5-Methyl-THF

Mechanistic aspects of the action of folate-requiring enzymes involve one-carbon unit transfer at the oxidation level of formaldehyde, formate and methyl (78ACR314, 8OMI2I6OO) and are exemplified in pyrimidine and purine biosynthesis. A more complex mechanism has to be suggested for the methyl transfer from 5-methyl-THF (322) to homocysteine, since this transmethylation reaction is cobalamine-dependent to form methionine in E. coli. 

Natnral folate, introduced through the diet, are mostly reduced folates, i.e., derivatives of tetra-hydrofolate (THE), such as 5-methyl-THF (5-MTHF), 5-formyl-THF, and 5,10-methylene-THF, and exist mainly as pteroylpolyglntamates, with np to nine additional glutamate molecules attached to the pteridine ring. 

Mammalian methylene-THF reductase is a FAD-containing flavoprotein that utilizes NADPH for the reduction to 5-methyl-THF.427,428 Matthews429 suggested that the mechanism of this reaction involves an internal oxidation-reduction reaction that generates a 5-methyl-quinonoid dihydro-THF (Eq. 15-45). Methylene-THF reductase of acetogenic bacteria is also a flavoprotein but it contains Fe-S centers as well. The 237-kDa a4P4 oligomer contains two molecules of FAD and four to six of both Fe and S2 ions.430 431... 

The anaerobic bacterium Clostridium thermoaceticum obtains its energy for growth by reduction of C02 with hydrogen (Eq. 16-46). One of the C02 molecules is reduced to formate which is converted via 5-methyl-THF to the methyl corrinoid 5-methoxybenzimidazolyl-... 

Tetrahydrofolate functions as a carrier of one-carbon units. There are numerous metabolic reactions that require either the addition or removal of a one-carbon unit of some specific oxidation state. THF binds one-carbon units of three oxidation levels the methanol, formaldehyde, and formate states. These are shown in Table 6.4 along with their origins and uses. The various one-carbon units are interconvertible, as shown in Figure 6.5. Nicotinamide coenzymes are involved. In addition, the one-carbon unit may be released as C02. The methanol-level THF-bound one-carbon unit 5-methyl-THF is the storage and transport form. Once formed, its main pathway of metabolism is to form methionine from homocysteine, a reaction that requires vitamin B12 in the form of methylcobalamin (see Figure 6.2 and Chapter 20) ... 

In vitamin B12 deficiency, a functional folate deficiency is observed because folate is "trapped" in the form of 5-methyl-THF. Both folate and vitamin B12 deficiencies result in megaloblastic anemia. Actual folate deficiency is often observed in alcoholics and pregnant women. 

The conversion of homocysteine to methionine requires 5-methyl-THF and the vitamin B12 coenzyme. 

The most reduced coenzyme is 5-methyl tetrahydrofolate poly glutamate. It is the source of the methyl group added to homocysteine regenerating methionine and tetrahydrofolate ready to accept a one-carbon unit from formate or serine. This last reaction is where folic acid and vitamin come together (Figs. 8.49, 8.52, and 8.53). The implications of this reaction and how folic acid can mask pernicious anemia are discussed in the seetion on vitamin Big (cyanocobalamin). Note that the formation of 5-methyl-THF nor-mdly is not reversible. Tetrahydrofolate can be regenerated only if there is adequate methyl cobalamin coenzyme. 

Fig. 3 Structural features of methionine synthase. Methionine synthase is comprised of five domains, which bind homocysteine (HCY), methylfolate (5-methyl THF), cobalamin, and S-adenosylmethionine (SAM). The Cap domain restricts oxidation of cobalamin in its vulnerable Cbl(I) state. Strucmres from E.coli (Bandarian et al., 2002 Dixon et al. 1996) and T.maritima (Evans et al. 2004) (PDB codes 1Q8J, 1K98 and IMSK, respectively) were used to construct this composite model. An uncharacteiized linker segment between the folate and cap domains is absent...

FGAR, 5 -phosphoribosyl Ai-formylglycineamide FTHF, A/ -formyl-tetrahydrofolate GAR, 5-phosphoribosylglycineamide hCys, homocysteine MTHF, 5-methyl-THF Q, quinone such as... 

Homocyslinuiia is an autosomal recessive condition caused by a deficiency of the enzyme, cystalhionine-p-synthase (CBS), which results in the accumulation of homocysteine and methionine and a deficiency of cystathionine and cysteine. There are other disorders to consider when an elevated homocysteine concentration is identified. These disorders include vitanun Bn uptake or activation defects, which may or may not have associated elevated methylmalonic acid, severe 5,10-methylenetetrahydrofolate reductase deficiency, and 5-methyl-THF-homocysteine meth-yltransferase deficiency. The latter two are typically associated with an elevated homocysteine, but low methionine concentrations, so it is relatively easy to disaiminate these conditions from homocystinuria. It is also important to consider that nongenetic causes of hyperhomocyste-inemia exist, such as dietary deficiencies, end-stage renal disease, and administration of several drugs [6]. 

Folic acid exhibits substantially greater stability than most reduced folates and is consequently the form of the vitamin normally used in supplements and to fortify foods. The order of stability of these latter forms is 5-formyl-THF>5-methyl-THF>10-formyl-THF>THF. However, it is important to note that the stability is pH-dependent, with the reduced folates being most stable at pH >8 and pH >2 and least stable between pH 4-6. 

The one-carbon cycle involves generation of one-carbon units by converting tetrahydrofolate to 5,10-methylene-THF from serine. 5,10-Methylene THF is subsequently used for synthesis of thymidine from uracil and purines (DNA synthesis). It is also utilized in regeneration of methionine from homocysteine by donation of the methyl group from 5-methyl THF which is formed by... 

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