EXCHANGE OF K-RANGE CHANGES IN THE 3-DIMENSIONAL STRUCTURE OF THE TRYPTOPHAN SYNTHASE ALPHA(2)BETA(2) COMPLEX( OR CS+ FOR NA+ INDUCES LOCALAND LONG)

Monovalent cations activate the pyridoxal phosphate-dependent reaction s of tryptophan synthase and affect intersubunit communication in the alpha(2) beta(2) complex. We report refined crystal structures of the tryptophan synthase alpha(2) beta(2) complex from Salmonella typhimuri um in the presence of K+ at 2.0 Angstrom and of Cs+ at 2.3 Angstrom. C omparison of these structures with the recently refined structure in t he presence of Na+ shows that each monovalent cation binds at approxim ately the same position about 8 Angstrom from the phosphate of pyridox al phosphate. Na+ and K+ are coordinated to the carbonyl oxygens of be ta Phe-306, beta Ser-308, and beta Gly-232 and to two or one water mol ecule, respectively. Cs+ is coordinated to the carbonyl oxygens of bet a 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 pip int erface on the 2-fold axis with four carbonyl oxygens in the coordinati on sphere. In addition to local changes in structure close to the cati on binding site, a number of long-range changes are observed. The K+ a nd Cs+ structures differ from the Na+ structure with respect to the po sitions of beta Asp-305, beta Lys-167, and alpha Asp-56. One unexpecte d result of this investigation is the movement of the side chains of b eta Phe-280 and beta Tyr-279 from a position partially blocking the tu nnel in the Na+ structure to a position lining the surface of the tunn el in the K+ and Cs+ structures. The results provide a structural basi s for understanding the effects of cations on activity and intersubuni t communication.