Threonine metabolism

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 [1].

Enzyme	EC Number	Gene id
Homoserine dehydrogenase	1.1.1.3	Missing in annotation
Aspartate semialdehyde dehydrogenase	1.2.1.11	NCLIV_020270
Homoserine O-acetyltransferase	2.3.1.31	NCLIV_009550
Homoserine kinase	2.7.1.39	NCLIV_059630
Aspartate kinase	2.7.2.4	NCLIV_045940
Threonine synthase	4.2.3.1	NCLIV_004630
Threonine-tRNA ligase	6.1.1.3	NCLIV_060420
Threonine-tRNA ligase	6.1.1.3	NCLIV_061140