Mechanism of Inactivation of γ-Cystathionase by β,β,β-Trifluoroalanine

β,β,β-Trifluoroalanine irreversibly inactivates γ-cystathionase. The inactivator becomes covalently attached to the enzyme. Two moles of inactivator is incorporated per tetramer (Silverman, R. B., and Abeles, R. PI. (1976), Biochemistry 15, 4718). The rate of inactivation, the rate of [^l4C]trifluoroalanine incorporation into the protein, and the rate of release of F^- from trifluoroalanine are equal. F^-, 2.7 mol, is released per mol of inactivator which becomes covalently attached to the protein. In the course of the inactivation, the β carbon of trifluoroalanine is converted to an acyl group which is bonded to an amino group of the protein. (See Scheme I.) It is proposed that trifluoroalanine labels the functional group, probably a lysine ϵ-amino group, which protonates or deprotonates the α and β positions of the substrate in the normal catalytic process. Propargylglycine inactivates and covalently labels the native enzyme (Washtien, W., and Abeles, R. H. (1977), Biochemistry 16 (in press)). Enzyme inactivated with trifluoroalanine does not react with propargylglycine. However, enzyme inactivated with 2 mol of propargylglycine per tetramer reacts with trifluoroalanine and 2 mol of trifluoroalanine become covalently bonded per mol of enzyme (tetramer). It is proposed that propargylglycine does not affect the functional group required for reaction with trifluoroalanine. Trifluoroalanine labels a group required for the activation of propargylglycine and thus prevents reaction of the enzyme with propargylglycine.