IUBMB Enzyme Nomenclature


Accepted name: cystathionine γ-lyase

Reaction: L-cystathionine + H2O = L-cysteine + 2-oxobutanoate + NH3 (overall reaction)
(1a) L-cystathionine = L-cysteine + 2-aminobut-2-enoate
(1b) 2-aminobut-2-enoate = 2-iminobutanoate (spontaneous)
(1c) 2-iminobutanoate + H2O = 2-oxobutanoate + NH3 (spontaneous)

For diagram of reaction click hereand mechanism click here.

Other name(s): homoserine deaminase; homoserine dehydratase; cystine desulfhydrase; cysteine desulfhydrase; γ-cystathionase; cystathionase; homoserine deaminase-cystathionase; γ-CTL; cystalysin; cysteine lyase; L-cystathionine cysteine-lyase (deaminating)

Systematic name: L-cystathionine cysteine-lyase (deaminating; 2-oxobutanoate-forming)

Comments: A multifunctional pyridoxal-phosphate protein. The enzyme cleaves a carbon-sulfur bond, releasing L-cysteine and an unstable enamine product that tautomerizes to an imine form, which undergoes a hydrolytic deamination to form 2-oxobutanoate and ammonia. The latter reaction, which can occur spontaneously, can also be catalysed by EC, 2-iminobutanoate/2-iminopropanoate deaminase. Also catalyses the conversion of L-homoserine to 2-oxobutanoate and ammonia, of L-cystine to thiocysteine, pyruvate and ammonia, and of L-cysteine to pyruvate, hydrogen sulfide and ammonia.

Links to other databases: BRENDA, EXPASY, KEGG, Metacyc, PDB, CAS registry number: 9012-96-8


1. Braunstein, A.E. and Azarkh, R.M. [Participation of vitamin B6 in enzymic formation of hydrogen sulfide from L-cysteine.] Dokl. Akad. Nauk. S.S.S.R. 71 (1950) 93-96. (in Russian)

2. Braunstein, A.E. and Azarkh, R.M. [Phosphopyridoxal in aerobic deamination of homoserine and serine.] Dokl. Akad. Nauk. S.S.S.R. 85 (1952) 385-388. (in Russian)

3. Flavin, M. and Segal, A. Purification and properties of the cystathionine γ-cleavage enzyme of Neurospora. J. Biol. Chem. 239 (1964) 2220-2227. [PMID: 14209951]

4. Matsuo, Y. and Greenberg, D.M. A crystalline enzyme that cleaves homoserine and cystathionine. III. Coenzyme resolution, activation, and inhibitors. J. Biol. Chem. 234 (1959) 507-515. [PMID: 13641250]

5. Matsuo, Y. and Greenberg, D.M. A crystalline enzyme that cleaves homoserine and cystathionine. IV. Mechanism of action, reversibility, and substrate specificity. J. Biol. Chem. 234 (1959) 516-519. [PMID: 13641251]

[EC created 1961 (EC created 1961, incorporated 1972)]

Return to EC 4.4.1 home page
Return to EC 4.4 home page
Return to EC 4 home page
Return to Enzymes home page
Return to IUBMB Biochemical Nomenclature home page