Porphyrin metabolism

Porphyrins are organic aromatic compounds composed of four pyrrole rings interconnected to each other and to the Fe2+ ion. Porphyrins are essential cofactors of many proteins including cytochrome proteins and haemoglobin and myoglobin in humans. The first step in the production of porphyrin in animals is the mitochondrial formation of delta-aminolevulinate from glycine and succinyl-CoA. In humans, this aminolevulinate is then transported to cytosol where the 4-step conversion into Coproporphyrinogen III occurs. The remaining processing of this intermediate to protoporphyrin IX and then to heme takes place in mitochondrion.

 

The cytochrome proteins in Plasmodium falciparum and Toxoplasma gondii require de novo synthesis of porphyrin. Although P. falciparum obtains heme from host haemoglobin digestion, it cannot utilise them for biosynthesis of its own porphyrin-containing proteins [1] and converts them to hemozoin [2]. The enzymes of de novo heme biosynthesis in P. falciparum (MPMP Porphyrin metabolism pathway) is distributed in mitochondria, cytosol and apicoplast. The first enzyme aminolevulinate synthase localises to mitochondria, whereas the next four enzymes localise to the apicoplast. The enzyme Coproporphyrinogen III oxidase lacks target signals and the last two enzymes catalysing protoporphyrinogen IX to heme are proposed to target mitochondria [3, 4, 5]. The enzymes of T. gondii are also proposed to be distributed in mitochondria, cytosol and apicoplast as in P. falciparum [6].

 

Various enzymes of this pathway in either P. falciparum or T. gondii have been biochemically characterised and their functions are verified. The P. falciparum ortholog of the enzyme aminolevulinate synthase has first been biochemically characterised by Wilson et al [7]. This is suggested to be a drug target as this enzyme is immunologically distinct, showed differences in inhibitor specificity to mammalian enzymes and it is important for parasite survival [8]. P. falciparum aminolevulinate dehydratase (catalyses conversion of aminolevulinate to porphobilinogen) was cloned, expressed and characterised by Dhanasekaran et al and this study showed that this enzyme is active and functions as an octamer. In addition, this protein does not require metal ion for activity, although it is stimulated to certain extent with Mg2+ ions [9]. The crystal structure of T. gondii aminolevulinate dehydratase was elucidated by Jaffe et al which demonstrated that T. gondii enzyme also behave as octamer and does not contain any metal ions in the active site, although Mg2+ ions are present at the intersections between pro-octamer dimers [10]. Although Plasmodium and Toxoplasma aminolevulinate dehydratase shares some similarity with plant enzymes, the non-dpendence of metal for catalysis is unique to these species. The differences in structure, localisation and metal dependence to human enzyme in addition to the fact that the treatment with succinylacetone kills T. gondii suggests it as a good drug target although it is challenging to design small molecule inhibitors which selectively target parasite enzyme active sites and not those of host [6].

 

All enzymes required for the catalysis of heme synthesis from glycine and succinyl-CoA are present in T. gondii genome. A hypothetical protein was previously annotated as UROS (4.2.1.75) for P. falciparum in PlasmoDB/MPMP. This had no sequence similarity to any other UROS, but InterProScan showed the presence of the HEM4 domain. It has been experimentally shown that P. falciparum porphobilinogen deaminase (PBD, 2.5.1.61) is bifunctional and can also catalyse UROS activity [11]. Therefore, the annotation has been updated in MPMP. The T. gondii genome has no high confidence orthologs to the hypothetical HEM4 domain containing protein from P. falciparum or to any other UROS enzymes from model organisms. A 1276 amino acid putative protein present in T. gondii is predicted to possess the HemD domain found in UROS enzymes. The presence of UROS activity in Toxoplasma PBD has yet be verified experimentally.

 

There are two different types of Coproporphyrinogen oxidase (CPOase) and protoporphyrinogen oxidase (PPOase) and they catalyse the reactions in an oxygen-dependent and oxygen-independent manner. Toxoplasma also has the 1.3.99.22 enzyme (oxygen-independent CPOase, HEMN), an enzyme which catalyses the reaction with S-adenosyl methionine as a co-substrate instead of O2 [12] in addition to oxygen-dependent CPOase (1.3.3.3) present in Plasmodium. The oxygen-independent form of PPOase is not present in Toxoplasma. The oxygen-dependent form of CPOase has recently been characterised and this demonstrated that this enzyme is active under aerobic conditions. This study also confirmed its cytosolic localisation using immuno-labelling and western blot analysis of lysates [13]. The enzymes heme oxygenase and biliverdin reductase, present in P. falciparum are absent in T. gondii suggesting the absence of heme degradation (biliverdin and bilirubin synthesis) in T. gondii.

 

Protein EC Number Gene id Protein localisation Localisation data source
Coproporphyrinogen oxidase 1.3.3.3 TGME49_223020 Cytoplasm-nuclear Previous publication
Protoporphyrinogen oxidase 1.3.3.4 TGME49_272490 Nucleus? Previous publication
oxygen-independent coproporphyrinogen-III oxidase 1.3.99.22 TGME49_288640    
Aminolevulinate synthase 2.3.1.37 TGME49_258690    
Porphobilinogen deaminase 2.5.1.61 TGME49_271420 Apicoplast Orthology transformation from P. falciparum
Uroporphyrinogen decarboxylase 4.1.1.37 TGME49_289940    
Aminolevulinate dehydratase 4.2.1.24 TGME49_253900 Apicoplast; Extracellular? Orthology transformation from P. falciparum; Previous publication
Uroporphyrinogen-III synthase 4.2.1.75 TGME49_264200    
Holocytochrome-c synthase 4.4.1.17 TGME49_293390    
Holocytochrome-c synthase 4.4.1.17 TGME49_314042    
Ferrochelatase 4.99.1.1 TGME49_258650    
Cytochrome c none TGME49_219750 Mitochondrion Previous publication

 

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Sources and fates of metabolites

 

Substrate Source pathways Product Fate pathways
Glycine Glycine, serine and cysteine metabolism    
Succinyl-CoA Tricarboxylic acid (TCA) cycle 5'-deoxyadenosine Purine metabolism
S-adenosylmethionine Methionine metabolism L-methionine Methionine metabolism




Examples of other protoheme containing proteins in Toxoplasma gondii genome

 

Protein EC Number Gene id Protein localisation Localisation data source
Guanylate cyclase 4.6.1.2 TGME49_254370 Plasma membrane Previous publication
Cytochrome c2 none TGME49_229420 Mitochondrion Previous publication
BCS1 none TGME49_243490 mitochondrion Previous publication
Cytochrome c1 none TGME49_246540 Mitochondrion Previous publication
Cytochrome b5 none TGME49_276110 Cytosol Previous publication