Cystathionine γ-synthase: Studies on the γ replacement reaction

Abstract

Cystathionine γ-synthase from Salmonella catalyzes the γ replacement reaction in which O- succinyl- l-homoserine and l-cysteine form succinic acid and cystathionine, an important intermediate in methionine biosynthesis. In the absence of l-cysteine the enzyme catalyzes a γ elimination reaction in which O-succinylhomoserine is decomposed to α-ketobutyrate, NH 3 and succinic acid. β-Mercaptopropionic acid, a non-competitive inhibitor of γ elimination, has been shown to be a substrate for the γ replacement reaction. The reaction product has been identified, by mass spectroscopy, as S- carboxyethyl- l-homocysteine . A kinetic study of this γ replacement reaction has demonstrated that β-mercaptopropionic acid has no effect on the enzyme's affinity for O-succinylhomoserine. It is suggested that the γ replacement reaction with l-cysteine is kinetically similar to that with β-mercaptopropionic acid.