The Cryptosporidia pathway of Pyruvate metabolism differs from the pathway of Plasmodium falciparum and Toxoplasma gondii in many aspects. The main differences between Cryptosporidia and Toxoplasma are the absence of pyruvate dehydrogenase complex and the inability to synthesise alanine from pyruvate, which is also absent in Plasmodium species. The pathway of pyruvate metabolism in Cryptosporidia has four separate components.
- Pyruvate oxidation into acetyl-CoA – In most eukaryotes, this oxidation is catalysed by the multi-protein pyruvate dehydrogenase complex localised generally to mitochondria and to apicoplast in Plasmodium and Coccidian species. This complex is absent in Cryptosporidia as is the relic plastid apicoplast. Some eukaryotes that lack mitochondria and pyruvate dehydrogenase complex possess oxygen -sensitive pyruvate:ferredoxin oxidoreductase (PFO). Alternatively, pyruvate:NADP(+) oxidoreductase is present in Cryptosporidium species and Euglena gracilis
. The enzyme of E. gracilis possess a mitochondrial transit peptide followed by an N-terminal PFO domain and C-terminal NADPH-cytochrome P450 reductase (CPR) domain. This suggests that this enzyme localises to mitochondria in the facultative anaerobe. In Cryptosporidium parvum, the mitochondrial transit peptide is absent, whereas the fused PFO and CPR domains are present . The expression of the entire fusion protein was observed in C. parvum sporozoites and the protein localises to cytosol rather than the mitosome .
- Anaplerotic (filling up) reactions – As the intermediates of citrate cycle are used up in biosynthetic reactions, oxaloacetate will be used up and need to be replenished. The enzymes PEP carboxykinase (both Toxoplasma and Plasmodium), PEP carboxylase (Plasmodium and Cryptosporidia) and pyruvate carboxylase (Toxoplasma) catalyse synthesis of oxaloacetate. Of these, the first two do not require energy source as they break energy rich phosphoenolpyruvate, whereas last enzyme require energy in the form of ATP. Although TCA cycle is only present in Cryptosporidium muris and absent in C. parvum and Cryptosporidium hominis, the oxaloacetate synthesised can be converted to malate with the action of malate dehydrogenase. Malate dehydrogenase is present in Plasmodium, Toxoplasma and Cryptosporidium species. The enzyme, malate dehydrogenase (decarboxylating) is only present in Cryptosporidia and absent in other Apicomplexa and decarboxylates malate to pyruvate. All these enzymatic activities were characterised in the C. parvum oocysts previously .
- Acetyl-CoA synthase and acetyltransferases - Acetyl-CoA synthase, acetate-CoA ligase, acetoacetyl-CoA reductase and peptide alpha-N-acetyltransferase are the other enzymes of this pathway present in Toxoplasma, Plasmodium and Cryptosporidium species. The enzyme acetyl-CoA C-acetyltransferase is present in first two and missing in the gene models of Cryptosporidium species
- Fermentation reactions – The genes coding for the enzymes pyruvate decarboxylase, bifunctional aldehyde/alcohol dehydrogenase and alcohol dehydrogenase are present in Cryptosporidia genomes and absent in other apicomplexan genomes.
|Enzyme||EC Number||Gene id|
|Bifunctional alcohol dehydrogenase/ aldehyde dehydrogenase||18.104.22.168; 22.214.171.124||cgd8_1720|
|Pyruvate dehydrogenase (NADP(+))||126.96.36.199||cgd4_690|
|Acetate-CoA ligase/Acetyl-CoA synthetase||188.8.131.52; 184.108.40.206||cgd1_3710|
Sources and fates of metabolites
|Substrate||Source pathways||Product||Fate pathways|
|Acetyl-CoA||Fatty acid elongation in the cytosol, Fatty acid elongation in the ER|