Lysine biosyntheis

Lysine biosynthesis is one of the unique metabolic capabilities of cyst forming Coccidia such as Toxoplasma gondii and Neospora caninum. Toxoplasma and Neospora genome analysis shows that they have the unique metabolic capability to synthesise lysine, an essential amino acid in humans. This capability is absent in other Apicomplexa. It has been suggested that Toxoplasma can convert aspartate into lysine via diaminopimelate pathway [1]. This pathway takes place via four different routes in different organisms (KEGG Lysine biosynthesis pathway). Of these, three variants belong to different groups of prokaryotes and the plant variant of the pathway was identified recently in Arabidopsis thaliana [2]. The first four enzymes (aspartate kinase, aspartate semialdehyde dehydrogenase, dihydrodipicolinate synthase and dihydrodipicolinate reductase) and the last enzyme (diaminopimelate decarboxylase) is the same in all variants of the pathway and these five enzymes are present in T. gondii and N. caninum genomes. The homology-based analysis of the Toxoplasma genome has not led to identification of middle enzymes of any routes even with low sequence similarity. These enzymes are not identifiable in N. caninum gene models as well. The reason for this is unknown which could be absence of the functional pathway, poor gene models or the pathway may take place via another route in these cyst forming Coccidians and the enzymes involved may be present as hypothetical proteins in the genomes.

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Sources and fates of metabolites