Oxidation 10-formyl-tetrahydrofolate

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. 

Formyl-tetrahydrofolate is more stable to atmospheric oxidation than folic acid itself and is commonly used in pharmaceutical preparations it is also known as folinic acid and the synthetic (racemic) compound as leucov-orin. Although the [6S, 67 ] racemic mixture might be expected to have only 50% of the biological activity of the naturally occurring 6S isomer, between 10% to 40% of the 67 isomer is biologically active (Baggott et al., 2001). 

Disposal of Surplus One-Carbon Fragments With the exception of serine hydroxymethyltransferase (Secdon 10.3.1.1), aU of the reactions shown in Figure 10.4 as sources of one-carbon subsdtuted folates are essentially catabolic reactions. When there is a greater entry of single carbon units into the folate pool than is required for biosynthetic reactions, the surplus can be oxidized to carbon dioxide byway of 10-formyl-tetrahydrofolate, thus ensuring the avaUabUity of tetrahydrofolate for catabolic reactions. 

The activity of 10-formyl-tetrahydrofolate dehydrogenase, which catalyzes the oxidation of 10-formyl tetrahydrofolate to CO2 and tetrahydrofolate, is reduced at times of low methionine availability as a means of conserving valuable one-carbon fragments. Therefore, there is no sink for one-carbon substituted tetrahydrofolate, and increasing amounts of folate are trapped as methyl-tetrahydrofolate that cannot be used because of the lack ofvitantin B12 (Krebs etal., 1976). 

Administration of diphenylhydantoin leads to decreased activity of methylene tetrahydrofolate reductase and an increased rate of oxidation of formyl tetrahydrofolate (increased oxidation of formate and histidine), with a fall in methylene- and methyl-tetrahydrofolate - the reverse of the effect of the methyl folate trap (Billings, 1984a, 1984b). 

In experimental animals and with isolated tissue preparations and organ cultures, the test can be refined by measuring the production of G02 from [ C]histidine in the presence and absence of added methionine. If the impairment of histidine metabolism is the result of primary folate deficiency, the addition of methionine wUl have no effect. By contrast, if the problem is trapping of folate as methyl-tetrahydrofolate, the addition of methionine will restore normal histidine oxidation as a result of restoring the inhibition of methylene-tetrahydrofolate reductase by S-adenosylmethionine and restoring the activity of 10-formyl-tetrahydrofolate dehydrogenase, thus permitting more normal folate metabolism (Section 10.3.4.1). 

Methylene-, methenyl- and 10-formyl-tetrahydrofolates are freely interconvertible. This means that when one-carbon folates are not required for synthetic reactions, the oxidation of formyl-tetrahydrofolate to carbon dioxide and folate provides a means of maintaining an adequate tissue pool of free folate. 

In living systems, folinic acid can be synthesized ultimately from folic acid by reduction to tetrahydrofolic acid followed by addition of a 1-carbon fragment to the molecule (N5.N1°-methylenetetrahydrofolate, V). After a 2-step oxidation, the formyl group resides either at the N5 or N10 position or as an equilibrium mixture. The essential reactions are summarized below 32... 

The one-carbon group carried by tetrahydrofolate is bonded to its N-5 or N-10 nitrogen atom (denoted as TV and TV i ) or to both. This unit can exist in three oxidation states (Table 24.2). The most-reduced form carries a methyl group, whereas the intermediate form carries a methylene group. More-oxidized forms carry a formyl, formimino, or methenyl group. The fully oxidized one-carbon unit, CO2, is carried by biotin rather than by tetrahydrofolate. 

Tetrahydrofolate, a carrier of activated one-carbon units, plays an important role in the metabolism of amino acids and nucleotides. This coenzyme carries one-carbon units at three oxidation states, which are interconvertible most reduced—methyl intermediate—methylene and most oxidized—formyl, formimino, and methenyl. The major donor of activated methyl groups is -adenosylmethionine, which is synthesized by the transfer of an adenosyl group from ATP to the sulfur atom of methionine. -Adenosylhomocysteine is formed when the activated methyl group is transferred to an acceptor. It is hydrolyzed to adenosine and homocysteine, the latter of which is then methylated to methionine to complete the activated methyl cycle. 

Thus, acetyl-CoA oxidation via the acetyl-CoA pathway in Archaeoglobus fulgidus differs from that of eubacterial sulfate reducers in several respects It involves tetrahydromethanopterin rather than tetrahydrofolate (H4F) as Ci carrier, and formyl-methanofiiran rather than free formate as an intermediate. Furthermore, coenzyme F420 serves as electron acceptor of two dehydrogenases. In eubacterial sulfate reducers the oxidation of acetyl-CoA to CO2 involves the exergonic formyl-H4F conversion to formate and H4F, which is catalyzed by formyl-H4F synthetase this reaction is coupled with ATP synthesis by the mechanism of substrate level phosphorylation (for literature see refs. [90,209]). The different mechanism of formyl-H4MPT conversion to... 

Folic acid is itself inactive it is converted into the biologically active coenzyme, tetrahydrofolic acid, which is important in the biosynthesis of amino acids and DNA and therefore in cell division. The formyl derivative of tetrahydrofolic acid is folinic acid and this is used to bypass the block when the body fails to effect the conversion of folic acid (see Folic acid antagonists, p. 606). Ascorbic acid protects the active tetrahydrofolic acid from oxidation the anaemia of scurvy, although usually normoblastic, may be megaloblastic due to deficiency of tetrahydrofolic acid. 

Metabolic Roles. There are five forms of tetrahydrofolate polyglutamate, some of which are coenzymes (Fig. 8.49). The most highly oxidized is 10-formyl tetrahydro-... 

The six one-carbon substituents of tetrahydrofolate. The oxidation state is the same in the one-carbon moiety of N -formyl, N -formyl, and N, N" -methenyl FH4. N -FormyI FH4 is required for de novo synthesis of purine nucleotides, whereas N, N -methylene FH4 is needed for formation of thymidilic acid. 

The one-carbon units carried by tetrahydrofolate are interconvertible (Figure 24.10). N NP b]vfet/)ylenetetrahydrofolate can be reduced to N -methyItetrahydrofolate or oxidized to N N -met/iL ny[tetrahydrofolate. N, N -MethenyltetrdihydYo(ohxt( can be converted into N -/ormimtnotetrahydrofolate or formyl-... 

Tetrahydrofolate (THF) is the major source of 1-carbon units used in the biosynthesis of many important biological molecules. This cofactor is derived from the vitamin folic acid and is a carrier of activated 1-carbon units at various oxidation levels (methyl, methylene, formyl, formimino, and methenyl). These compounds can be interconverted as required by the cellular process. The major donor of the 1-carbon unit is serine in the foUowing reaction ... 

Fig. 40.1. Overview of the one-carbon pool. FH4 C indicates tetrahydrofolate (FH4) containing a one-carbon unit that is at the formyl, methylene, or methyl level of oxidation (see Fig. 40.3). The origin of the carbons is indicated, as are the final products after a one-carbon transfer.

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