Lipoic acid incorporation
Lipoic acid is an essential cofactor of dehydrogenase enzymes, mainly the E2 subunit of pyruvate dehydrogenase, branched-chain keto-acid dehydrogenase and 2-oxoglutarate dehydrogenase complexes. Lipoic acid is an organic sulphur compound derived from the 8-carbon fatty acid, octanoic acid. In most eukaryotes, lipoic acid is synthesised in mitochondrion and it is consistent with the localisation of above mentioned three complexes in mitochondria. In apicomplexan parasites, pyruvate dehydrogenase complex is localised in the plastid organelle, apicoplast. The pyruvate dehydrogenase enzyme complex catalysing pyruvate oxidation is absent in Piroplasma species. In addition to the absence of the requirement for lipoic acid in apicoplast, the de novo fatty acid synthesis pathway which provides the precursor for lipoic acid synthesis is also absent in Piroplasma. The only mitochondrial dehydrogenase complex present in Piroplasma species is 2-oxoglutarate dehydrogenase of TCA cycle. The work by Crawford et al in Toxoplasma gondii demonstrated that apicoplast synthesises lipoic acid de novo, whereas mitochondria salvages it from host . The identification of lipoic acid salvage enzyme in Piroplasma genomes suggests that the salvage pathway is present in these species.
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