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

L-homocysteine hydrogen-sulfide-lyase (deaminating; 2-oxobutanoate-forming)

L-homocysteine + H2O = hydrogen sulfide + NH3 + 2-oxobutanoate (overall reaction) [RN:R01283];
(1a) L-homocysteine = hydrogen sulfide + 2-aminobut-2-enoate [RN:R08633];
(1b) 2-aminobut-2-enoate = 2-iminobutanoate (spontaneous);
(1c) 2-iminobutanoate + H2O = 2-oxobutanoate + NH3 (spontaneous)

R01283 R08633
Reaction

L-homocysteine [CPD:C00155];
H2O [CPD:C00001];
2-aminobut-2-enoate [CPD:C17234];
2-iminobutanoate [CPD:C20905]

A pyridoxal-phosphate protein. The enzyme cleaves a carbon-sulfur bond, releasing hydrogen sulfide 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

EC 4.4.1.2 created 1961

1  [PMID:14917685]