title: Identification of the geometric requirements for allosteric communication between the alpha- and beta-subunits of tryptophan synthase.
abstract: The pyridoxal 5'-phosphate-dependent tryptophan synthase alpha2beta2 complex is a paradigmatic protein for substrate channeling and allosteric regulation. The enzymatic activity is modulated by a ligand-mediated equilibrium between open (inactive) and closed (active) conformations of the alpha- and beta-subunit, predominantly involving the mobile alpha loop 6 and the beta-COMM domain that contains beta helix 6. The alpha ligand-triggered intersubunit communication seems to rely on a single hydrogen bond formed between the carbonyl oxygen of betaSer-178 of beta helix 6 and the NH group of alphaGly-181 of alpha loop 6. We investigated whether and to what extent mutations of alphaGly-181 and betaSer-178 affect allosteric regulation by the replacement of betaSer-178 with Pro or Ala and of alphaGly-181 with either Pro to remove the amidic proton that forms the hydrogen bond or Ala, Val, and Phe to analyze the dependence on steric hindrance of the open-closed conformational transition. The alpha and beta activity assays and the equilibrium distribution of beta-subunit catalytic intermediates indicate that mutations do not significantly influence the intersubunit catalytic activation but completely abolish ligand-induced alpha-to beta-subunit signaling, demonstrating distinct pathways for alpha-beta-site communication. Limited proteolysis experiments indicate that the removal of the interaction between betaSer-178 and alphaGly-181 strongly favors the more trypsin-accessible open conformation of the alpha-active site. When the hydrogen bond cannot be formed, the alpha-subunit is unable to attain the closed conformation, and consequently, the allosteric signal is aborted at the subunit interface.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Parma, 43100 Parma, Italy.
authors.forename: Samanta
authors.initials: S
authors.lastname: Raboni
authors.forename: Stefano
authors.initials: S
authors.lastname: Bettati
authors.forename: Andrea
authors.initials: A
authors.lastname: Mozzarelli
mesh_terms: D000494:Allosteric Regulation; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism; D001665:Binding Sites; D008958:Models, Molecular; D015394:Molecular Structure; D016297:Mutagenesis, Site-Directed; D011487:Protein Conformation; D021122:Protein Subunits / Q000737:chemistry* / Q000378:metabolism*; D012486:Salmonella typhimurium / Q000201:enzymology; D012694:Serine / Q000378:metabolism; D014367:Tryptophan Synthase / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D021122:Protein Subunits; D012694:Serine; D014367:Tryptophan Synthase