5,10-Methenyl-tetrahydrofolates

Pomper BK, Vorholt JA, Chistoserdova L, et al. 1999. A methenyl tetrahydromethanopterin cyclohydrolase and a methenyl tetrahydrofolate cyclohydrolase in Methylobacterium extorquens AMI. Eur J Biochem 261 475-80. 

Methylene-, methenyl-, and 10-formyl-tetrahydrofolates are freely interconvertible. The two activities involved - methylene-tetrahydrofolate dehydrogenase and methenyl-tetrahydrofolate cyclohydrolase - form a trifunctional enzyme with 10-formyl-tetrahydrofolate synthetase (Paukert et al., 1976). This means that single-carbon fragments entering the folate pool in any form other than as methyl-tetrahydrofolate can be readily available for any of the biosynthetic reactions shown in Figure 10.4. 

The conversion of 5-formyl-tetrahydrofolate to methenyl-tetrahydrofolate, catalyzed by 5-formyl-tetrahydrofolate cyclohydrolase, is important. Although 5-formyl-tetrahydrofolate is the most commonly used pharmaceutical preparation of the vitamin, a relatively large proportion of orally administered... 

The conversion of 5-formyl-tetrahydrofolate to methenyl-tetrahydrofolate, catalyzed by 5-formyl-tetrahydrofolate cyclohydrolase, is important. Although... 

The entire glycine molecule is added to the growing purine precursor. Then C8 is added by methenyl tetrahydrofolate, N3 by glutamine, C6 by C02, N1 by aspartate, and C2 by formyl tetrahydrofolate (see Figure 7-20, bottom). 

Human IMP synthase has a subunit molecular weight of 62.1 kDa and associates as a dimer. A trifunctional enzyme, Cj-THF synthase, containing (V °-methenyl tetrahydrofolate (5,10-CH-THF) cyclohydrolase and A °-formyl tetrahydrofolate (lO-CHO-THF) synthetase, catalyzes the reactions 5,10-CH2-THF 5,10-CH-THF and THF —> lO-CHO-THF. The A °-CH-THF produced is a substrate for GAR and AICAR transformylases catalyzing reactions 3 and 9 of the pathway. In higher eukaryotes, the dehydrogenase and cyclohydrolase activities are located in one domain of the protein, which is fused to a larger synthetase domain, forming a trifunctional enzyme. 

The catabolism of histidine leads to the formation of formiminoglutamate (section 11.11.6). The formimino group is transferred onto tetrahydrofolate to form formimino-tetrahydrofolate, which is subsequently deaminated to form methenyl-tetrahydrofolate. 

The coenzymes are formed from dihydrofolic acid by the enzyme dihydrofolate hydrogenase which gives tetrahydrofolic acid (Osborn, Freeman and Huennekens, 1958), and this is then modified by various one-carbon substituents. The coenzyme for the insertion of C-2 into purines, i.e. for the formylation of the ribotide 9 14) to give (9.1 s)y is -formyl-5563738-tetrahydrofolic acid. For the earlier stage of the insertion of C-8, i.e. for the formylation of glycinamide ribotide to form (eventually) (9./ ), the coenzyme is -methenyl-tetrahydrofolic acid (9-/6). The... 

Tetrahydrofolate can carry one-carbon fragments attached to N-5 (formyl, formimino, or methyl groups), N-10 (formyl group), or bridging N-5 to N-10 (methylene or methenyl groups). 5-Formyl-tetrahydrofolate is more stable than folate and is therefore used pharma-... 

Methanogenesis from CO2 via methenyl H MPT, methylene H MPT, and methyl H MPT using coenzyme F420H2 (5,6,7,8-tetrahydrofolate). 

To prepare 10-formyl TF1F (formyl donor for the formylation of fMet-tRNAfMet), 10 mg of 5-formyl-tetrahydrofolate (5-formyl THF) are dissolved in 1 ml of 0.1 MHC1 that has been flushed extensively with N2 and kept overnight at 4° in a closed vial. The resulting yellow suspension of 5,10-methenyl THF is agitated and an aliquot withdrawn for spectropho-tometric determination of the concentration (6 mM solution = 85 A355). The expected 5,10-methenyl THF concentration is approximately 20 mM. The cyclic methenyl THF can be stored for several weeks at —20°. The 10-formyl THF is stable for only a few weeks and is freshly prepared hydrolyzing the cyclic methenyl THF. For this purpose, the stock solution is diluted in 100 mM Tris-HCl (pH 8.0) extensively flushed with N2 and supplemented with 100 mM 2-mercaptoethanol. After 15 min incubation at 20°, the solution is divided in aliquots, which are stored at —20°. 

One-carbon units in different oxidation states are required in the pathways producing purines, thymidine, and many other compounds. When a biochemical reaction requires a methyl group (methylation), S-adenos dmethionme (SAM) is generally the methyl donor. If a one-carbon unit in another oxidation state is required (methylene, methenyl, formyl), tetrahydrofolate (THF) typically serves as its donor. 

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