title: Exchange of K+ or Cs+ for Na+ induces local and long-range changes in the three-dimensional structure of the tryptophan synthase alpha2beta2 complex.
abstract: Monovalent cations activate the pyridoxal phosphate-dependent reactions of tryptophan synthase and affect intersubunit communication in the alpha2beta2 complex. We report refined crystal structures of the tryptophan synthase alpha2beta2 complex from Salmonella typhimurium in the presence of K+ at 2.0 angstrom and of Cs+ at 2.3 angstrom. Comparison of these structures with the recently refined structure in the presence of Na+ shows that each monovalent cation binds at approximately the same position about 8 angstrom from the phosphate of pyridoxal phosphate. Na+ and K+ are coordinated to the carbonyl oxygens of beta Phe-306, beta Ser-308, and beta Gly-232 and to two or one water molecule, respectively. Cs+ is coordinated to the carbonyl oxygens of beta Phe-306, beta Ser-308, beta Gly-232, beta Val-231, beta Gly-268 and beta Leu-304. A second binding site for Cs+ is located in the beta/beta interface on the 2-fold axis with four carbonyl oxygens in the coordination sphere. In addition to local changes in structure close to the cation binding site, a number of long-range changes are observed. The K+ and Cs+ structures differ from the Na+ structure with respect to the positions of beta Asp-305, beta Lys-167, and alpha Asp-56. One unexpected result of this investigation is the movement of the side chains of beta Phe-280 and beta Tyr-279 from a position partially blocking the tunnel in the Na+ structure to a position lining the surface of the tunnel in the K+ and Cs+ structures. The results provide a structural basis for understanding the effects of cations on activity and intersubunit communication.
authors.affiliation: Laboratory of Molecular Biology and Laboratory of Biochemical Pharmacology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA.
authors.forename: S
authors.initials: S
authors.lastname: Rhee
authors.forename: K D
authors.initials: KD
authors.lastname: Parris
authors.forename: S A
authors.initials: SA
authors.lastname: Ahmed
authors.forename: E W
authors.initials: EW
authors.lastname: Miles
authors.forename: D R
authors.initials: DR
authors.lastname: Davies
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002414:Cations, Monovalent / Q000494:pharmacology; D002586:Cesium / Q000494:pharmacology*; D003460:Crystallization; D018360:Crystallography, X-Ray; D046911:Macromolecular Substances; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011188:Potassium / Q000494:pharmacology*; D011487:Protein Conformation / Q000187:drug effects; D012486:Salmonella typhimurium / Q000201:enzymology; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D014367:Tryptophan Synthase / Q000737:chemistry* / Q000187:drug effects / Q000378:metabolism
chemical_list: D002414:Cations, Monovalent; D046911:Macromolecular Substances; D002586:Cesium; D012964:Sodium; D014367:Tryptophan Synthase; D011188:Potassium