Thymidine biosynthesis

The inability to absorb Vitamin B12 occms in pernicious anemia. In pernicious anemia intrinsic factor is missing. The anemia results from impaired DNA synthesis due to a block in purine and thymidine biosynthesis. The block in nucleotide biosynthesis is a consequence of the effect of vitamin B12 on folate metabolism. When vitamin B-12 is deficient essentially all of the folate becomes trapped as the N -methyltetrahydrofolate derivative as a result of the loss of functional methionine synthase. This trapping prevents the synthesis of other tetrahydrofolate derivatives. required for the purine and thymidine nucleotide biosynthesis pathways. 

A further step in the pathway leading from the pteroates to folic acid and on to DNA bases requires the enzyme dihydrofolate reductase. Exogenous folic acid must be reduced stepwise to dihydrofolic acid and then to tetrahydrofolic acid, an important cofactor essential for supplying a 1-carbon unit in thymidine biosynthesis and, ultimately, for DNA synthesis (Fig. 38.5). The same enzyme also must reduce endogenously produced dihydrofolate. Inhibition of this key... 

Function To donate methyl groups to phospholipid, biogenic amines, thymidine, and amino acid biosynthesis To provide one-carbon fragments at the level of formaldehyde and formic acid for purine and pyrimidine biosynthesis Location Most everywhere... 

Microbial growth studies also gave an important clue to the intracellular role of vitamin B12 when it was observed that the presence of thymidine overcame the need for B12 in the culture medium of Lactobacillus lactis dorner, suggesting B12 was required for the biosynthesis of thymidine. 

Folic acid is a vitamin, as we developed in chapter 15. It is a complex molecule that serves as an essential precursor for coenzymes involved in the metabolism of one-carbon units. For example, folic acid-derived coenzymes are critically involved in the biosynthesis of thymidine for nucleic acid synthesis and methionine for protein biosynthesis. The synthesis of both demands donation of a methyl group and they come from folic acid-derived coenzymes. 

Many DOHs, such as L-daunosamine, L-epivancosamine or L-ristosamine, contain an amino group at C3, which is introduced by an aminotransferase. The substrate for this reaction is the 3-keto sugar intermediate that arises as a consequence of the action of a 2,3-dehydratase. This transaminahon reaction has been biochemically characterized in the biosynthesis of L-epivancosamine [10]. Using a coupled reaction with EvaB (2,3-dehydratase) and EvaC (aminotransferase), with pyridoxal-5-phosphate (PEP) as a coenzyme and L-glutamate as a cosubstrate, they were able to show conversion of TDP-4-keto-2,6-dideoxyglucose into thymidine-5 -diphospho-3-amino-2,3,6-trideoxy-D-threo-hexopyranos-4-ulose. 

Many DOHs, such as L-mycarose, L-epivancosamine, L-noviose or L-dauno-samine, show an L configuration. Formation of L-DOH requires the action of a 5- or 3,5-epimerase. The epimerase EvaD from the biosynthesis pathway of L-epivancosamine was shown to act as a 5-epimerase on the intermediate thymidine-5 -diphospho-3-amino-2,3,6-trideoxy-D-tIireo-hexopyranos-4-ulose [10]. On the... 

In Fig. 1 various targets of some important cytostatic agents are depicted. Their main mechanisms of action can be briefly summarized as follows. Pentostatin blocks purine nucleotides by inhibiting adenosine deaminase. 6-Mercaptopurine and 6-thioguanine inhibit purine ring biosynthesis and they inhibit nucleotide interconversions. Methotrexate by inhibiting dihydrofolate reduction blocks thymidine monophosphate and purine synthesis. 5-Fluorouracil also blocks thymidine monophosphate synthesis. Dactinomycin, daunorubicin, doxorubicin and mitoxantrone intercalate with DNA and inhibit RNA synthesis. L-asparaginase deaminates... 

The pyrimidine antagonists inhibit the biosynthesis of pyrimidine nucleotides or interfere with vital cellular functions, such as the synthesis or function of nucleic acids. The analogues of deoxycytidine and thymidine that are used are inhibitors of DNA synthesis while 5-fluorouracil (5-FU) an analogue of uracil, is an inhibitor of both RNA function and of the synthesis of thymidylate (see Fig. 2). PALA (N-phosphonoacetyl-L-aspartate), an inhibitor of as-... 

Thymidylate synthase (TS) is the enzyme that converts 2-deoxyuridine monophosphate into thymidine monophosphate. This is a key step in the biosynthesis of DNA. This enzymatic reaction of methylation involves the formation of a ternary complex between the substrate, the enzyme, and tetrahydrofolic acid (CH2FAH4). The catalytic cycle involves the dissociation of this complex and the elimination of FAH4. It is initiated by pulling out the proton H-5, thus generating an exocyclic methylene compound. As the release of a F" " ion is energetically forbidden, the fluorine atom that replaces the proton H-5 cannot be pulled out by the base. This leads to inhibition of the enzyme (Figure 7.2). 

The sugar nucleotides (an uninformative name that has been used for glycosyl nucleotides, or more strictly, glycosyl esters of nucleoside di- or mono-phosphates) were discussed in this Series12 in 1973. Since then, accumulation of new data about these derivatives has continued, and now, about 35 representatives of this class are known to participate in the biosynthesis of polysaccharide chains of bacterial polymers (for a survey, see Ref. 13). These include glycosyl esters of uridine 5 -diphosphate (UDP), thymidine 5 -diphosphate (dTDP), guanosine 5 -diphosphate (GDP), cytidine 5 -diphosphate (CDP), cytidine 5 -monophosphate (CMP), and adenosine 5 -diphosphate (ADP). 

The stimulatory effect of l-(chloromethyl)silatrane on the DNA synthesis in the cells of the regenerating liver is shown in Table 10 and Fig. 3. In a similar way (according to incorporation of 3H-thymidine, 14C-orotic acid and 3H-leucine) it has been found that l-(chloromethyl)silatrane intensifies the DNA, RNA and protein biosynthesis in other developing cells (by 20—60%). This has shown the importance of further investigation of this preparation as a stimulator of cell division and biosynthesis of nucleic acids and proteins. 

Tetrahydrofolic acid (THF) is a coenzyme in the synthesis of purine bases and thymidine. These are constituents of DNA and RNA and are required for cell growth and replication. Lack of THF leads to inhibition of cell proliferation. Formation of THF from dihydrofolate (DHF) is catalyzed by the enzyme dihydrofolate reductase. DHF is made from folic acid, a vitamin that cannot be synthesized in the body but must be taken up from exogenous sources. Most bacteria do not have a requirement for folate, because they are capable of synthesizing it-more precisely DHF-ffom precursors. Selective interference with bacterial biosynthesis of THF can be achieved with sulfonamides and trimethoprim. 

With the introduction of radiochemical methods, DNA biosynthesis and its inhibition is usually followed either by measuring the incorporation of radioactive thymine into thymine auxotrophs of bacteria or the incorporation of radioactive thymidine into prototrophic organisms. In the latter instance, it is practical to include in the experimental medium a large excess of non-radioactive deoxyadenosine in order... 

Kornberg s work on the biosynthesis of deoxyribonucleic acid has shown that enzymes in Escherichia coli extracts catalyze the formation of the 5-triphosphates of 2-deoxyadenosine, 2-deoxyguanosine, 2-deoxycyti-dine, and thymidine from the corresponding monophosphates in the presence of adenosine 5-triphosphate, but fail to catalyze phosphorylation of deoxyuridine 5-phosphate this finding could explain why uracil is not a constituent of deoxyribonucleic acid. 

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