title: Crystal structure of the beta Ser178--> Pro mutant of tryptophan synthase. A "knock-out" allosteric enzyme.
abstract: The catalytic activity of the pyridoxal 5'-phosphate-dependent tryptophan synthase alpha(2)beta(2) complex is allosterically regulated. The hydrogen bond between the helix betaH6 residue betaSer(178) and the loop alphaL6 residue Gly(181) was shown to be critical in ligand-induced intersubunit signaling, with the alpha-beta communication being completely lost in the mutant betaSer(178) --> Pro (Marabotti, A., De Biase, D., Tramonti, A., Bettati, S., and Mozzarelli, A. (2001) J. Biol. Chem. 276, 17747-17753). The structural basis of the impaired allosteric regulation was investigated by determining the crystal structures of the mutant betaSer(178) --> Pro in the absence and presence of the alpha-subunit ligands indole-3-acetylglycine and glycerol 3-phosphate. The mutation causes local and distant conformational changes especially in the beta-subunit. The ligand-free structure exhibits larger differences at the N-terminal part of helix betaH6, whereas the enzyme ligand complexes show differences at the C-terminal side. In contrast to the wild-type enzyme loop alphaL6 remains in an open conformation even in the presence of alpha-ligands. This effects the equilibrium between active and inactive conformations of the alpha-active site, altering k(cat) and K(m), and forms the structural basis for the missing allosteric communication between the alpha- and beta-subunits.
authors.affiliation: Max-Planck-Institut für Molekulare Physiologie, Abteilung für Physikalische Biochemie, D-44227 Dortmund, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Weyand
authors.forename: Ilme
authors.initials: I
authors.lastname: Schlichting
authors.forename: Petra
authors.initials: P
authors.lastname: Herde
authors.forename: Anna
authors.initials: A
authors.lastname: Marabotti
authors.forename: Andrea
authors.initials: A
authors.lastname: Mozzarelli
mesh_terms: D000495:Allosteric Site; D001665:Binding Sites; D003198:Computer Simulation; D018360:Crystallography, X-Ray; D005994:Glycerophosphates / Q000737:chemistry; D005998:Glycine / Q000031:analogs & derivatives* / Q000737:chemistry; D006860:Hydrogen Bonding; D007211:Indoles / Q000737:chemistry; D007700:Kinetics; D008024:Ligands; D008956:Models, Chemical; D008958:Models, Molecular; D009154:Mutation; D011392:Proline / Q000737:chemistry*; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D012694:Serine / Q000737:chemistry*; D014367:Tryptophan Synthase / Q000737:chemistry* / Q000235:genetics*
chemical_list: D005994:Glycerophosphates; D007211:Indoles; D008024:Ligands; C116425:indole-3-acetylglycine; D012694:Serine; D011392:Proline; C029620:alpha-glycerophosphoric acid; D014367:Tryptophan Synthase; D005998:Glycine