Tetrahydrofolic acid 5,10-methylene

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]. 

MTHF (Fig. 6.3.1) is synthesized biologically from 5,10-methylene-tetrahydrofolic acid through the action of MTE1FR. Commercially, it is available as a calcium salt. The 5MTHF calcium salt is soluble as 500 mg in 100 ml water. A 1 mM solution... 

Phosphorylation of dCDP to dCTP (step k, Fig. 25-14) completes the biosynthesis of the first of the pyrimidine precursors of DNA. The uridine nucleotides arise in two ways. Reduction of UDP yields dUDP (step), Fig. 25-14). However, the deoxycytidine nucleotides are more often hydrolytically deaminated (reactions / and / ) 274 Methylation of dUMP to form thymidylate, dTMP (step n, Fig. 25-14), is catalyzed by thymidylate synthase. The reaction involves transfer of a 1-carbon unit from methylene tetrahydrofolic acid with subsequent reduction using THF as the electron donor. A probable mechanism is shown in Fig. 15-21. See also Box 15-E. Some bacterial transfer RNAs contain 4-thiouridine (Fig. 5-33). The sulfur atom is introduced by a sulfurtransferase (the Thil gene product in E. coli). The same protein is essential for thiamin biosynthesis (Fig. 25-21)274a... 

A metabolic pathway that has received considerable attention is the conversion of 2 -deoxyuridine 5 -monophosphate (dUMP, 6.60) to thymidine 5 -monophosphate (TMP, 6.61) (Scheme 6.13). Without an adequate supply of TMP, a cell or bacterium cannot create DNA for cell division. Therefore, blocking TMP synthesis is an attractive method for slowing the advancement of certain cancers and bacterial infections. Important molecules in the methylation of dUMP are the various folic acid derivatives folic acid (FA, 6.62), dihydrofolic acid (DHF, 6.63), tetrahydrofolic acid (THF, 6.64), and N5, A1 "-methylene tetrahydrofolic acid (MTHF, 6.65) (Figure 6.23). These structures... 

Site of action 5-FU per se is devoid of antineoplastic activity and must be converted to the corresponding deoxynucleotide (5-FdUMP, Figure 38.9), which competes with deoxyuridine monophosphate (dUMP) for thymidylate synthetase. 5-FdUMP acts as a pseudosubstrate and is entrapped with the enzyme and its N5,N10-methylene tetrahydrofolic acid coenzyme in a ternary complex that cannot proceed to products. DNA synthesis decreases due to lack of thymidine, leading to imbalanced cell growth and cell death. [Note Leucovorin is given with 5-FU because the reduced folate coenzyme is required in the thymidylate synthetase reaction. Lack of sufficient coenzyme reduces the effectiveness of the antipyrimidine.] 5-FU is also incorporated into RNA and low levels have been detected in DNA. 

The chiralities at C-6 of natural 5,6,7,8-tetrahydrofolic acid and related folates, e.g. 5,10-methylene-, 5-methyl- and 5-formyl-5,6,7,8-tetrahydrofolic acid, from various biological systems are the same and possess the absolute configuration (S) at C-6 as deduced from an X-ray study of the ion (51) (79JA6114). 

Tetrahydrofolic acid then functions as a carrier of one-carbon groups for amino acid and nucleotide metabolism. The basic ring system is able to transfer methyl, methylene, methenyl, or formyl groups, and it utilizes slightly different reagents as appropriate. These are shown here for convenience, we have left out the benzoic acid-glutamic acid portion of the structure. These compounds are all interrelated, but we are not going to delve any deeper into the actual biochemical relationships. 

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). 

ZD-9331 is a non-nucleosidic inhibitor of thymidylate synthase. It is also an antifolate, in which the quinazoline moiety replaces the pteridine entity, structurally close to methylene tetrahydrofolate (i.e., the second substrate of thymidylate synthase). Moreover, replacement of the acid function of glutamic acid by a tetrazole renders polyglutamination impossible. Consequently, ZD-9331 is active on tumors that are resistant to the usual antifolates. ... 

He isoleucine, Kyn kynurenine, Leu leucine, Lys lysine, Met methionine, MTHFR 5,10-methylene tetrahydrofolate reductase, Orn ornithine, p plasma, P5C pyrroline-5-carboxylic acid, PEA phosphoethanolamine, Phe phenylalanine, P-Hyl O-phosphohydroxylysine, Pip pipecolic acid, Pro proline, Sacch saccharopine, Sar sarcosine, Ser serine,... 

Tetrahydrofolic acid Folic acid (B9) Glutamate Methyl, formyl, residues methylene and formimino groups Bacteria, archaea and eukaryotes... 

Tetrahydrofolate accepts methyl groups, usually from serine. The product, N5,N10-methylene-tetrahydrofolate, is the central compound in 1-carbon metabolism. Tetrahydrofolate can also accept a methyl group from the complete breakdown of glycine. In Figure 5-2, only the N5, C6, and N10 atoms of the pteroic acid are shown for clarity. 

There is at present no unifying concept which would explain the presence of optically active TIQs, TIQ-1-carboxylic acids, and noralkaloids in mammalian systems. The conclusion reached by several investigators that methylenetetrahydrofolate is responsible for the N-methylation of phenethylamines and indolylethylamines (221,222) makes it likely that the one-carbon unit present at C-1 in the noralkaloids is derived from formaldehyde, formed via nonenzymatic disassociation of methylene-tetrahydrofolates (223,224). 

The cause of megaloblastosis is depressed DNA synthesis, as a result of impaired methylation of dCDP to TDP, catalyzed by thymidylate synthetase, but more or less normal synthesis of RNA. As discussed in Section 10.3.3, thymidylate synthetase uses methylene tetrahydrofolate as the methyl donor it is obvious that folic acid deficiency will result in unpaired thymidylate synthesis. It is less easy to see how vitamin B12 deficiency results in impaired thymidylate synthesis without invoking the methyl folate trap hypothesis (Section 10.3.4.1). The main circulating form of folic acid is methyl-tetrahydrofolate before this can be used for other reactions in tissues, it must be demethylated to yield free folic acid. The only reaction that achieves this is the reaction of methionine synthetase (Section 10.8.1). Thus, vitamin B12 deficiency results in a functional deficiency of folate. 

Supplements of400 /xg per day of folic acid, begun before conception, halve the risk of neural tube defect (Section 10.9.4), and similar supplements reduce the plasma concentration of homocysteine in people homozygous for the ther-molabile variant of methylene-tetrahydrofolate reductase (Section 10.3.4.2), although it is not known whether or not this will reduce their risk of cardiovascular disease. A number of manufacturers voluntarily enrich foods with folic acid. In the United States and other countries, there is mandatory enrichment of cereal products with folic acid. 

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