10-Formyl tetrahydrofolate synthetase

Tremblay GB, Mejia NR, MacKenzie RE (1992), The NADP-dependent methylenete-trahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase-formyl-tetrahydrofolate synthetase is not expressed in Spodoptera frugiperda cells, J. Biol. Chem. 267 8281-8285. 

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. 

Fig. 1. Main routes involved in the synthesis and interconversion of glycine and serine in plants. The various steps are numbered, and the necessary enzymes are as follows 1, glycolate oxidase, E.C. 1.1.3.1 2, aminotransferases, serine, E.C. 2.6.1.45, and glutamate, E.C. 2.6.1.4, glyoxylate aminotransferases 3, enzyme complex in mitochondria (see Fig. 2) 4, serine-glyoxylate aminotransferase, E.C. 2.6.1.45 5, glycerate dehydrogenase, E.C. 1.1.1.29 6, glycerate kinase E.C. 2.7.1.31 7, D-3-phosphoglycerate phosphatase, E.C. 3.1.3.38 8, d-3-phosphoglycerate dehydrogenase, E.C. 1.1.1.95 9, phosphoserine aminotransferase, E.C. 2.6.1.52 10, phosphoserine phosphatase, E.C. 3.1.3.3 11, serine hydroxymethyltransferase E.C. 2.1.2.1 12, nonenzymatic decarboxylation 13, formyl tetrahydrofolate synthetase, E.C. 6.3.4.3 14, isocitrate iyase, E.C. 4.1.3.1.

In addition to the control of one carbon metabolism by feedback control, regulation of synthesis of the enzymes metabolizing folate derivatives is probably also of importance. The activities of DHF reductase, (Hillcoat et al., 1973) thymidylate synthetase (Rosenberg et al., 1971 Conrad and Ruddle, 1972), serine hydroxymethyl transferase and 10-formyl tetrahydrofolate synthetase (Rosenblatt and Erbe, 1973) increase 4-20 fold as mammalian tissue cultures pass from the lag to logarithmic phase. Control can also occur at the level of entry of the folate cofactors into the cell. 

The enzymatic activity of amido phosphoribosyltransferase (P-Rib-PP— PR A) is low and flux through the de novo pathway in vivo is regulated by the end-products, AMP, IMP and GMP. Inhibition of reaction 1 by dihydrofolate polyglutamates would signal the unavailability of /V1()-formyl tetrahydrofolate, required as a substrate at reactions 3 and 9 of the pathway. The purine pathway is subject to further regulation at the branch point from IMP XMP is a potent inhibitor of IMP cyclohydrolase (FAICAR—> IMP), AMP inhibits adenylosuccinate synthetase (IMP—> sAMP) and GMP inhibits IMP dehydrogenase (IMP— XMP). 

Fig. 15-16 The de novo purine biosynthetic pathway. Rib-5-P, ribose 5-phosphate P-Rib-PP, 5-phosphoribosyl 1-pyrophosphate PRA, 5-phosphoribosylamine IO-CHO-FH4, /Vl0-formyl tetrahydrofolate GAR, glycineamide ribotide FGAR. /V-formylglycineamide ribotide FGAM, /V-formylglycineamidine ribotide AIR, 5-aminoimidazole ribotide CAIR, 4-carboxy-5-aminoimidazole ribotide SAICAR, iV-succino-5-aminoimidazole-4-carboxamide ribotide AICAR, 5-aminoimidazole-4-carboxamide ribotide FAICAR, 5-formamidoimidazole-4-carboxamide ribotide sAMP, /V-succino-AMP. Enzymes (1) amido phosphoribosyltransferase (2) GAR synthetase (3) GAR transformylase (4) FGAM synthetase (5) AIR synthetase (6) AIR carboxylase (7) SAICAR synthetase (8) adenylosuecinase (9) AICAR transformylase (10) IMP cyclohydrolase (11) sAMP synthetase (12) adenylosuecinasc (13) IMP dehydrogenase (14) GMP synthetase.

Methotrexate is a potent inhibitor of dihydrofolate reductase, with an affinity 1,000-fold greater than that of dUiydrofolate. Chemotherapy consists of alternating periods of administration of methotrexate and folate (normally as 5-formyl-tetrahydrofolate, leucovorin) to replete the normal tissues and avoid induction of folate deficiency- so-called leucovorin rescue. As well as depleting tissue pools of tetrahydrofolate, methotrexate leads to the accumulation of relatively large amounts of 10-formyl-dihydrofolate, which is apotentinhibitor of both thymidylate synthetase and glycinamide ribotide transformylase, an intermediate step in purine nucleotide synthesis. It is likely that this, rather than simple depletion of tetrahydrofolate, is the basis of the cytotoxic action of methotrexate (Barametal., 1988). 

In the case of linear gramicidin, the N-terminus of the nonribosomal peptide carries a formyl group (10). Just like in the bacterial ribosomal synthesis, only a formylated first building block is processed additionally by the corresponding enzymatic machinery. Thus, one can find a distinct formylation (F) domain at the very N-terminus of the synthetase. Another formylated NRPS product is coelichelin whose N-terminal ornithine residue is believed to be Nj-formylated in trans by a formyltransferase genetically associated with the NRPS (17). Formyl-tetrahydrofolate is used as source of the formyl group by these enzymes. 

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... 

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. 

Several processes described above use one-carbon derivatives of tetrahydrofolic acid (Fig. 14-22). E.g., the synthesis of the purine ring (Eig. 14-18) requires N °-formyl tetrahydrofolate. Thymidylate synthetase, a key enzyme in pyrimidine synthesis, uses FP,N -methylene tetrahydrofolate both as a donor of a methyl group... 

---