Pyridoxal 5′-phosphate dependent α,β-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 reaction represents the final step in the biosynthesis of L-cysteine in enteric bacteria and plants. A quinonoid intermediate has not been detected using a variety of kinetic and spectroscopic probes for the wild-type or mutant enzymes, ruling out an E-1 mechanism. The structure of the external Schiff base 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.