Lyases;
Carbon-sulfur lyases;
Carbon-sulfur lyases (only sub-subclass identified to date)

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

L-cystathionine + H2O = L-cysteine + 2-oxobutanoate + NH3 (overall reaction) [RN:R01001];
(1a) L-cystathionine = L-cysteine + 2-aminobut-2-enoate [RN:R08632];
(1b) 2-aminobut-2-enoate = 2-iminobutanoate (spontaneous);
(1c) 2-iminobutanoate + H2O = 2-oxobutanoate + NH3 (spontaneous)

R01001 R08632;
(other) R00782 R02408 R04770 R04930 R09366

L-cystathionine [CPD:C02291];
H2O [CPD:C00001];
2-aminobut-2-enoate [CPD:C17234];
2-iminobutanoate [CPD:C20905]

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 3.5.99.10, 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.

EC 4.4.1.1 created 1961 (EC 4.2.1.15 created 1961, incorporated 1972)