title: Allosteric regulation of tryptophan synthase channeling: the internal aldimine probed by trans-3-indole-3'-acrylate binding.
abstract: Substrate channeling in the tryptophan synthase bienzyme complex from Salmonella typhimurium is regulated by allosteric interactions triggered by binding of ligand to the alpha-site and covalent reaction at the beta-site. These interactions switch the enzyme between low-activity forms with open conformations and high-activity forms with closed conformations. Previously, allosteric interactions have been demonstrated between the alpha-site and the external aldimine, alpha-aminoacrylate, and quinonoid forms of the beta-site. Here we employ the chromophoric l-Trp analogue, trans-3-indole-3'-acrylate (IA), and noncleavable alpha-site ligands (ASLs) to probe the allosteric properties of the internal aldimine, E(Ain). The ASLs studied are alpha-d,l-glycerol phosphate (GP) and d-glyceraldehyde 3-phosphate (G3P), and examples of two new classes of high-affinity alpha-site ligands, N-(4'-trifluoromethoxybenzoyl)-2-aminoethyl phosphate (F6) and N-(4'-trifluoromethoxybenzenesulfonyl)-2-aminoethyl phosphate (F9), that were previously shown to bind to the alpha-site by optical spectroscopy and X-ray crystal structures [Ngo, H., Harris, R., Kimmich, N., Casino, P., Niks, D., Blumenstein, L., Barends, T. R., Kulik, V., Weyand, M., Schlichting, I., and Dunn, M. F. (2007) Synthesis and characterization of allosteric probes of substrate channeling in the tryptophan synthase bienzyme complex, Biochemistry 46, 7713-7727]. The binding of IA to the beta-site is stimulated by the binding of GP, G3P, F6, or F9 to the alpha-site. The binding of ASLs was found to increase the affinity of the beta-site of E(Ain) for IA by 4-5-fold, demonstrating for the first time that the beta-subunit of the E(Ain) species undergoes a switching between low- and high-affinity states in response to the binding of ASLs.
authors.affiliation: Department of Biochemistry, University of California, Riverside, California 92521, USA.
authors.forename: Patricia
authors.initials: P
authors.lastname: Casino
authors.forename: Dimitri
authors.initials: D
authors.lastname: Niks
authors.forename: Huu
authors.initials: H
authors.lastname: Ngo
authors.forename: Peng
authors.initials: P
authors.lastname: Pan
authors.forename: Peter
authors.initials: P
authors.lastname: Brzovic
authors.forename: Lars
authors.initials: L
authors.lastname: Blumenstein
authors.forename: Thomas Reinier
authors.initials: TR
authors.lastname: Barends
authors.forename: Ilme
authors.initials: I
authors.lastname: Schlichting
authors.forename: Michael F
authors.initials: MF
authors.lastname: Dunn
mesh_terms: D000494:Allosteric Regulation / Q000502:physiology*; D001665:Binding Sites; D002942:Circular Dichroism; D003460:Crystallization; D007211:Indoles / Q000378:metabolism*; D007700:Kinetics; D011487:Protein Conformation; D013056:Spectrophotometry, Ultraviolet; D014367:Tryptophan Synthase / Q000737:chemistry / Q000378:metabolism*
chemical_list: D007211:Indoles; C001446:indoleacrylic acid; D014367:Tryptophan Synthase