Folates polyglutamyl

Figure 14.2 Chemical structure of folates. Folate molecules consist of pteridine, para-aminobenzoate (pABA), and glutamate moieties. Plants usually contain polyglutamylated forms of folates that are made by the addition of up to about six glutamate residues (which form the y-glutamate tail) attached to the first glutamate, each linked by amide bonds to the preceding molecule of glutamate through the y-carboxyl of the latter. Cl units at various levels of oxidation can be attached to NS and/or N1 0, as indicated by Ri and R2.

Folic acid and its polyglutamyl derivatives can be reduced to the THF coenzymes in two stages the first step is a slow reduction with NADPH to 7,8-dihydro-folate (step a, Fig. 15-18). The same enzyme that catalyzes this reaction rapidly reduces the dihydrofolates... 

Fluoroglutamate-Containing Folates and Antifols Probing the Role of Polyglutamylation in Vitro ... 

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. 

Zeng H, Chen ZS, Belinsky MG, et al. Transport of methotrexate (MTX) and folates by multidrug resistance protein (MRP) 3 and MRP1 effect of polyglutamylation on MTX transport. Cancer Res 2001 61 7225-7232. 

Unlike most enzymes utilizing or metabolizing tetrahydrofolate, methionine synthetase has equal activity toward methyl-tetrahydrofolate mono- and polyglutamates. As discussed in Section 10.2.2, demethylation of methyl-tetrahydrofolate is essendal for the polyglutamylation and intracellular accumulation of folate. 

Experimental animals that have been exposed to ititrous oxide to deplete vitamin B12 show an increase in the proportion of liver folate present as methyl-tetrahydrofolate (85% rather than the normal 45%), largely at the expense of unsubstituted tetrahydrofolate and increased urinary loss of methyl-tetrahydrofolate (Horne et al., 1989). Tissue retention of folate is impaired because methyl-tetrahydrofolate is a poor substrate for polyglutamyl-folate synthetase, compared with unsubstituted tetrahydrofolate (Section 10.2.2.1). As a result of this, vitamin B12 deficiency is frequently accompanied by biochemical evidence of functional folate deficiency, including impaired metabolism of histidine (excretion of formiminoglutamate Section 10.3.1.2) and impaired thymidylate synthetase activity (as shown by abnormally low dUMP suppression Section 10.3.3.3), although plasma concentrations of methyl-tetrahydrofolate are normal or elevated. 

Plasma folylmonoglutamates are transported into cells, where they are converted to the polyglutamyl form, an event catalyzed by folylpoly glutamate synthase. This enzyme uses telrahydrofolates as a substrate. It seems not to recognize or use S-methyl-Hifolates. Hence, the 5-methy 1-Hifolate absorbed from the bloodstream must be converted to folate prior to polyglutarnation. The reaction (KCurs in a... 

Folate is a generic term referring to a family of related compoimds. All of these compoimds represent modifications of the simplest form of the vitamin, folic acid (pteroylglutamic add, PteGlu). Folic add does not occur in natme in appredable amoimts, though it is readily assimilated by the body and converted to the active cofactor forms of the vitamin. Folic acid is the form of the vitamin used in folate supplements. Folates are modified by reduction and by a polyglutamyl chain or tail. The reduced folates include dihydrofolate and tetrahydrofolate. 

Structure of folic acid showing its components. The numbered part participates in one-carbon transfer reactions. In nature, folate occurs largely as polyglutamyl derivatives in which the glutamate residues are attached by isopeptide linkages via the y-carboxyl group. The pteridine ring structure is also present in tetrahydrobiopterin, a coenzyme in the hydroxylation of phenylalanine, tyrosine, and tryptophan (Chapter 17). 

In the tissues, tetrahydrofolate is converted to polyg-lutamyl forms by an ATP-dependent synthetase. In the liver, the major form is pteroyl pentaglutamate. Reduced polyglutamyl forms, each substituted with one of several one-carbon moieties, are the preferred coenzymes of folate-dependent enzymes. Reduction of folate (F) to tetrahydrofolate (FH4) occurs in two steps F is reduced to 7,8-dihydrofolate (FH2), and FH2 is reduced to 5,6,7,8-tetrahydrofolate (FH4). Both of these reactions are catalyzed by a single NADPH-linked enzyme, dihydrofolate reductase (Figure 27-2). 

Kamb A, Finer-Moore J, Calvert AH, et al. Structural basis for recognition of polyglutamyl folates by thymidylate synthase. Biochemistry 1992 31 9883-9890. 

FIG. 1. Chemical structuies of folic acid and tetrahydrofolates. Tetrahydrofolates are shown as the polyglutamyl form, n = 2-9. R represents the one-carbon substituent for the folates shown, and R = H for unsubslituted tetrahydrofolates. 

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