In enzymology, a cystathionine gamma-synthase (EC 2.5.1.48) is an enzyme that catalyzes the formation of cystathionine from cysteine and an activated derivative of homoserine, e.g.: In microorganisms, the activated substrate of this enzyme is O4-succinyl-L-homoserine or O4-acetyl-L-homoserine.

Cystathionine gamma-synthase from plants uses L-homoserine phosphate instead.

[1] This enzyme belongs to the family of transferases, specifically those transferring aryl or alkyl groups other than methyl groups.

The systematic name of this enzyme class is O4-succinyl-L-homoserine:L-cysteine S-(3-amino-3-carboxypropyl)transferase.

Other names in common use include O-succinyl-L-homoserine succinate-lyase (adding cysteine), O-succinylhomoserine (thiol)-lyase, homoserine O-transsuccinylase, O-succinylhomoserine synthase, O-succinylhomoserine synthetase, cystathionine synthase, cystathionine synthetase, homoserine transsuccinylase, 4-O-succinyl-L-homoserine:L-cysteine, and S-(3-amino-3-carboxypropyl)transferase.