Tetrahydrofolate Synthesis

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. 

Trimethoprim inhibits bacterial DHF reductase, the human enzyme being significantly less sensitive than the bacterial one (rarely, bone marrow depression). A 2,4-diaminopyrimidine, trimethoprim has bacteriostatic activity against a broad spectrum of pathogens. It is used mostly as a component of co-trimoxazole. 

Co-trimoxazole is a combination of trimethoprim and the sulfonamide sulfomethoxa-zole. Since THF synthesis is inhibited at two successive steps, the antibacterial effect of co-trimoxazole is better than that of the individual components. Resistant pathogens are infrequent a bactericidal effect may occur. Adverse effects correspond to those of the components. 

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Tetrahydrofolic acid (THF) is a co-en-zyme in the synthesis of purine bases and thymidine. These are constituents of DNA and RNA and required for cell growth and repUcation. Lack of THF leads to inhibition of cell proUferation. 

Introduced in 1935, they were the first broad-spectrum chemotherapeutics. 

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Trimethoprim (TMP), a folate analog and inhibitor of dihydrofolate reductase (Figure V-1-3), is usually used together with sulfamethoxazole (SMX). The simultaneous inhibition of the tetrahydrofolate synthesis pathway at two steps has a synergistic effect and prevents the rapid generation of resistance. The clinical uses and side effects of TMP-SMX are discussed. 

We are familiar with several examples of chemical activation as a strategy for group transfer reactions. Acetyl-CoA is an activated form of acetate, biotin and tetrahydrofolate activate one-carbon groups for transfer, and ATP is an activated form of phosphate. Luis Leloir, a biochemist in Argentina, showed in the 1950s that glycogen synthesis depended upon sugar nucleotides, which may be... 

Methotrexate (MTX, chemical structure shown in Fig. 1.) competitively inhibits the dehyrofolate reductase, an enzyme that plays an essential role in purine synthesis. The dehydrofolate reductase regenerates reduced folates when thymidine monophosphate is formed from deoxyuridine monophosphate. Without reduced folates cells are unable to synthesize thymine. Administration of N-5 tetrahydrofolate or N-5 formyl-tetrahydrofolate (folinic acid) can bypass this block and rescue cells from methotrexate activity by serving as antidote. 

However, an almost classical attempt to generate an a, -unsaturated e-capro-lactam 87 involved a Knoevenagel condensation of 86 as the ring forming step. The material was employed in a synthesis of conformationally restricted analogs of some anti cancer agents (tetrahydrofolate analog) (Scheme 17) [24]. 

The carbons added in reactions 4 and 5 of Figure 34-2 are contributed by derivatives of tetrahydrofolate. Purine deficiency states, which are rare in humans, generally reflect a deficiency of folic acid. Compounds that inhibit formation of tetrahydrofolates and therefore block purine synthesis have been used in cancer chemotherapy. Inhibitory compounds and the reactions they inhibit include azaserine (reaction 5, Figure 34—2), diazanorleucine (reaction 2), 6-mercaptopurine (reactions 13 and 14), and mycophenofic acid (reaction 14). 

Since biosynthesis of IMP consumes glycine, glutamine, tetrahydrofolate derivatives, aspartate, and ATP, it is advantageous to regulate purine biosynthesis. The major determinant of the rate of de novo purine nucleotide biosynthesis is the concentration of PRPP, whose pool size depends on its rates of synthesis, utilization, and degradation. The rate of PRPP synthesis depends on the availabihty of ribose 5-phosphate and on the activity of PRPP synthase, an enzyme sensitive to feedback inhibition by AMP, ADP, GMP, and GDP. 

Since the end products of pyrimidine catabolism are highly water-soluble, pyrimidine overproduction results in few clinical signs or symptoms. In hypemricemia associated with severe overproduction of PRPP, there is overproduction of pyrimidine nucleotides and increased excretion of p-alanine. Since A, A -methyl-ene-tetrahydrofolate is required for thymidylate synthesis, disorders of folate and vitamin Bjj metabofism result in deficiencies of TMP. 

The methylation of deoxyuridine monophosphate (dUMP) to thymidine monophosphate (TMP), catalyzed by thymidylate synthase, is essential for the synthesis of DNA. The one-carbon fragment of methy-lene-tetrahydrofolate is reduced to a methyl group with release of dihydrofolate, which is then reduced back to tetrahydrofolate by dihydrofolate reductase. Thymidylate synthase and dihydrofolate reductase are especially active in tissues with a high rate of cell division. Methotrexate, an analog of 10-methyl-tetrahydrofolate, inhibits dihydrofolate reductase and has been exploited as an anticancer drug. The dihydrofolate reductases of some bacteria and parasites differ from the human enzyme inhibitors of these enzymes can be used as antibacterial drugs, eg, trimethoprim, and anti-malarial drugs, eg, pyrimethamine. 

Folic acid antagonist inhibits dihydrofolate reductase (DHFR) blocks reduction of folate to tetrahydrofolate inhibits de novo purine synthesis results in arrest of DNA, RNA, and protein synthesis... 

DHFR has been the object of intense research for the last few decades. The enzyme catalyses the NADPH-dependent reduction of 7,8-dihydrofolate to 5,6,7,8 tetrahydrofolate, a chemical which participates in the thymidilate synthesis cycle. Thus, the enzyme is crucial in the synthesis of thymidine monophosphate as well as in various one-carbon unit transfer reactions. 

Ribonucleotide reductase works on ribo-A, -U, -G, -C diphosphates to give the deoxynucleotide. The deoxyuridine, which is useless for RNA synthesis, is converted to deoxythymidine by the enzyme thymidylate synthase, which uses methylene tetrahydrofolate as a one-carbon donor. The odd thing here is that ribonucleotide reductase uses the UDP as a substrate to give the dUDP. This must then be hydrolyzed to the dUMP before thymidylate synthase will use it to make dTMP. Then the dTMP has to be kinased (phosphorylated) up to dTTP before DNA can be made. 

The answer is c. (Hardman, pp 1058-1059. Katzung, pp 793-795.) Trimethoprim inhibits dihydro folic acid reductase. Sulfamethoxazole inhibits p-aminobenzoic acid (PABA) from being incorporated into folic acid by competitive inhibition of dihydropteroate synthase. Either action inhibits the synthesis of tetrahydrofolic acid. 

With the synthesis of the doubly labeled radioactive compound,34 5-formyl-14C-tetrahydrofolate-3H, investigators have been able to trace the metabolic products when the drug is administered orally or intravenously.50... 

Group-transfer reactions often involve vitamins3, which humans need to have in then-diet, since we are incapable of realizing their synthesis. These include nicotinamide (derived from the vitamin nicotinic acid) and riboflavin (vitamin B2) derivatives, required for electron transfer reactions, biotin for the transfer of C02, pantothenate for acyl group transfer, thiamine (vitamin as thiamine pyrophosphate) for transfer of aldehyde groups and folic acid (as tetrahydrofolate) for exchange of one-carbon fragments. Lipoic acid (not a vitamin) is both an acyl and an electron carrier. In addition, vitamins such as pyridoxine (vitamin B6, as pyridoxal phosphate), vitamin B12 and vitamin C (ascorbic acid) participate as cofactors in an important number of metabolic reactions. 

Fluorouracil to tetrahydrofolate for thymidine and purine synthesis Generates two active metabolites one prevents synthesis of... 

Cotrimoxazole contains the synergstic antibiotics sulfamethoxazole and trimethoprim, which inhibit different steps in the prokaryotic synthesis of tetrahydrofolate. 

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