Leucine, isoleucine and valine metabolism

Toxoplasma gondii possesses the pathway for degradation of branched chain amino acids leucine, isoleucine and valine, a pathway present in both animals and plants. These amino acids are essential to all the apicomplexans including Toxoplasma and Plasmodium species. This amino acids degradation pathway may well provide a source of acetyl-CoA for the Krebs cycle in starving animals. The T. gondii genome possesses the genes for enzymes which catalyse the degradation of these branched chain amino acids into acetyl-CoA, (R)-methyl-malonyl-CoA and propionyl-CoA. The absence of mitochondrial pyruvate dehydrogenase complex suggests that this mitochondrial pathway may provide an alternative source of acetyl-CoA for the TCA cycle in Toxoplasma. Although propionyl-CoA, another product of this process is toxic, the 2-methylcitrate cycle can metabolise it to pyruvate.

The only enzyme missing in the T. gondii genome is methylglutaconyl-CoA hydratase (4.2.1.18). Although the pathway is complete in Toxoplasma, Plasmodium falciparum does not possess a complete pathway. The Leucine, isoleucine and valine metabolism pathway in P. falciparum has the enzymes that participate in the beginning of the pathway including branched chain alpha-keto acid dehydrogenase complex and branched chain amino acid transaminase. The enzymes Enoyl-CoA hydratase and 3-hydroxy isobutyryl dehydrogenase are also present in the genome. The acyl-CoA dehydrogenases such as isovaleryl-CoA dehydrogenase and the enzymes catalysing the downstream reactions involved in degradation of 3-hydroxyisovaleryl-CoA, 3-hydroxy-2-isobutyrate and (S)-hydroxy-2-methylbutyryl-CoA to acetyl-CoA, propionyl-CoA and methyl-malonyl-CoA are absent.

List of genes annotated as tRNA-Leu in T. gondii genome

List of genes annotated as tRNA-Ile in T. gondii genome

List of genes annotated as tRNA-Val in T. gondii genome

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Sources and fates of metabolites