Threonine synthase is a pyridoxal phosphate (PLP) dependent enzyme that catalyses the last reaction in the synthesis of threonine from aspartate. It proceeds by converting O-phospho-L-homoserine (OPH) into threonine and inorganic phosphate. In plants, OPH is an intermediate between the methionine and threonine/isoleucine pathways. Thus threonine synthase competes for OPH with cystathionine-gamma-synthase, the first enzyme in the methionine pathway. These enzymes are in general dimers. Members of this CD, Thr-synth_1, are widely distributed in bacteria, archaea and higher plants.