Enzymatic synthesis of homocysteine or methionine directly from

The bacterial biosynthesis of methionine has previously been shown to involved formation of cystathionine from cysteine and O-succinyl-hormoserine, catalyzed by cystathionine γ-synthase, followed by cleavage of cystathionine to yield homocysteine. This report presents evidence that hydrogen sulfide can replace cysteine as a substrate for cystathionine γ-synthase, yielding homocysteine directly. The maximum velocity of homocysteine formation is about half that of cystathionine formation, but the Km for hydrogen sulfide is 50 times higher than that for cysteine. The leakiness of Salmonella mutants blocked in the conversion of cystathionine to homocysteine suggests that direct synthesis of homocysteine from H2S may take place in the cell, although only to a limited extent. Cystathionine γ-synthase has also been shown to catalyze formation of methionine directly from O-saccinyl homoserine and methyl mercaptan. This reaction is presumed to have no role in the utilization of inorganic sulfur compounds for methionine biosynthesis, but may provide an explanation for the ability of S-methyl cysteine to support the growth of methionine auxotrophs of some microorganisms.