A number of pyridoxal-dependent enzymes involved in the metabolism of
cysteine, homocysteine and methionine have been shown [1,2] to be evolutionary
related. These are:
Cystathionine γ-lyase (EC 22.214.171.124) (γ-cystathionase), which
catalyzes the transformation of cystathionine into cysteine, oxobutanoate
and ammonia. This is the final reaction in the transulfuration pathway that
leads from methionine to cysteine in eukaryotes.
Cystathionine γ-synthase (EC 126.96.36.199), which catalyzes the conversion
of cysteine and succinyl-homoserine into cystathionine and succinate: the
first step in the biosynthesis of methionine from cysteine in bacteria
Cystathionine β-lyase (EC 188.8.131.52) (β-cystathionase), which catalyzes
the conversion of cystathionine into homocysteine, pyruvate and ammonia:
the second step in the biosynthesis of methionine from cysteine in bacteria
Methionine γ-lyase (EC 184.108.40.206) (L-methioninase) which catalyzes the
transformation of methionine into methanethiol, oxobutanoate and ammonia.
OAH/OAS sulfhydrylase, which catalyzes the conversion of acetylhomoserine
into homocysteine and that of acetylserine into cysteine (gene MET17 or
MET25 in yeast).
O-succinylhomoserine sulfhydrylase (EC 4.2.99.-).
Yeast hypothetical protein YGL184c.
Yeast hypothetical protein YHR112c.
These enzymes are proteins of about 400 amino-acid residues. The pyridoxal-P
group is attached to a lysine residue located in the central section of these
enzymes; the sequence around this residue is highly conserved and can be used
as a signature pattern to detect this class of enzymes.
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