Purine metabolism (salvage)

The purine nucleotides are not only required as components of nucleic acids, but also as cofactors of metabolic processes and as a source of energy (ATP). Apicomplexans cannot synthesise purine rings de novo and salvage them from host. The putative transporters involved in the uptake of purine bases and nucleosides from parasitophorous vacuole are present in apicomplexans including Toxoplasma gondii, Plasmodium falciparum and Cryptosporidium parvum. There were four transporters identified in T. gondii of which three are present in the genome of ME49 strain (genome analysed), whereas all four are present in VEG and GT1 strain genomes. The work of De Koning et al in T. gondii demonstrated the presence high affinity purine nucleoside transporter (TgAT2) and purine base transporter (TgNT1) in addition to the already characterised low affinity nucleoside transporter (TgAT1) [1]. TgAT1 has substrate specificity to adenosine and inosine and TgAT2 possess broad substrate specificity. TgNT1 can transport the bases hypoxanthine, guanine and xanthine. The presence of high affinity transporters TgAT2 and TgNT1 suggests that these parasites are capable of utilising very low concentration of nucleobases and nucleosides available in the parasitophorous vacuole.


The two most important enzymes involved in salvage of purine nucleotides in apicomplexans are hypoxanthine-guanine phosphoribosyltransferase (HGXPRTase) and adenosine kinase leading to guanine nucleotide generation and adenine nucleotide generation respectively. HGXPRTase can catalyse the synthesis of IMP, XMP and GMP from salvaged hypoxanthine, xanthine and guanine respectively. IMP dehydrogenase and GMP synthase are the other enzymes which are involved in guanine nucleotide generation. Of the 4 purine nucleosides incorporated into nucleic acids (adenosine, guanosine, inosine and xanthosine), adenosine is incorporated more than 12-folds higher than others. Adenosine kinase is the the most active of all enzymes and is the main route of adenosine incorporation and AMP generation [2] although there are other routes available for both adenosine incorporation and AMP generation. Adenosine can also be utilised for IMP generation catalysed by adenosine deaminase [3] and these IMP can either be converted to guanine nucleotide (HGXPRTase route) or to AMP via the action of adenylosuccinate synthase and adenylosuccinate lyase. In addition, the activity of purine nucleoside phosphorylase was detected with guanosine and inosine. The enzyme AMP deaminase catalysing conversion of AMP to IMP is also present in T. gondii. This and 10-fold higher adenylate kinase activity suggests that adenine nucleotide serves as substrate for generation of guanine nucleotides [2]. The enzyme adenosine kinase is absent in P. falciparum (MPMP purine metabolism pathway), where AMP generation depends on adenylosuccinate synthase and lyase activities. Many of the enzymes of purine salvage pathway in T. gondii were biochemically characterised and their kinetic properties were studied. The reduction of ribonucleotides to deoxyribonucleotides is catalysed by ribonucleoside diphosphate reductase enzyme by oxidising the co-substrate thioredoxin.


Enzyme EC Number Gene id Protein localisation Localisation data source
IMP dehydrogenase TGME49_233110 Cytosol Apiloc; Previous publication
Ribonucleotide di-P reductase TGME49_207060 Cytosol Previous publication
Ribonucleotide di-P reductase TGME49_294640    
TRX reductase TGME49_309730 Cytosol Previous publication
Purine nucleoside phosphorylase TGME49_307030    
Purine nucleoside phosphorylase TGME49_310640 Mitochondrion Previous publication
Hypoxanthine-guanine phosphoribosyltransferase TGME49_200320 Cytosol; Inner membrane complex Apiloc; Previous publication
Adenosine kinase TGME49_250880 Extracellular space Previous publication
Adenylate kinase TGME49_224900 Mitochondrion Previous publication
Adenylate kinase TGME49_230460    
Adenylate kinase TGME49_233400 Mitochondrion Previous publication
Adenylate kinase TGME49_258640    
Adenylate kinase TGME49_269050    
Adenylate kinase TGME49_281970    
Adenylate kinase TGME49_288320    
Nucleoside-diphosphate kinase TGME49_202210    
Nucleoside-diphosphate kinase TGME49_269035    
Guanylate kinase TGME49_242730    
5'-nucleotidase TGME49_216810    
IMP-specific 5'-nucleotidase TGME49_222710    
5'-nucleotidase TGME49_239620 Mitochondrion Previous publication
3',5'-cyclic-nucleotide phosphodiesterase TGME49_202540    
3',5'-cyclic-nucleotide phosphodiesterase TGME49_220420    
3',5'-cyclic-nucleotide phosphodiesterase TGME49_224840    
3',5'-cyclic-nucleotide phosphodiesterase TGME49_226755    
3',5'-cyclic-nucleotide phosphodiesterase TGME49_228500    
3',5'-cyclic-nucleotide phosphodiesterase TGME49_229405    
3',5'-cyclic-nucleotide phosphodiesterase TGME49_233040    
3',5'-cyclic-nucleotide phosphodiesterase TGME49_233065    
3',5'-cyclic-nucleotide phosphodiesterase TGME49_241880 Plasma membrane Previous publication
3',5'-cyclic-nucleotide phosphodiesterase TGME49_257080    
3',5'-cyclic-nucleotide phosphodiesterase TGME49_257945    
3',5'-cyclic-nucleotide phosphodiesterase TGME49_258512    
3',5'-cyclic-nucleotide phosphodiesterase TGME49_266920    
3',5'-cyclic-nucleotide phosphodiesterase TGME49_272650    
3',5'-cyclic-nucleotide phosphodiesterase TGME49_280410 Plasma membrane Previous publication
3',5'-cyclic-nucleotide phosphodiesterase TGME49_293000 Nucleus Previous publication
3',5'-cyclic-nucleotide phosphodiesterase TGME49_310520    
3',5'-cyclic-nucleotide phosphodiesterase TGME49_311060    
3',5'-cyclic-nucleotide phosphodiesterase TGME49_318675    
Adenosine deaminase TGME49_205720 Cytoskeleton Previous publication
Adenosine deaminase TGME49_215480    
AMP deaminase TGME49_234280 Cytosol Previous publication
Inorganic diphosphatase TGME49_283830 Peroxisome Previous publication
Ecto-nucleoside triphosphate diphosphohydrolase TGME49_225290    
Ecto-nucleoside triphosphate diphosphohydrolase TGME49_277240 Parasitophorous vacuole (Extracellular space) Apiloc; Previous publication
Ecto-nucleoside triphosphate diphosphohydrolase TGME49_314410 Cytosol Previous publication
Nucleoside-triphosphate pyrophosphatase TGME49_202300 Cytosol Previous publication
Apyrase TGME49_307800    
Apyrase TGME49_310810    
Nucleoside-diphosphatase TGME49_259960    
Adenylosuccinate lyase TGME49_263190 Nucleus Previous publication
Adenylate cyclase TGME49_216280    
Adenylate cyclase TGME49_230555    
Adenylate cyclase TGME49_270865 Cytosol Previous publication
Adenylate cyclase TGME49_289170    
Guanylate cyclase TGME49_254370 Plasma membrane Previous publication
Adenylosuccinate synthase TGME49_279450 Cytosol Previous publication
GMP synthase TGME49_230450 Cytosol Previous publication
Nucleoside (adenosine) transporter none TGME49_233130 Membrane GO annotation
Nucleoside (adenosine) transporter none TGME49_244440 Membrane GO annotation
Equilibrative nucleoside transporter none TGME49_288540 Membrane GO annotation


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Sources and fates of metabolites


Substrate Source pathways Product Fate pathways
Adenosine Host Ribose-1P Pentose phosphate cycle
Hypoxanthine/xanthine/guanine Host dATP/dGTP DNA replication
Guanosine/inosine Host ATP/GTP Transcription, Many metabolic pathways
PRPP Pentose phosphate cycle    
Glutamine Glutamate metabolism Glutamate Glutamate metabolism
Aspartate Host Fumarate Tricarboxylic acid (TCA) cycle


Nucleoside catabolism


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