Cystathionine γ-Synthase from Salmonella

The mechanism of cystathionine γ-synthase is of interest because it catalyzes the replacement and elimination of substituents at the γ as well as the β position of amino acids. Decomposition of succinylhomoserine by γ elimination at 3–5° was accompanied by quenching and a shift to longer wave lengths of the enzyme's absorption maximum at 422 mµ and the appearance of a new absorption maximum at 485 mµ. These changes persisted as long as saturating levels of succinylhomoserine were present. In the presence of both cysteine and succinylhomoserine, the enzyme formed cystathionine by γ replacement; and α-ketobutyrate formation by γ elimination was undetectable until all the cysteine had been consumed. No spectral changes were observed during cystathionine synthesis. During pyruvate formation from succinylserine by β-elimination, the absorption band at 422 mµ was quenched and shifted to 430 mµ and transient absorption maxima appeared at 465 and 485 mµ. The maximum velocity of γ elimination from succinyl-homoserine was 22 times faster at 37° than at 5°, but the apparent Km for succinylhomoserine was constant over this range of temperature. The enthalpy and entropy of activation for the γ elimination reaction were calculated to be +16,800 cal per mole and +0.2 e.u., respectively. The optical rotatory dispersion spectrum of cystathionine γ-synthase showed a positive extrinsic Cotton effect centered at 422 mµ and a negative intrinsic Cotton effect centered near 222 mµ. The α helix content of the enzyme, calculated from the depth of the trough at 233 mµ, was 43%.