Ligand binding induces a large conformational change in O-acetylserine sulfhydrylase from Salmonella typhimurium

Abstract

Covalent binding of l-methionine as an external aldimine to the pyridoxal 5′-phosphate-cofactor in the K41A mutant of O-acetylserine sulfhydrylase from Salmonella typhimurium induces a large conformational change in the protein. Methionine mimics the action of the substrate O-acetyl- l-serine during catalysis. The α-carboxylate moiety of l-methionine in external aldimine linkage with the active site pyridoxal 5′-phosphate forms a hydrogen bonding network to the “asparagine-loop” P67-T68-N69-G70 which adopts a different conformation than in the native protein. The side-chain nitrogen of Asn69 moves more than 7 Å to make a hydrogen bond to the α-carboxylate group of the inhibitor. As the external aldimine is formed, the PLP tilts by 13 ° along its longitudinal axis such that C4′ moves toward the entrance to the active site and the side-chain of the methionine is directed toward the active site entrance. The local rearrangement acts as a trigger to induce a large global conformational change in the protein. A subdomain comprised of β-strand 4, α-helix 3, β-strand 5 and α-helix 4 moves towards the active site by a rotation of 7 °. This subdomain movement results in a reduction of the severe twist of its central β-sheet and reduces the active site entrance to a small hole, giving access only to small molecules like sulfide, the second substrate, or acetate, the first product.