Pyridoxal 5 '-phosphate dependent alpha,beta-elimination reactions: Mechanism of O-acetylserine sulfhydrylase

O-Acetylserine sulfhydrylase catalyzes the replacement of the beta -acetoxy group of O-acetyl-L-serine with sulfide to generate L-cysteine. The reacti on represents the final step in the biosynthesis of L-cysteine in enteric b acteria and plants. A quinonoid intermediate has not been detected using a variety of kinetic and spectroscopic probes for the wild-type or mutant enz ymes, ruling out an E-1 mechanism. The structure of the external Schiff bas e intermediate indicates an anti elimination. O-Acetylserine sulfhydrylase is the only known pyridoxal 5'-phosphate-dependent enzyme that catalyzes a beta -elimination reaction to have an anti E-2 mechanism.