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 Toxoplasma gondii. In addition to the de novo folate biosynthesis pathway, T. gondii 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 Apicomplexa genomes [1].

 

T. gondii also possesses 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 T. gondii (phenylalanine metabolism) and absent in P. falciparum. 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 (Refer to folate biosynthesis pathway in MPMP). The two aromatic amino acid hydroxylase enzymes present in T. gondii genome utilises tetrahydrobiopterin as cofactor. Homology-based search has also led to the identification of five enzymes catalysing biosynthesis of molybdopterin from GTP. Sulfite oxidase, an enzyme utilising molybdopterin as cofactor is also present in T. gondii.

 

Enzyme EC Number Gene id Protein localisation Localisation data source
Sepiapterin reductase 1.1.1.153 TGME49_231920 Cytoplasm-nuclear Previous publication
6,7-dihydropteridine reductase 1.5.1.34 TGME49_285750    
Bifunctional dihydrofolate reductase/ thymidylate synthase 1.5.1.3; 2.1.1.45 TGME49_249180 Cytosol Previous publication
Bifunctional methylenetetrahydrofolate dehydrogenase/methenyltetrahydro folate cyclohydrolase 1.5.1.5; 3.5.4.9 TGME49_257625    
Molybdopterin cofactor synthesis protein  2 (MOCS2/MoaE) 2.-.-.- TGME49_273350    
Serine hydroxymethyltransferase 2.1.2.1 TGME49_234190 Cytoskeleton Previous publication
Methionyl-tRNA formyltransferase 2.1.2.9 TGME49_235640    
Bifunctional dihydropteroate synthase/dihydro-hydroxymethylpterin pyrophosphokinase 2.5.1.15; 2.7.6.3 TGME49_259550 Mitochondrion Previous publication
Aminodeoxychorismate synthase 2.6.1.85 TGME49_202920    
Alkaline phosphatase 3.1.3.1 TGME49_252380 Plasma membrane Previous publication
Alkaline phosphatase 3.1.3.1 TGME49_265830    
Gamma-glutamyl hydrolase 3.4.19.9 TGME49_243350    
GTP cyclohydrolase 3.5.4.16 TGME49_253780    
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 TGME49_305800    
Bifunctional tetrahydrofolylpolyglutamate synthase/dihydrofolate synthase 6.3.2.12; 6.3.2.17 TGME49_271350 Cytosol Previous publication
Pterine (BT1 family) transporter none TGME49_201690 Membrane GO annotation
Folate/biopterin (BT1 family) transporter none TGME49_205490 Membrane GO annotation
Molybdopterin cofactor synthesis protein 3 (MOCS3/MoaB) none TGME49_212100    
Molybdopterin cofactor synthesis protein 1 (MOCS1) none TGME49_213780    
Folate/Methotrexate transporter none TGME49_225140 Membrane GO annotation
Pterine (BT1 family) transporter none TGME49_231420 Membrane GO annotation
Molybdopterin cofactor synthesis protein 1 (MOCS1/MoaC) none TGME49_240930    
Folate/Methotrexate transporter none TGME49_260220 Membrane GO annotation
Pterine (BT1 family) transporter none TGME49_266366 Membrane GO annotation
Molybdopterin cofactor synthesis protein 1 (MOCS1/MoaA) none TGME49_293480 Nucleus Previous publication

 

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Molybdopterin dependent enzymes in T. gondii genome

 

Enzyme EC Number Gene id
Sulfite oxidase 1.8.3.1 TGME49_295720

 

 

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