Note: a dashed line (without arrowheads) between two compound names is meant to imply that the two names are just different instantiations of the same compound -- i.e. one may be a specific name and the other a general name, or they may both represent the same compound in different stages of a polymerization-type pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Locations of Mapped Genes:
|Superclasses:||Biosynthesis → Cofactors, Prosthetic Groups, Electron Carriers Biosynthesis → Vitamins Biosynthesis → Folate Biosynthesis|
Folates are required in a variety of reactions (known as one-carbon metabolism) in both bacterial and mammalian tissues, where they act as carriers of one-carbon units in various oxidation states. These one-carbon units are utilized in the biosynthesis of various cellular components, including glycine, methionine, formylmethionine, thymidylate, pantothenate and purine nucleotides.
During folate biosynthesis (as described in superpathway of tetrahydrofolate biosynthesis and salvage) the enzyme dihydrofolate synthetase (encoded in E. coli by folC) adds a glutamate residue to 7,8-dihydropteroate, resulting in 7,8-dihydrofolate, also known as H2PteGlu1. This molecule in turn is reduced by dihydrofolate reductase (FolA) to tetrahydrofolate (H4PteGlu1, or THF). THF can then be converted to several other folate molecules [Sun01a]. However, most folate molecules are further modified in cells by successive additions of glutamate residues, forming folate polyglutamtes (or folylpoly-γ-glutamates). Most of the glutamates are added by γ-carboxy-linked polyglutamylation via an amide linkage to the γ-carboxylate group of the folate or folate derivative. Since these isopeptide bonds are not the normal amide bonds they are not hydrolyzed by peptidases or proteases that are specific for α-carboxyl-linked peptide bonds.
The addition of glutamyl residues probably occurs after the reduction of newly synthesized dihydrofolate to tetrahydrofolate and its conversion to other tetrahydrofolate derivatives.
Apparently, the glutamylation of folate residues serves several goals: it prevents the efflux of folates out of the cell, it increases the binding of folate cofactors to the enzymes of folate interconversion and biosynthesis, and in mammals, it allows the accumulation of folates in the mitochondria, which is required for glycine synthesis [Moran99]. Folylpolyglutamates are generally better substrates for folate-dependent enzymes than their monoglutamyl counterparts. Km values decrease with increasing oligo-γ-glutamyl chain length [Shane89]. At least in one case, the vitamin B12-independent methionine synthetase, there is an absolute requirement for the polyglutamate cofactor [Bognar85].
In addition, many folate enzymes are multifunctional and channel one-carbon units between reactions without achieving equilibrium with the cell medium. Therefore, the conjugated oligo-γ-glutamyl chain can potentially regulate the reaction rates, and allows channeling of the substrate between enzymes in a way which controls biosynthetic pathways [Shane89].
Folylpoly-γ-glutamate synthetase (FPGS), the enzyme that catalyzes the conversion of folates to polyglutamates, has been purified from several organisms, including E. coli [Bognar85]. It is a MgATP-dependent enzyme present in all cells. FPGS forms a complex with MgATP, a folate derivative, and glutamate, in an ordered manner whereby the three substrates are added sequentially [Sun01a]. In E. coli, FPGS is a bi-functional enzyme, which also catalyzes the addition of glutamate to 7,8-dihydropteroate, generating 7,8,-dihydrofolate (dihydrofolate synthetase, (E.C# 22.214.171.124).
In exponentially growing cells of E. coli folylpoly-γ- glutamates have short glutamate chain lengths: mono- and triglutamate derivatives are most abundant, with tetra-, penta- and hexaglutamate derivatives also present (in order of decreasing abundance). However, in stationary phase, cells contain longer-chain-length folypolyglutamates, with the predominant chain length containing six or seven glutamyl residues. These longer chains are generated by a second enzyme, which adds glutamate moieties in α-linkage to tetrahydropteroyl- triglutamates [Ferone86]. However, this enzyme has not been purified, nor has the gene encoding it been identified.
Both folylpolyglutamate synthetases can accept several different folate derivatives as substrates. It seems that the preferred substrate for the addition of a second glutamate residue is 10-formyl-THF (10-formyl-H4PteGlu1), while the preferred substrate for the addition of a third glutamate residue is the glutamated form of 5,10-methylene-THF (5,10-methylene-H4PteGlu2).
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Ferone86: Ferone R, Singer SC, Hunt DF (1986). "In vitro synthesis of alpha-carboxyl-linked folylpolyglutamates by an enzyme preparation from Escherichia coli." J Biol Chem 261(35);16363-71. PMID: 3536926
Bognar87: Bognar AL, Osborne C, Shane B (1987). "Primary structure of the Escherichia coli folC gene and its folylpolyglutamate synthetase-dihydrofolate synthetase product and regulation of expression by an upstream gene." J Biol Chem 262(25);12337-43. PMID: 3040739
Cai95: Cai K, Schirch D, Schirch V (1995). "The affinity of pyridoxal 5'-phosphate for folding intermediates of Escherichia coli serine hydroxymethyltransferase." J Biol Chem 270(33);19294-9. PMID: 7642604
Contestabile00: Contestabile R, Angelaccio S, Bossa F, Wright HT, Scarsdale N, Kazanina G, Schirch V (2000). "Role of tyrosine 65 in the mechanism of serine hydroxymethyltransferase." Biochemistry 39(25);7492-500. PMID: 10858298
Delle94: Delle Fratte S, Iurescia S, Angelaccio S, Bossa F, Schirch V (1994). "The function of arginine 363 as the substrate carboxyl-binding site in Escherichia coli serine hydroxymethyltransferase." Eur J Biochem 225(1);395-401. PMID: 7925461
DiazMejia09: Diaz-Mejia JJ, Babu M, Emili A (2009). "Computational and experimental approaches to chart the Escherichia coli cell-envelope-associated proteome and interactome." FEMS Microbiol Rev 33(1);66-97. PMID: 19054114
Fitzpatrick98: Fitzpatrick TB, Malthouse JP (1998). "A substrate-induced change in the stereospecificity of the serine-hydroxymethyltransferase-catalysed exchange of the alpha-protons of amino acids--evidence for a second catalytic site." Eur J Biochem 252(1);113-7. PMID: 9523719
Florio09: Florio R, Chiaraluce R, Consalvi V, Paiardini A, Catacchio B, Bossa F, Contestabile R (2009). "The role of evolutionarily conserved hydrophobic contacts in the quaternary structure stability of Escherichia coli serine hydroxymethyltransferase." FEBS J 276(1);132-43. PMID: 19019081
Florio09a: Florio R, Chiaraluce R, Consalvi V, Paiardini A, Catacchio B, Bossa F, Contestabile R (2009). "Structural stability of the cofactor binding site in Escherichia coli serine hydroxymethyltransferase--the role of evolutionarily conserved hydrophobic contacts." FEBS J 276(24);7319-28. PMID: 19909338
Griffin64: Griffin MJ, Brown GM (1964). "The biosynthesis of folic acid. III. Enzymatic formation of dihydrofolic acid from dihydropteroic acid and of tetrahydropteroylpolyglutamic acid compounds from tetrahydrofolic acid." J Biol Chem 239;310-6. PMID: 14114858
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