The enzyme threonine synthase (EC 4.2.3.1) catalyzes the chemical reaction

O-phospho-L-homoserine + H₂O ${\displaystyle \rightleftharpoons }$ L-threonine + phosphate

This enzyme belongs to the family of lyases, specifically those carbon-oxygen lyases acting on phosphates. The systematic name of this enzyme class is O-phospho-L-homoserine phosphate-lyase (adding water L-threonine-forming). Other names in common use include threonine synthetase, and O-phospho-L-homoserine phospho-lyase (adding water). This enzyme participates in glycine, serine and threonine metabolism, and vitamin B₆ metabolism. It employs one cofactor, pyridoxal phosphate.

As of late 2007, 7 structures have been solved for this class of enzymes, with PDB accession codes 1UIM, 1UIN, 1V7C, 1VB3, 2C2B, 2C2G, and 2D1F.