Deoxythymidine 5’-monophosphate dTMP

Thymine Deoxythymidme Deoxythymidylic acid Deoxythymidine monophosphate (dTMP) Deoxythymidine diphosphate (dTDP) Deoxythymidine triphosphate (dTTP)... 

The enzyme tetrahydrofolate reductase, which is essential for the synthesises deoxythymidine monophosphate (dTMP) from deoxyuridine monophosphate, a process essential for DNA synthesis. This enzyme catalyses formation of methylene tetrahydrofate (CH3-FH4) a necessary co-substrate for synthesis of d-TMP catalysed by thymidylate synthase (See Figure 20.12(a) and p. 477). 

Trifluridine (Viroptic) is a fluorinated pyrimidine nucleoside that has in vitro activity against HSV-1 and HSV-2, vaccinia, and to a lesser extent, some adenoviruses. Activation of trifluridine requires its conversion to the 5 monophosphate form by cellular enzymes. Trifluridine monophosphate inhibits the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP) by thymidylate synthetase. In addition, it competes with deoxythymidine triphosphate (dTTP) for incorporation by both viral and cellular DNA polymerases. Trifluridine-resistant mutants have been found to have alterations in thymidylate synthetase specificity. 

The TS mediates the conversion of 2-deoxyuridine monophosphate (dUMP) into deoxythymidine monophosphate (dTMP). This enzymatic methylation reaction is a key step in the synthesis of DNA and involves a ternary complex between the substrate, the enzyme and the co-factor [methylene tetrahydrofolic acid (CH2FAH4)] (Fig. 24) [8,80,81], This transformation represents the sole de novo source of dTMP, a building block for DNA synthesis and repair [82]. 

The other major class of antimalarials are the folate synthesis antagonists. There is a considerable difference in the drug sensitivity and affinity of dihydrofolate reductase enzyme (DHFR) between humans and the Plasmodium parasite. The parasite can therefore be eliminated successfully without excessive toxic effects to the human host. DHFR inhibitors block the reaction that transforms deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP) at the end of the pyrimidine-synthetic pathway. This reaction, a methylation, requires N °-methylene-tetrahydrofolate as a carbon carrier, which is oxidized to dihydrofolate. If the dihydrofolate cannot then be reduced back to tetrahydrofolate (THF), this essential step in DNA synthesis will come to a standstill. 

The folic acid-dependent conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP) carried out by thymidylate synthase is an absolute requirement for DNA synthesis. An unusually high demand for uracil (ura) by certain tumor cells suggested that such an inhibitor of this process could have tumor cell selectivity. 5-Fluorouracil (fl5ura) (80), along with 5-fluorocytosine (fl5cyt) (81) and 5-fluoroorotic acid (fl5oro) (82), were synthesized by Heidelberger in 1957 as part of a... 

Figure 7-16. The transfer of a one-carbon unit from serine to deoxyuridine monophosphate (dUMP) to form deoxythymidine monophosphate (dTMP). FH4 is oxidized to FH2 (dihydrofolate) in this reaction. FH2 is reduced to FH4 by dihydrofolate reductase. Hi indicate the steps at which the antimetabolites methotrexate and 5-fluo-rouracil (5-FU) act.

Folic acid serves as a carrier of one-carbon groups in many metabolic reactions. It is required for the biosynthesis of compounds such as choline, serine, glycine, purines, and deoxythymidine monophosphate (dTMP). 

The best-known example of this type of inhibition is 5 -fluorouracil, a rather old cytostatic agent. Fluorouracil is first converted metabolically into the corresponding phosphodeoxyriboside. This, in turn, blocks DNA biosynthesis by inhibiting thy-midylate synthase, an enzyme which methylates deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP), one of the four building blocks of DNA [77]. 

All the pyrimidine nucleotides can proceed to the deoxy compounds as diphosphates, similar to the purines. However, uridine does not occur in DNA therefore, it must be converted to a thymine derivative. This occurs by the following basic pathway CDP is converted to dCDP. This is converted to dCMP, which can produce dUMP. The dUMP is then methylated in the number 5 position, via N5, N10-methylene tetrahydrofolate. This produces deoxythymidine monophosphate (dTMP) and dihydrofolate. The dTMP then can be converted to dTDP and dTTP, which can be incorporated into DNA (Fig. 20.9). 

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. 

The one-carbon groups carried by FH4 are used for many biosynthetic reactions. For example, one-carbon units are transferred to the pyrimidine base of deoxyuridine monophosphate (dUMP) to form deoxythymidine monophosphate (dTMP), to the amino acid glycine to form serine, to precursors of the purine bases to produce carbons C2 and C8 of the purine ring, and to vitamin B12. 

The nucleotide deoxythymidine monophosphate (dTMP) is produced from deoxyuridine monophosphate (dUMP) by a reaction in which dUMP is methylated to form dTMP (Fig. 40.5). The source of carbon is N, N °-methylene FH4. Two hydrogen atoms from FH4 are used to reduce the donated carbon to the methyl level. Consequently, dihydrofolate (FH2) is produced. Reduction of FH2 by NADPH in a... 

Decreased methylation of deoxyuridine monophosphate (dUMP) to form deoxythymidine monophosphate (dTMP), a reaction that requires N, N °-methyl-ene tetrahydrofolate as a coenzyme (see Fig. 40.5), leads to an increase in the intracellular dUTP/dTTP ratio. This change causes a significant increase in the incorporation of uracil into DNA. Although much of this uracil can be removed by DNA repair enzymes, the lack of available dTTP blocks the step of DNA repair catalyzed by DNA polymerase. The result is fragmentation of DNA as well as blockade of normal DNA rephcation. 

Pyrimidine bases are first synthesized as the free base and then converted to a nucleotide. Aspartate and carbamoyl phosphate form all components of the pyrimidine ring. Ribose 5-phosphate, which is converted to phosphoribosyl pyrophosphate (PRPP), is required to donate the sugar phosphate to form a nucleotide. The first pyrimidine nucleotide produced is orotate monophosphate (OMP). The OMP is converted to uridine monophosphate (UMP), which will become the precursor for both cytidine triphosphate (CTP) and deoxythymidine monophosphate (dTMP) production. 

Figure 6.54 Biosynthetic pathway of 2 -deoxythymidine monophosphate (dTMP).

The de novo pathway to 2 -deoxythymidine monophosphate (dTMP) synthesis first requires the use of dUMP (2 -deoxyuridine-5 -monophosphate) from the metabolism of either UDP or CDP (cytidine diphosphate). The hydrolysis of dUTP (2 -deoxyuridine-5 -triphosphate) to dUMP and subsequent methylation at C-5 by the action of thymidylate synthase, using A, A i°-methylenetetrahydrofolate (THF) as the methyl donor, generate dTMP (Figure 6.54). The latter is subsequently phosphorylated to deoxy-thymidine triphosphate (dTTP) used in DNA synthesis and repair. 

The addition of one or more phosphates forms a nucleotide, e.g. adenosine (the nucleoside) can form the nucleotides adenosine monophosphate (AMP), adenosine diphosphate (ADP) and adenosine triphosphate (ATP) (Fig. 58.3). Alternatively, the corresponding deoxyribonucleotides are formed, e.g. deoxythymidine monophosphate (dTMP). 

Thymidine monophosphate (deoxythymidine monophosphate [dTMP]) is generated from deoxyuridine monophosphate (dUMP). The methyl group added to the ring is derived from the CIO methylene unit of tetrahydrofolate as described in Chapter 12 (Scheme 12.9), and reduction is brought about by nicotine adenine dinucleotide (NADH, NAD+). A representation of the process is provided in Scheme 14.12. 

Scheme 14.12. A representation of a potential pathway for the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP) while tetrahydrofolate is oxidized to dihydrofolate using nicotinamide adenine dinucleotide (NAD /NADH). EC numbers and some graphic materials in this scheme have been taken from appropriate links in a URL starting with http //www.chem.qmul.ac.uk/iubmb/enzyme.

Thymidylate synthase belongs to a class of enzymes required for DNA replication, and its activity is higher in rapidly proliferating cells. In particular, thymidylate synthase is responsible for methylation of deoxyuridine monophosphate (dUMP, 21) to deoxythymidine monophosphate (dTMP, 22) with the use of 5,10-methylenetet-rahydrofolate (23) as a cofactor (Scheme 2) [12], With fluorodeoxyuridine monophosphate, a slowly-reversible ternary complex 24 is formed instead. Inhibition of thymidylate synthase leads to deoxyribonucleotide imbalance, and hence to interference with DNA synthesis and repair. Alternative mechanism of DNA-directed Fluorouracil effect is misincorporation of fluorodeoxyuridine triphosphate (20) into DNA. Analogously, fluorouridine triphosphate (17) is extensively incorporated into different RNA species, disrapting their normal processing and function [7,8,11]. 

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