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

O-Acetylserine sulfhydrylase catalyzes the replacement of the β-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 E1 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 β-elimination reaction to have an anti E2 mechanism.