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 and Coccidian species such as Toxoplasma and Neospora 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 Toxoplasma 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.

All enzymes required for the catalysis of heme synthesis from glycine and succinyl-CoA are present in T. gondii and N. caninum. 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 [7]. Therefore, the annotation has been updated in MPMP. The T. gondii and N. caninum genomes have no high confidence orthologs to the hypothetical HEM4 domain containing protein from P. falciparum or to any other UROS enzymes from model organisms. A 751 amino acid putative protein present in N. caninum is predicted to possess the HemD domain found in UROS enzymes. The presence of UROS activity in Toxoplasma and Neospora PBD has yet to 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 and Neospora also have 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 [8] in addition to oxygen-dependent CPOase (1.3.3.3) present in Plasmodium. The oxygen-independent form of PPOase is not present in these coccidian genomes. The enzymes heme oxygenase and biliverdin reductase, present in P. falciparum are absent in T. gondii and N. caninum suggesting the absence of heme degradation (biliverdin and bilirubin synthesis) in Coccidia.

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Sources and fates of metabolites

Examples of other protoheme containing proteins in Neospora caninum genome