Threonine metabolism

Threonine is one of the ten essential amino acids in human, which needs to be obtained from the 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 Apicomplexa. The first two enzymes are common to the 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 [1].

Enzyme	EC Number	Gene id	Protein localisation	Localisation data source
Homoserine dehydrogenase	1.1.1.3	Missing in annotation
Aspartate semialdehyde dehydrogenase	1.2.1.11	TGME49_205420	Cytosol	GO annotation
Homoserine O-acetyltransferase	2.3.1.31	TGME49_320730
Homoserine kinase	2.7.1.39	TGME49_216640
Aspartate kinase	2.7.2.4	TGME49_227090
Threonine synthase	4.2.3.1	TGME49_220840
Threonine-tRNA ligase	6.1.1.3	TGME49_219430
Threonine-tRNA ligase	6.1.1.3	TGME49_300260	Apicoplast	Previous publication