The Active Site of O- Acetylserine Sulfhydrylase Is the Anchor Point for Bienzyme Complex Formation with Serine Acetyltransferase

Abstract

ABSTRACT The biosynthesis of cysteine in bacteria and plants is carried out by a two-step pathway, catalyzed by serine acetyltransferase (SAT) and O -acetylserine sulfhydrylase (OASS; O- acetylserine [thiol] lyase). The aerobic form of OASS forms a tight bienzyme complex with SAT in vivo, termed cysteine synthase. We have determined the crystal structure of OASS in complex with a C-terminal peptide of SAT required for bienzyme complex formation. The binding site of the peptide is at the active site of OASS, and its C-terminal carboxyl group occupies the same anion binding pocket as the α-carboxylate of the O -acetylserine substrate of OASS. These results explain the partial inhibition of OASS by SAT on complex formation as well as the competitive dissociation of the complex by O -acetylserine.