O-acetylserine sulfhydrylase.

The 31P NMR data suggest slight differences in the structures around the 5'-P for the internal Schiff base and the lanthionine external Schiff base (both largely ketoeneamine) and a large difference for enolimine portion of the serine external Schiff base. Addition of cysteine or serine increase delayed fluorescence and triplet to singlet energy transfer. Addition of OAS exhibits a splitting of the 0,0 vibronic, the result of two distinct conformations, likely enolimine and ketoeneamine tautomers. Nonetheless, the alpha-amino-acrylate Schiff base conformation differs from either the internal or external Schiff base conformations. All of the time-resolved fluorescence data are consistent with conformation changes reflecting redistribution of ketoeneamine and enolimine tautomers as catalysis occurs. It is important to remember that the structural changes are substantial. The native structure (internal Schiff base) is active site open, while the K41A mutant enzyme (ketoeneamine external Schiff base) is active site closed. The trigger for the conformational change from open to closed as one goes from the internal to external Schiff base is the occupancy of the alpha-carboxyl subsite of the active site (Burkhard et al., 1999). Associated with this, as observed in pH-rate profiles, pH-dependent changes in phosphorescence, and pH-dependent changes in fluorescence enhancement upon binding acetate or cysteine is an enzyme group with a pK in the range 7-8. Dependent on the protonation state of the enzyme group, structural changes likely occur that also reflect a redistribution of the tautomeric equilibrium. Finally, the minimal catalytic cycle can likely be pictured as shown in Fig. 20. The changes may be pH dependent, and the open conformations for the internal Schiff base and the alpha-aminoacrylate Schiff base are not identical structurally, as expected because of the increased stability of the latter.