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 4.4.1.1) (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 2.5.1.48), 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 (gene metB).
• Cystathionine β-lyase (EC 4.4.1.8) (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 (gene metC).
• Methionine γ-lyase (EC 4.4.1.11) (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.