O-Acetylserine is an ñ-amino acid with the chemical formula HO<sub>2</sub>CCH(NH<sub>2</sub>)CH<sub>2</sub>OC(O)CH<sub>3</sub>. It is an intermediate in the biosynthesis of the common amino acid cysteine in bacteria and plants. O-Acetylserine is biosynthesized by acetylation of the serine by the enzyme serine transacetylase. The enzyme O-acetylserine (thiol)-lyase, using sulfide sources, converts this ester into cysteine, releasing acetate: