Threonine is one of the ten essential amino acids in human, which needs to be obtained from diet. Toxoplasma gondii and Neospora caninum possess the genes for four of the five enzymes involved in threonine biosynthesis from aspartate. The missing enzyme in the pathway is homoserine dehydrogenase. Lysine and threonine are the only amino acids which can be synthesised by cyst forming Coccidia, but absent in other Apicomplexans. The first two enzymes are common to threonine biosynthesis and lysine biosynthesis pathways as the precursor substrate for these amino acids is aspartate. Of the 4 enzymes of threonine biosynthesis identified in the genome, homoserine kinase is the only enzyme identified by MS-based proteomics studies at least in the oocyst stage of T. gondii .
|Enzyme||EC Number||Gene id|
|Homoserine dehydrogenase||18.104.22.168||Missing in annotation|
|Aspartate semialdehyde dehydrogenase||22.214.171.124||NCLIV_020270|
List of genes annotated as tRNA-Thr in N. caninum genome
Sources and fates of metabolites
|Substrate||Source pathways||Product||Fate pathways|
|L-Aspartate||Asparagine and aspartate metabolism, Host||O-acetyl-L-homoserine||?|