In enzymology, a cysteine synthase (EC 2.5.1.47) is an enzyme that catalyzes the chemical reaction

O³-acetyl-L-serine + hydrogen sulfide ${\displaystyle \rightleftharpoons }$ L-cysteine + acetate

Thus, the two substrates of this enzyme are O³-acetyl-L-serine and hydrogen sulfide, whereas its two products are L-cysteine and acetate.

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 O3-acetyl-L-serine:hydrogen-sulfide 2-amino-2-carboxyethyltransferase. Other names in common use include O-acetyl-L-serine sulfhydrylase, O-acetyl-L-serine sulfohydrolase, O-acetylserine (thiol)-lyase, O-acetylserine (thiol)-lyase A, O-acetylserine sulfhydrylase, O3-acetyl-L-serine acetate-lyase (adding hydrogen-sulfide), acetylserine sulfhydrylase, cysteine synthetase, S-sulfocysteine synthase, 3-O-acetyl-L-serine:hydrogen-sulfide, and 2-amino-2-carboxyethyltransferase. This enzyme participates in 3 metabolic pathways: cysteine metabolism, selenoamino acid metabolism, and sulfur metabolism. It employs one cofactor, pyridoxal phosphate.

As of late 2007, 12 structures have been solved for this class of enzymes, with PDB accession codes 1O58, 1VE1, 1Y7L, 1Z7W, 1Z7Y, 2BHS, 2BHT, 2EGU, 2ISQ, 2Q3B, 2Q3C, and 2Q3D.

• Becker MA, Kredich NM, Tomkins GM (1969). "The purification and characterization of O-acetylserine sulfhydrylase-A from Salmonella typhimurium". J. Biol. Chem. 244 (9): 2418–27. doi:10.1016/S0021-9258(19)78240-4. PMID 4891157.
• Hara S, Payne MA, Schnackerz KD, Cook PF (1990). "A rapid purification procedure and computer-assisted sulfide ion selective electrode assay for O-acetylserine sulfhydrylase from Salmonella typhimurium". Protein Expr. Purif. 1 (1): 70–6. doi:10.1016/1046-5928(90)90048-4. PMID 2152186.
• Ikegami F, Kaneko M, Lambein F, Kuo Y-H, Murakoshi I (1987). "Difference between uracilylalanine synthases and cysteine synthases in Pisum sativum". Phytochemistry. 26 (10): 2699–2704. Bibcode:1987PChem..26.2699I. doi:10.1016/S0031-9422(00)83575-X.
• Murakoshi I, Kaneko M, Koide C, Ikegami F (1986). "Enzymatic-synthesis of the neuroexcitatory amino-acid quisqualic by cysteine synthase". Phytochemistry. 25 (12): 2759–2763. doi:10.1016/S0031-9422(00)83736-X.
• Tai CH, Burkhard P, Gani D, Jenn T, Johnson C, Cook PF (2001). "Characterization of the allosteric anion-binding site of O-acetylserine sulfhydrylase". Biochemistry. 40 (25): 7446–52. doi:10.1021/bi015511s. PMID 11412097.
• Schnackerz KD, Hazlett TL, Gratton E, Mozzarelli A (2000). "Role of pyridoxal 5'-phosphate in the structural stabilization of O-acetylserine sulfhydrylase". J. Biol. Chem. 275 (51): 40244–51. doi:10.1074/jbc.M007015200. PMID 10995767.