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 three transporters identified in ME49 strain were also identified in Neospora caninum genome. 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 (T. gondii purine metabolism pathway). The orthologs of all these enzymes are also present in Neospora caninum. The enzyme AMP deaminase catalysing conversion of AMP to IMP is also present in T. gondii and N. caninum. This and 10-fold higher adenylate kinase activity suggests that adenine nucleotide serves as substrate for generation of guanine nucleotides in T. gondii [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
IMP dehydrogenase NCLIV_032920
Ribonucleotide di-P reductase NCLIV_052980
TRX reductase NCLIV_053860
TRX reductase NCLIV_063590
Purine nucleoside phosphorylase NCLIV_053130
Purine nucleoside phosphorylase NCLIV_054700
Hypoxanthine-guanine phosphoribosyltransferase NCLIV_038170
Adenosine kinase NCLIV_066410
Adenylate kinase NCLIV_023650
Adenylate kinase NCLIV_027870
Adenylate kinase NCLIV_030980
Adenylate kinase NCLIV_033190
Adenylate kinase NCLIV_040820
Adenylate kinase NCLIV_047810
Nucleoside-diphosphate kinase NCLIV_022680
Nucleoside-diphosphate kinase NCLIV_037320
Guanylate kinase NCLIV_017730
IMP-specific 5'-nucleotidase NCLIV_005860
5'-nucleotidase NCLIV_016240
5'-nucleotidase NCLIV_059500
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_000060
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_000970
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_010880
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_012520
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_017300
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_019740
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_022390
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_028010
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_028410
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_028420
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_029870
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_030330
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_032860
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_034660
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_039140
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_044990
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_046230
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_047870
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_054600
3',5'-cyclic-nucleotide phosphodiesterase NCLIV_055040
Adenosine deaminase NCLIV_019950
Adenosine deaminase NCLIV_052400
AMP deaminase NCLIV_049460
Inorganic diphosphatase NCLIV_028850
Ecto-nucleoside triphosphate diphosphohydrolase NCLIV_047410
Ecto-nucleoside triphosphate diphosphohydrolase NCLIV_057470
Ecto-nucleoside triphosphate diphosphohydrolase NCLIV_067130
Ecto-nucleoside triphosphate diphosphohydrolase NCLIV_068400
Ecto-nucleoside triphosphate diphosphohydrolase NCLIV_068460
Nucleoside-triphosphate pyrophosphatase NCLIV_022610
Apyrase NCLIV_054860
Apyrase NCLIV_060550
Nucleoside-diphosphatase NCLIV_026950
Adenylosuccinate lyase NCLIV_024730
Adenylate cyclase NCLIV_031070
Adenylate cyclase NCLIV_035920
Adenylate cyclase NCLIV_041540
Adenylate cyclase NCLIV_059900
Guanylate cyclase NCLIV_008660
Adenylosuccinate synthase NCLIV_038330
GMP synthase NCLIV_030970
Nucleoside (adenosine) transporter none NCLIV_019000
Nucleoside (adenosine) transporter none NCLIV_032940
Equilibrative nucleoside transporter none NCLIV_041010


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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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