, the remainder is AMP, adenosine monophosphate, one of the four building blocks of the important biological macromolecules, the nucleic acids. There are two types of nucleic acids (26) ribonucleic acid (RNA), and deoxyribonucleic acid (DNA). RNA is a polymer of four different nucleotides, one of which is AMP, the ribose phosphate of adenine. The other three nucleotides are also ribose phosphates of heterocyclic bases, guanine, cytosine, and uracil. The structure of the four bases is shown in Figure 6. 

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... 

The biosynthetic pathway to pyrimidine nucleotides is simpler than that for purine nucleotides, reflecting the simpler structure of the base. In contrast to the biosynthetic pathway for purine nucleotides, in the pyrimidine pathway the pyrimidine ring is constructed before ribose-5-phosphate is incorporated into the nucleotide. The first pyrimidine mononucleotide to be synthesized is orotidine-5 -monophosphate (OMP), and from this compound, pathways lead to nucleotides of uracil, cytosine, and thymine. OMP thus occupies a central role in pyrimidine nucleotide biosynthesis, somewhat analogous to the position of IMP in purine nucleotide biosynthesis. Like IMP, OMP is found only in low concentrations in cells and is not a constituent of RNA. 

How is the other major pyrimidine ribonucleotide, cytidine, formed It is synthesized from the uracil base of UMP, but UMP is converted into UTP before the synthesis can take place. Recall that the diphosphates and triphosphates are the active forms of nucleotides in biosynthesis and energy conversions. Nucleoside monophosphates are converted into nucleoside triphosphates in stages. First, nucleoside monophosphates are converted into diphosphates by specific nucleoside monophosphate kinases that utilize ATP as the phosphoryl-group donor (Section 9.4). For example, UMP is phosphorylated to UDP by UMP kinase. 

The fluorescence decay of dinucleoside monophosphates containing the modified base ethanoadenine (EAD) (see Table 4) and the natural bases guanine and uracil have been investigated by Kubota et al. [31c]. The fluorescence quantum yields are lower for the conjugates than for the nucleotide of EAD (<0.1 versus 0.52). The observation of multiple exponential fluorescence decay was attributed to multiple conformations. A small component with a decay time similar to that of unmodified EAD was attributed to an extended conformation and two shorter components to loosely stacked folded conformations. While the mechanism of fluorescence quenching was not addressed, an electron transfer mechanism in which singlet EAD can serve as an electron acceptor (with guanine) or electron donor (with uracil) is... 

Purines, pyrimidines and their nucleosides and nucleoside triphosphates are synthesized in the cytoplasm. At this stage the antifolate drugs (sulphonamides and dihydrofolate reductase inhibitors) act by interfering with the synthesis and recycling of the co-factor dihydrofolic acid (DHF). Thymidylic acid (2-deoxy-thymidine monophosphate, dTMP) is an essential nucleotide precursor of DNA synthesis. It is produced by the enzyme thymidylate synthetase by transfer of a methyl group from tetrahydrofolic acid (THF) to the uracil base on uridylic acid (2-deoxyuridine monophosphate, dUMP) (Fig. 12.5). THF is converted to DHF in this process and must be reverted to THF by the enzyme dihydrofolate reductase (DHFR) before... 

Fluorouracil (5-FU) is a fluorinated analog of the naturally occurring pyrimidine uracil, originally synthesized in the late 1950s (Table 124—11). It is a prodrug and must be metabolized to the nucleotide form, fluorodeoxyuridine monophosphate (FdUMP), to be active. In the presence of folates, FdUMP binds tightly to and interferes with... 

Colin Tuma is being treated with 5-fluorouracil (5-FU), a pyrimidine base similar to uracil and thymine. 5-FU inhibits the synthesis of the thymine nucleotides required for DNA replication. Thymine is normally produced by a reaction catalyzed by thymidylate synthase, an enzyme that converts deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP). 5-FU is converted in the body to F-dUMP, which binds tightly to thymidylate synthase in a transition state complex and inhibits the reaction (recall that thymine is 5-methyl uracil). Thus, thymine nucleotides cannot be generated for DNA synthesis, and the rate of cell proliferation decreases. 

Studies of CIO2 reactions with nucleic acid constituents showed that it is virtually inert toward nucleotides of adenine, cytosine, or uracil, but that guanosine monophosphate (110) was rapidly attacked at pH 7.1 (less rapidly at lower pHs). 

Fig. 11.1.4. Separation of uracil and 5-fluorouracil bases, nucleosides and nucleotides by reversed phase ion-pair HPLC. Chromatographic conditions column, Bondapak Cig (300 x 4 mm) mobile phase, (from 0-30 min) 0.1 mM tetrabutylammonium hydrogen sulphate (Cjg), 2.5 mM tetraethylammonium bromide (Cg) and 2% methanol in 2 mM sodium acetate, 1.5 mM phosphate buffer, pH 6.0 (Buffer A) (from 30-50 min) Buffer A-i-30 mM phosphate detection, UV at 254 nm. Peaks FU, fluorouracil FUR, fluorouracU riboside/ FUdR, fluorouracil deoxyriboside FUMP, fluorouridine 5 -monophosphate 5 dFUR, 5 -deoxyfluorouracil riboside FdUMP, deoxyfluorouri-dine monophosphate UDPG, uridine diphosphoglucose UDP, uridine diphosphate dUDP, deoxyuridine monophosphate UTP, uridine triphosphate. Reproduced from Au et al. (1982), with permission.

How do we target DNA synthesis Two typical drugs (Fig. 40.1) are 5-fluorouradl and methotrexate, and both of them target the supply of thymine nucleotides. As we saw in Topics 33 and 34, thymine is one of the four DNA bases. RNA has uracil instead of thymine, and the difference between them is that thymine has an extra CH3 group (Fig. 40.2). Uracil thus supplies building blocks for RNA, but it is also a precursor in the pathway to make the thymine building blocks for DNA. Uracil is converted into deoxyuridine monophosphate, and in the next step the CH3 group is added to make deoxythymidine monophosphate (Fig. 40.3). The... 

Cytidine monophosphate (CMP) and dCMP are deaminated to uracil and dTMP is degraded to thymine. Both uracil and thymine can be recycled to nucleotides by the salvage pathway. Alternatively, they can be degraded to -alanine and P-aminoisohutyrate, respectively. 

Mechanism of action Fluorouracil is an analogue of uracil. After entry into cells it undergoes conversion to active metabolites fluorodeoxyuridine monophosphate (FdUMP), fluorodeoxyuridine triphosphate (FdUTP) and fluorouridine triphosphate (FUTP). FdUMP directly inhibits thymidy-late synthetase, reducing the availability of thymidine nucleotides, which are required for DNA synthesis, until new enzyme can be synthesized. FUTP is incorporated into RNA and causes impaired RNA processing and functioning, which disrupts cellular metabolism and viability [71 ]. 

On the other hand, as seen in this chapter and in earlier chapters, the formation of phosphates of adenine (e.g., AMP, ADP, and ATP), guanidine (e.g., GTP), cytosine (e.g.,cytidine monophosphate [CMP]), uracil (e.g., uridine monophosphate [UMP]), and dTMP have all involved the carbohydrate scaffold as a building block for the formation of the finished heterocyclic base (purine or pyrimidine). It is also important to realize that, as part of nucleotide salvage pathways, it has been found that a family of enzymes collectively known as phosphorylases serves to catalyze reactions between free bases and phosphate esters of carbohydrates (and related compounds). For example, as shown in Scheme 14.13, the generalized enzyme, purine nucleoside phosphorylase (EC 2.4.2.1), catalyzes the conversion of a purine with... 

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