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4.4.1.1

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 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); CGL

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 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.

Links to other databases:

BRENDA, EXPASY, KEGG, MetaCyc, PDB, CAS registry number: 9012-96-8

References:

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. [PMID: 12998462] (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 4.4.1.1 created 1961 (EC 4.2.1.15 created 1961, incorporated 1972)]

4.4.1.2

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 of reaction mechanism, 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 3.5.99.10, 2-iminobutanoate/2-iminopropanoate deaminase

Links to other databases:

BRENDA, EXPASY, KEGG, MetaCyc, PDB, CAS registry number: 9024-41-3

References:

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

[EC 4.4.1.2 created 1961]

4.4.1.11

Accepted name:

methionine γ-lyase

Reaction:

L-methionine + H₂O = methanethiol + NH₃ + 2-oxobutanoate (overall reaction)
(1a) L-methionine = methanethiol + 2-aminobut-2-enoate
(1b) 2-aminobut-2-enoate = 2-iminobutanoate (spontaneous)
(1c) 2-iminobutanoate + H₂O = 2-oxobutanoate + NH₃ (spontaneous)

For diagram of reaction mechanism, click here

Other name(s):

L-methioninase; methionine lyase; methioninase; methionine dethiomethylase; L-methionine γ-lyase; L-methionine methanethiol-lyase (deaminating)

Systematic name:

L-methionine methanethiol-lyase (deaminating; 2-oxobutanoate-forming)

Comments:

A pyridoxal-phosphate protein. The enzyme cleaves a carbon-sulfur bond, releasing methanethiol 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. The enzyme is involved in L-methionine catabolism.

Links to other databases:

BRENDA, EAWAG-BBD, EXPASY, KEGG, MetaCyc, PDB, CAS registry number: 42616-25-1

References:

1.	Kreis, W. and Hession, C. Isolation and purification of L-methionine-α-deamino-γ-mercaptomethane-lyase (L-methioninase) from Clostridium sporogenes. Cancer Res. 33 (1973) 1862–1865. [PMID: 4720797]

[EC 4.4.1.11 created 1976]

5.4.99.3

Accepted name:

2-acetolactate mutase

Reaction:

2-acetolactate = 3-hydroxy-3-methyl-2-oxobutanoate

Other name(s):

acetolactate mutase; acetohydroxy acid isomerase

Systematic name:

2-acetolactate methylmutase

Comments:

Requires ascorbic acid; also converts 2-aceto-2-hydroxybutanoate to 3-hydroxy-3-methyl-2-oxopentanoate.

Links to other databases:

BRENDA, EXPASY, KEGG, MetaCyc, CAS registry number: 37318-52-8

References:

1.	Allaudeen, H.S. and Ramakrishnan, T. Biosynthesis of isoleucine and valine in Mycobacterium tuberculosis H37 Rv. Arch. Biochem. Biophys. 125 (1968) 199–209. [DOI] [PMID: 4384955]

[EC 5.4.99.3 created 1972]