Folate biosynthesis

Folic acid, also known as Vitamin B9 is important to several biological functions. The folate derivative, 5,10-methylene-tetrahydrofolate is essential for the synthesis of dTMP from dUMP and it is therefore crucial for DNA replication and cell division. Tetrahydrofolate is an essential substrate in the biosynthesis of amino acid, glycine. Drugs targeting folate biosynthesis pathway has long been prescribed as anti-malarial agents. The two essential precursors of folate biosynthesis are 4-aminobenzoate (a product of shikimate biosynthesis pathway) and GTP. Thymidylate cycle, a part of folate biosynthesis pathway (below) plays important role in the generation of amino acid glycine and dTMP. Dihydrofolate reductase enzyme replenishes tetrahydrofolate from dihydrofolate for the above mentioned biosynthetic processes. The dihydrofolate reductase and thymidylate synthase activities are catalysed by a bifunctional enzyme in both Plasmodium falciparum and Coccidians Toxoplasma gondii and Neospora caninum. In addition to the de novo folate biosynthesis pathway, T. gondii and N. caninum can salvage folate from host. Massimine et al demonstrated the uptake of radio-labelled exogenous folic acid and revealed the presence of common folate transporter which has high affinity for folic acid. This transporter is suggested to be bidirectional and concentration-dependent. They also added that T. gondii and other apicomplexans encode folate transporters as there are putative transporters homologous to BT1 family proteins present in these apicomplexan genomes [1].

 

T. gondii and N. caninum also possess the enzymes for the synthesis of tetrahydrobiopterin and dihydrobiopterin. Tetrahydrobiopterin is an important cofactor in the hydroxylation of phenylalanine to tyrosine (catalysed by the enzyme phenylalanine hydroxylase), a reaction which is present in the Coccidians (phenylalanine metabolism) and absent in Plasmodium species. The enzymes 1.1.1.153 (catalyses conversion of 6-pyruvoyl-5,6,7,8-tetrahydropterin into tetrahydrobiopterin) and 1.5.1.34 (converts dihydrobiopterin to tetrahydrobiopterin) are absent in P. falciparum. The two aromatic amino acid hydroxylase enzymes present in these Coccidia genomes utilise tetrahydrobiopterin as cofactor. Homology-based search has also led to the identification of five enzymes catalysing biosynthesis of molybdopterin from GTP in T. gondii. The orthologs of four of the five molybdopterin synthesis enzymes in T. gondii were identified in N. caninum genome, whereas the other one is missing in the gene models. Sulfite oxidase, an enzyme utilising molybdopterin as cofactor is present in T. gondii. This sulfite oxidase enzyme is also missing in N. caninum gene models. It is not exactly known whether the sulfite oxidase function is absent in N. caninum.

 

Enzyme EC Number Gene id
Sepiapterin reductase 1.1.1.153 NCLIV_031930
6,7-dihydropteridine reductase 1.5.1.34 NCLIV_014630
Bifunctional dihydrofolate reductase/thymidylate synthase 1.5.1.3; 2.1.1.45 NCLIV_065390
Bifunctional methylenetetrahydrofolate dehydrogenase/methenyltetrahydrofolate cyclohydrolase 1.5.1.5; 3.5.4.9 NCLIV_029470
Molybdopterin cofactor synthesis protein  2 (MOCS2/MoaE) 2.-.-.- NCLIV_034285
Serine hydroxymethyltransferase 2.1.2.1 NCLIV_049400
Methionyl-tRNA formyltransferase 2.1.2.9 NCLIV_050010
Bifunctional dihydropteroate synthase/dihydro-hydroxymethylpterin pyrophosphokinase 2.5.1.15; 2.7.6.3 NCLIV_027230
Aminodeoxychorismate synthase 2.6.1.85 NCLIV_022000
Alkaline phosphatase 3.1.3.1 NCLIV_007470
Alkaline phosphatase 3.1.3.1 NCLIV_039680
Gamma-glutamyl hydrolase 3.4.19.9 NCLIV_018100
GTP cyclohydrolase 3.5.4.16 NCLIV_008130
Dihydroneopterin aldolase 4.1.2.25 Missing in annotation
Aminodeoxy-chorismate lyase 4.1.3.38 Missing in annotation
6-pyruvoyltetrahydro-pterin synthase 4.2.3.12 Missing in annotation
Bifunctional tetrahydrofolylpolyglutamate synthase/dihydrofolate synthase 6.3.2.12; 6.3.2.17 NCLIV_035540
Molybdopterin cofactor synthesis protein 1 (MOCS1/MoaA) none NCLIV_000440
Molybdopterin cofactor synthesis protein 1 (MOCS1) none NCLIV_013520
Molybdopterin cofactor synthesis protein 1 (MOCS1/MoaC) none NCLIV_017070
Folate/biopterin (BT1 family) transporter none NCLIV_020200
Pterin (BT1 family) transporter none NCLIV_023040
Folate/Methotrexate transporter none NCLIV_026780
Pterin (BT1 family) transporter none NCLIV_031680
Folate/biopterin (BT1 family) transporter none NCLIV_039560
Folate/Methotrexate transporter none NCLIV_047540
Molybdopterin cofactor synthesis protein 3 (MOCS3/MoaB) none NCLIV_049260
Pterin (BT1 family) transporter none NCLIV_050350

 

Open in a new window

 

 

Molybdopterin dependent enzymes in N. caninum genome

 

Enzyme EC Number Gene id
Sulfite oxidase 1.8.3.1 NCLIV_002115

 

 

Sources and fates of metabolites

 

Substrate Source pathways Product Fate pathways
GTP Purine metabolism Glycoaldehyde ?
Chorismate Shikimate biosynthesis Pyruvate Glycolysis, Pyruvate metabolism, Alanine metabolism
Glutamine Glutamate metabolism Glutamate Glutamate metabolism
Folate Host    
Serine Glycine, serine and cysteine metabolism Glycine Glycine, serine and cysteine metabolism
Methionyl-tRNA Methionine metabolism    
Dihydrobiopterin Phenylalanine and tyrosine metabolism Tetrahydrobiopterin Phenylalanine and tyrosine metabolism