IUBMB Enzyme Nomenclature


Accepted name: homocysteine desulfhydrase

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

For diagram click here.

Other name(s): homocysteine desulfurase; L-homocysteine hydrogen-sulfide-lyase (deaminating)

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

Comments: 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, 2-iminobutanoate/2-iminopropanoate deaminase

Links to other databases: BRENDA, EXPASY, KEGG, Metacyc, PDB, CAS registry number: 9024-41-3


1. Kallio, R.E. Function of pyridoxal phosphate in desulfhydrase systems of Proteus morganii. J. Biol. Chem. 192 (1951) 371-377. [PMID: 14917685]

[EC created 1961]

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