Pyrimidine metabolism (biosynthesis and salvage)

The apicomplexans Toxoplasma gondii and Plasmodium falciparum can synthesise pyrimidine de novo from aspartate and glutamine. The Cryptosporidium species cannot synthesise pyrimidines de novo and salvages it from host. T. gondii also possess the salvage mechanisms in addition to de novo synthesis, whereas P. falciparum does not possess salvaging ability. The measurement of activities of five of six enzymes catalysing pyrimidine biosynthesis by Hill et al in wide range of protozoan parasites demonstrated the presence of this pathway in P. berghei and T. gondii [1]. The work by same group in P. berghei also demonstrated the detection of all enzymes of pyrimidine biosynthesis pathway and it is identified that carbamoyl-phosphate synthase accepts glutamine rather than ammonia as amine group donor [2]. All the six enzymes leading to UMP generation were biochemically characterised in T. gondii [3]. The carbamoyl-phosphate synthase enzyme of T. gondii have bifunctional N-terminal glutamine amidotransferase domain fused with C-terminal carbamoyl-phosphate synthase domains. It is a unique feature of apicomplexans and it is suggested that it does not possess the allosteric regulation by PRPP as in mammalian enzymes. In addition, acivicin, a glutamine antagonist was demonstrated to inhibit T. gondii growth in vitro suggesting it to be a possible drug target [4, 5]. Pyrimidine biosynthesis can also be indirectly targeted with atovaquone, a ubiquinone analog as blocking mitochondrial electron flow will lead to inhibition of dihydroorotate dehydrogenase, the enzyme catalysing fourth step of UMP biosynthesis (refer to Electron transport chain pathway). The inhibition of thymidylate synthase enzyme (refer to folate biosynthesis pathway) also indirectly targets pyrimidine biosynthesis.

 

The enzyme CTP synthase is present in all apicomplexans and is the only enzyme involved in the conversion of uridine to cytidine nucleotides. In addition to these, the enzyme cytidine/dCTP deaminase, an enzyme involved in deamination of dCTP to dUTP is present in both P. falciparum and T. gondii. Other salvage enzymes present in T.gondii and absent in P. falciparum includes dCMP deaminase, uracil phosphoribosyltransferase (UPRT) and pyrimidine (uridine) phosphorylase. There are also biochemical evidence available for the presence of salvage pathway enzymes mainly UPRT in T. gondii. These also suggest that disruption of either biosynthesis or salvage pathway has no effect on growth in vitro although biosynthesis pathway is required for virulence in mammals [5].

 

Enzyme EC Number Gene id Protein localisation Localisation data source
Ribonucleotide reductase 1.17.4.1 TGME49_207060 Cytosol Previous publication
Ribonucleotide reductase 1.17.4.1 TGME49_294640    
Dihydroorotate dehydrogenase 1.3.5.2 TGME49_210790 Mitochondrion Previous publication
TRX reductase 1.8.1.9 TGME49_309730 Cytosol Previous publication; GO annotation
Thymidylate synthetase 2.1.1.45 TGME49_249180 Cytosol Previous publication
Aspartate carbamoyltransferase 2.1.3.2 TGME49_291640 Nucleus Previous publication
Orotate phosphoribosyl transferase 2.4.2.10 TGME49_259660 Mitochondrion Previous publication
Pyrimidine phosphorylase 2.4.2.3 TGME49_235000    
Pyrimidine phosphorylase 2.4.2.3 TGME49_307030    
Uracil phosphoribosyltransferase 2.4.2.9 TGME49_312480 Cytoskeleton Previous publication
Cytidylate kinase 2.7.4.14 TGME49_257740 Apicoplast Previous publication
Nucleoside-diphosphate kinase 2.7.4.6 TGME49_202210    
Nucleoside-diphosphate kinase 2.7.4.6 TGME49_269035    
Nucleoside-diphosphate kinase 2.7.4.6 TGME49_295350 Cytosol Previous publication
dTMP kinase 2.7.4.9 TGME49_306970 Cytosol Previous publication
UTP-glucose-1-P uridylyltransferase 2.7.7.9 TGME49_218200    
UTP-glucose-1-P uridylyltransferase 2.7.7.9 TGME49_264780    
5'-nucleotidase 3.1.3.5 TGME49_216810    
5'-nucleotidase 3.1.3.5 TGME49_239620    
Dihydroorotase 3.5.2.3 TGME49_293610    
dCMP deaminase 3.5.4.12 TGME49_200430    
Cytidine deaminase/dCTP deaminase 3.5.4.5; 3.5.4.13 TGME49_239630 Mitochondrion Previous publication
Nucleoside-triphosphate pyrophosphatase 3.6.1.19 TGME49_202300 Cytosol Previous publication
dUTP diphosphatase 3.6.1.23 TGME49_233140 Cytosol Previous publication
Orotidine-5'-phosphate decarboxylase 4.1.1.23 TGME49_259690    
Carbonic anhydrase 4.2.1.1 TGME49_259950 Nucleus Previous publication
Carbonic anhydrase 4.2.1.1 TGME49_297070 Plasma membrane Previous publication
CTP synthase 6.3.4.2 TGME49_299210 Nucleus Previous publication
Carbamoyl-P synthase 6.3.5.5 TGME49_215260 Cytosol Previous publication
Enhancer of rudimentary none TGME49_280750 Extracellular? Previous publication

 

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Sources and fates of metabolites

 

Substrate Source pathways Product Fate pathways
Glutamine Glutamate metabolism Glutamate Glutamate metabolism
Aspartate Host UTP/CTP Transcription, Many metabolic pathways
PRPP Pentose phosphate cycle dTTP/dCTP DNA replication
Methylene-THF Folate biosynthesis DHF Folate biosynthesis