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

Covalent binding of L-methionine as an external aldimine to the pyridoxal 5 '-phosphate-cofactor in the K41A mutant of O-acetylserine sulfhydrylase fro m Salmonella typhimurium induces a large conformational change in the prote in. Methionine mimics the action of the substrate O-acetyl-L-serine during catalysis. The alpha-carboxylate moiety of L-methionine in external aldimin e linkage with the active site pyridoxal 5'-phosphate forms a hydrogen bond ing network to the "asparagine-loop" P67-T68-N69-G70 which adopts a differe nt conformation than in the native protein. The side-chain nitrogen of Asn6 9 moves more than 7 Angstrom to make a hydrogen bond to the alpha-carboxyla te group of the inhibitor. As the external aldimine is formed, the PLP tilt s by 13 degrees along its longitudinal axis such that C4' moves toward the entrance to the active site and the side-chain of the methionine is directe d toward the active site entrance. The local rearrangement acts as a trigge r to induce a large global conformational change in the protein. A subdomai n comprised of beta-strand 4, alpha-helix 3, beta-strand 5 and alpha-helix 4 moves towards the active site by a rotation of 7 degrees. This subdomain movement results in a reduction of the severe twist of its central beta-she et and reduces the active site entrance to a small hole, giving access only to small molecules like sulfide, the second substrate, or acetate, the fir st product. (C) 1999 Academic Press.