Coenzymes folate

The N-5 position is considerably more basic than the N-10 position, and this basicity is one of several factors that control certain preferences in the course of reactions involving tetrahydrofolate. Thus, for-mylation occurs more readily at N-10 while alkylation occurs more readily at N-5. Benkovic and Bullard (1973) have reviewed evidence for an iminium cation at N-5 as the active donor in formaldehyde oxidation-level transfers. Recently, Barrows et al. (1976) have further studied such a mechanism for folic acid. The interconversion of these forms of folate coenzymes by enzymatic means has been reviewed by Stokstad and Koch (1967), and the reader is directed there for further details. Folate coenzymes are involved in a wide variety of biochemical reactions. These include purine and pyrimidine synthesis, conversion of glycine to serine, and utilization and generation of formate. In addition, the catabolism of histidine, with the formation of formiminoglu-tamic acid (FIGLU), is an important cellular reaction involving folate. 

---

H. W. Dickerman, The Role of Folate Coenzyme in the Initiation of Protein Synthesis in Folate Antagonists as Chemotherapeutic Agents, Ann. N.Y. Acad. Sci. 186, 70 (1971). 

P. F. Nixon and J. R. Bertino, Separation and identification of folate coenzymes on DEAE Sephadex. In Methods in Enzymology, XVII, Part B (eds. D. B. McCormick and L. D. Wright), Academic Press, New York,... 

Decreased levels of folate coenzymes needed for various reactions of de novo purine synthesis and thymine synthesis produce shortages of deoxyribonudeotides and consequent impaired DNA synthesis in many tissues. 

Folate coenzyme concentrations may also decline as a result of treatment with drugs that inhibit dihydrofolate reductase, eg, methotrexate. 

Figure 10-5. Conversion of deoxyuridylate (dUMP) to deoxythymidylate (dTMP) by thymidylate synthetase. The importance of folate coenzymes in this reaction is illustrated. NADPH + H provide the necessary reducing equivalents and serine is the source of one-carbon units present on N, N °-methylene tetrahydrofolate (THF).

The adverse effects of methotrexate include gastrointestinal complaints, bone marrow suppression, alopecia and nephrotoxicity. The toxic effects of methotrexate may be terminated by administering the fully reduced folate coenzyme leucovorin (folinic acid). Leucovorin rescue permits the administration of high doses of methotrexate, for example in situations where partially resistance has occurred or to obtain cytotoxic concentrations of methotrexate in the CNS. 

Inhibitors. Aside from its role in providing reduced folate coenzymes for cells, this enzyme has attracted a great deal of attention because it appears to be a site of action of the important anticancer drugs methotrexate (amethopterin) and aminopterin.293 364 365 These compounds inhibit dihydrofolate reductase in concentrations as low as 10 8 to 10 9 M. Methotrexate is also widely used as an immunosuppresant drug and in the treatment of parasitic infections. 

Folate coenzymes, 803, 804. See also Folic acid Folate receptors 805 3-Fold axis 333 Folding patterns... 

The trinucleotide ZTP also accumulates, not only in bacteria but also in many eukaryotic cells. Bochner and Ames suggested it may be an alarmone signaling a deficit of folate coenzymes in the cell and causing a shutdown of protein synthesis. ZTP is synthesized by an unusual reaction, transfer of a pyrophosphate group from PRPP (phosphoribosyl pyrophosphate). 

Reactions Requiring Acyl-Group Transfers Linked to Oxidation—Reduction Biotin Mediates Carboxylations Folate Coenzymes Are Used in Reactions for One-Carbon Transfers... 

Structures and enzymatic interconversions of folate coenzymes. The reactive centers of the coenzymes are shown in red. The most active forms of the coenzyme contain oligo- or polyglutamyl groups. 

Involvement of folate coenzymes in one-carbon metabolism. Shown in red are the one-carbon units of the end products that originate with the reactive one-carbon units of the folate coenzymes. 

Folate Coenzymes Are Used in Reactions for One-Carbon Transfers... 

Interestingly, dihydroMTX and tetrahydroMTX were both less potent than MTX itself as inhibitors of DHFR, but proved to be more potent as inhibitors of TS as in the case of the above AP derivatives, dihydroMTX was more inhibitory than the tetrahydro derivative in every system examined [270]. It is unsurprising, in view of our current knowledge of the folate coenzyme pathways, that the unnatural D-enantiomer of 5,10-methylene-5,6,7,8-tetrahydroFA had no activity either as a substrate for or as an inhibitor of DHFR [271],... 

Aminopterin and amethopterin (methotrexate) are 4-amino analogues of folic acid (Fig. 11.3) and interfere with the production of the active folate coenzyme by blocking the enzyme dihydrofolate reductase (reaction b in Fig. 13.1) (see also Fig. 11.6). 

The active form of folate is the tetrahydro-derivative that is formed through reduction by dihydrofolate reductase. This enzymatic reaction (Figure 29.5) is inhibited by trimethoprim, leading to a decrease in the folate coenzymes for purine, pyrimidine, and amino acid synthesis. Bacterial reductase has a much stronger affinity for trimethoprim than does the mammalian enzyme, which accounts for the drug s selective toxicity. [Note Examples of other folate reductase inhibitors include pyrimethamine, which is used with sulfonamides in parasitic infections (see p. 353), and methotrexate, which is used in cancer chemotherapy (see p. 378).]... 

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. 

Folate Coenzyme-Mediated Transfer of One-Carbon Groups Jeanne I. Rader and F. M. Huennekens... 

In the folate coenzymes, the pteridine ring is fully reduced to tetrahydro-folate, although the oxidized form, dihydrofolate, is an important metabolic intermediate. In the reactions of thymidylate synthetase (Section 10.3.3) and methylene tetrahydrofolate reductase (Section 10.3.2.1), the pteridine ring has a redox role in the reaction. The folate coenzymes are conjugated with up to six additional glutamate residues, finked by y-glutamyl peptide bonds. 

Horne DW, Patterson D, and Cook RJ (1989) Effect of nitrous oxide inactivation of vitamin B12-dependent methionine synthetase on the subcellular distribution of folate coenzymes In rat liver. Archives of Biochemistry and Biophysics 270, 729-33. 

The metabolism of folic acid involves reduction of the pterin ting to different forms of tetrahydrofolylglutamate. The reduction is catalyzed by dihydtofolate reductase and NADPH functions as a hydrogen donor. The metabolic roles of the folate coenzymes are to serve as acceptors or donors of one-carbon units in a variety of reactions. These one-carbon units exist in different oxidation states and include methanol, formaldehyde, and formate. The resulting tetrahydrofolylglutamate is an enzyme cofactor in amino acid metabolism and in the biosynthesis of purine and pyrimidines (10,96). The one-carbon unit is attached at either the N-5 or N-10 position. The activated one-carbon unit of 5,10-methylene-H folate (5) is a substrate of T-synthase, an important enzyme of growing cells. 5-10-Methylene-H folate (5) is reduced to 5-methyl-H,j folate (4) and is used in methionine biosynthesis. Alternatively, it can be oxidized to 10-formyl-H folate (7) for use in the purine biosynthetic pathway. 

Folate coenzymes are biusynthesized from dietary fulic acid in humans and other animals. Bacteria and protone must biusynthesize them from PABA and pteridine diphos-... 

If biosynthesis of bacterial (or proto/xral) folate coenzymes is blocked at more than one point in the pathway, the result will be a synergistic antimicrohial effect. This is beneficial because the microbe will not develop resistance a.s readily... 

---