Tetrahydrofolate reactions

L-Tyrosine metabohsm and catecholamine biosynthesis occur largely in the brain, central nervous tissue, and endocrine system, which have large pools of L-ascorbic acid (128). Catecholamine, a neurotransmitter, is the precursor in the formation of dopamine, which is converted to noradrenaline and adrenaline. The precise role of ascorbic acid has not been completely understood. Ascorbic acid has important biochemical functions with various hydroxylase enzymes in steroid, dmg, andhpid metabohsm. The cytochrome P-450 oxidase catalyzes the conversion of cholesterol to bUe acids and the detoxification process of aromatic dmgs and other xenobiotics, eg, carcinogens, poUutants, and pesticides, in the body (129). The effects of L-ascorbic acid on histamine metabohsm related to scurvy and anaphylactic shock have been investigated (130). Another ceUular reaction involving ascorbic acid is the conversion of folate to tetrahydrofolate. Ascorbic acid has many biochemical functions which affect the immune system of the body (131). 

Catalytic reduction of folic acid to 5,6,7,8-tetrahydrofolic acid (225) proceeds fast in trifluoroacetic acid (66HCA875), but a modified method using chemical reductants leads with sodium dithionite to 7,8-dihydrofolic acid (224). Further treatment with sodium borohydride gives (225) which has been converted into 5-formyl-(6i ,S)-5,6,7,8-tetrahydro-L-folic acid (leucovorin) (226) by reaction with methyl formate (equation 70) (80HCA2554). 

We are familiar with several examples of chemical activation as a strategy for group transfer reactions. Acetyl-CoA is an activated form of acetate, biotin and tetrahydrofolate activate one-carbon groups for transfer, and ATP is an activated form of phosphate. Luis Leloir, a biochemist in Argentina, showed in the 1950s that glycogen synthesis depended upon sugar nucleotides, which may be... 

N5-Methyltetrahydrofolate homocysteine methyl-transferase (= methionine synthase). This reaction is essential to restore tetrahydrofolate from N5-methyltetrahydrofolate (Fig. 2). 

The carbons added in reactions 4 and 5 of Figure 34-2 are contributed by derivatives of tetrahydrofolate. Purine deficiency states, which are rare in humans, generally reflect a deficiency of folic acid. Compounds that inhibit formation of tetrahydrofolates and therefore block purine synthesis have been used in cancer chemotherapy. Inhibitory compounds and the reactions they inhibit include azaserine (reaction 5, Figure 34—2), diazanorleucine (reaction 2), 6-mercaptopurine (reactions 13 and 14), and mycophenofic acid (reaction 14). 

Reaction 12 of Figure 34—7 is the only reaction of pyrimidine nucleotide biosynthesis that requires a tetrahydrofo-late derivative. The methylene group of A A Tmethyl-ene-tetrahydrofolate is reduced to the methyl group that is transferred, and tetrahydrofolate is oxidized to dihydro-... 

The major point of entry for one-carbon fragments into substimted folates is methylene tetrahydrofolate (Figure 45-16), which is formed by the reaction of glycine, serine, and choHne with tetrahydrofolate. Serine is the most important source of substituted folates for biosynthetic reactions, and the activity of serine hy-... 

Berman MH, AC Frazer (1992) Importance of tetrahydrofolate and ATP in the anaerobic O-demethylation reaction for phenylmethylethers. Appl Environ Microbiol 58 925-931. 

DHFR has been the object of intense research for the last few decades. The enzyme catalyses the NADPH-dependent reduction of 7,8-dihydrofolate to 5,6,7,8 tetrahydrofolate, a chemical which participates in the thymidilate synthesis cycle. Thus, the enzyme is crucial in the synthesis of thymidine monophosphate as well as in various one-carbon unit transfer reactions. 

FIGURE 40-3 Glycine cleavage system and some related reactions. Glycine and serine are readily interchangeable. Enzymes (1) Glycine cleavage system (2) and (4) Serine hydroxymethyltransferase (3) N5 10-methylenetetrahydrolate reductase. N5 I0-CH2-FH4, N5>10-methylenetetrahydrolate FH4, tetrahydrofolic acid. 

A relatively large number of agents have been utilized to treat this intractable disorder folinic acid (5-formyl-tetrahydrofolic acid), folic acid, methyltetrahydrofolic acid, betaine, methionine, pyridoxine, cobalamin and carnitine. Betaine, which provides methyl groups to the beta i ne ho mocystei ne methyltransferase reaction, is a safe treatment that lowers blood homocysteine and increases methionine. 

Fig. 2.4 The S-methylmethionine cycle and its interaction with the activated methyl cycle. The SMM cycle operates within the activated methyl cycle, and in effect short-circuits it. The reactions mediated by MMT and HMT are shown in bold. THF, tetrahydrofolate CH2-THF, 5,10-methylenetetrahydrofolate,...

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... 

Group-transfer reactions often involve vitamins3, which humans need to have in then-diet, since we are incapable of realizing their synthesis. These include nicotinamide (derived from the vitamin nicotinic acid) and riboflavin (vitamin B2) derivatives, required for electron transfer reactions, biotin for the transfer of C02, pantothenate for acyl group transfer, thiamine (vitamin as thiamine pyrophosphate) for transfer of aldehyde groups and folic acid (as tetrahydrofolate) for exchange of one-carbon fragments. Lipoic acid (not a vitamin) is both an acyl and an electron carrier. In addition, vitamins such as pyridoxine (vitamin B6, as pyridoxal phosphate), vitamin B12 and vitamin C (ascorbic acid) participate as cofactors in an important number of metabolic reactions. 

Two mechanisms have been proposed for the ATP-dependent enzymic formylation of tetrahydrofolate at N-10. In the first, ATP and formate react initially to generate formylphosphate, which is the active formylating species. In the second mechanism, a cyclic phosphorodiamidate (37) is formed in the initial reaction between ATP and... 

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. 

Figure 15.2 Structural formula of tetrahydrofolate and representation of derivatives involved in single carbon transfer. The tetrahydrofolate is always part of a complex with several glutamate residues. The parent compound, pteroylglutamate (folate) lacks four hydrogen atoms, one each from carbon atoms 5, 6, 7 and 8. Tetrahydrofolate can exist in any one of three oxidation states, as shown they are interconvertible through oxidereduction reactions. Each plays a individual and different role is synthesis of key compounds (See below).

The compounds that are the immediate methyl gronp donors are methyltetra hydrofate (CH3-FH4) and S-adenosyl methionine (SAM) (see Figure 15.2). These are involved in, at least, five key reactions or processes which are summarised in Figure 15.4. Complexity arises in the topic of methyl group transfer in formation and reformation of the methylating compounds 5-adenosylmethione and methyl tetrahydrofolate. There are four important reactions in the formation utilisation and then the reformation of 5-adenosylmethionine as follows ... 

Figure 15.5 Four reactions involved in methylatlon. The reactions are (1) formation of S-adenosylmethlonIne (SAM) (11) transfer of methyl group to an acceptor (111) conversion of S-adenosylmethlonIne to homocysteine (Iv) conversion of homocysteine to methionine using methyl tetrahydrofolate as the methyl donor with the formation of FH4.

Figure 15.6 Formation of methyl tetrahydrofolate and SAM from serine. Reaction (1) is described in Appendix 8.3. Reaction (ii) is Figure 15.5 and the several reactions represent in reaction (iv) are discribed in Figure 15.4.

The coenzyme tetrahydrofolate (THF) is the main agent by which Ci fragments are transferred in the metabolism. THF can bind this type of group in various oxidation states and pass it on (see p. 108). In addition, there is activated methyl, in the form of S-adenosyl methionine (SAM). SAM is involved in many methylation reactions—e. g., in creatine synthesis (see p. 336), the conversion of norepinephrine into epinephrine (see p. 352), the inactivation of norepinephrine by methylation of a phenolic OH group (see p. 316), and in the formation of the active form of the cytostatic drug 6-mercaptopurine (see p. 402). 

This enzyme [EC 2.1.2.7], also called 2-methylserine hy-droxymethyltransferase, catalyzes the reaction of o-ala-nine with water and 5,10-methylenetetrahydrofolate to produce tetrahydrofolate and 2-methylserine. The enzyme can also use 2-hydroxymethylserine as a substrate. E. W. Miles (1971) Meth. Enzymol. 17B, 341. 

This enzyme [EC 1.5.1.3], also called tetrahydrofolate dehydrogenase, catalyzes the reversible reaction of 7,8-dihydrofolate with NADPH to produce 5,6,7,8-tetrahy-drofolate and NADP+. The enzyme isolated from mammals and some microorganisms can also slowly catalyze... 

This pyridoxal-phosphate-dependent enzyme [EC 2.1.2.5], also known as glutamate formyltransferase, catalyzes the reaction of 5-formiminotetrahydrofolate with L-glutamate to produce tetrahydrofolate and A-formim-ino-L-glutamate. The enzyme will additionally catalyze the transfer of the formyl moiety from 5-formyltetrahy-drofolate to L-glutamate. This protein occurs in eukaryotes as a bifunctional enzyme also having a formiminote-trahydrofolate cyclodeaminase activity [EC 4.3.1.4]. 

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