title: Limited proteolysis of the beta 2-dimer of tryptophan synthase yields an enzymatically active derivative that binds alpha-subunits.
abstract: Proteolytic modification of tryptophan synthase holo-beta 2-subunit from Escherichia coli at the C-terminal side of E-296 leads to an active species (E-296-nicked holo-beta 2) capable of interacting with alpha-subunits. Although this heterologous subunit interaction is rather weak, it induces an increase in catalytic efficiency in E-296-nicked holo-beta 2 by a factor of about 150. Correspondingly, enzymatic activity of alpha-subunits is enhanced 180-fold. This is in striking contrast to the findings from earlier reports which demonstrated that proteolytic derivatives modified at other positions in the "hinge region" embedded in the C-domain of the beta 2-subunit (K-272, R-275, and K-283) are enzymatically inactive and cannot associate with alpha-subunits. The equilibrium binding curve for the cofactor pyridoxal 5'-phosphate to E-296-nicked apo-beta 2 is hyperbolic (i.e., noncooperative), yielding an apparent microscopic dissociation constant, Kd, of 5 x 10(-6) M. This value closely resembles the low-affinity dissociation constant of cooperative cofactor binding to the native beta 2-subunit, indicating that the conformational interactions between structural domains in the modified beta-protein seem to be disturbed considerably.
authors.affiliation: Institut für Physiologische Chemie, Universität Witten/Herdecke, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Kaufmann
authors.forename: T
authors.initials: T
authors.lastname: Schwarz
authors.forename: R
authors.initials: R
authors.lastname: Jaenicke
authors.forename: K D
authors.initials: KD
authors.lastname: Schnackerz
authors.forename: H E
authors.initials: HE
authors.lastname: Meyer
authors.forename: P
authors.initials: P
authors.lastname: Bartholmes
mesh_terms: D000595:Amino Acid Sequence; D002850:Chromatography, Gel; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000201:enzymology; D006868:Hydrolysis; D009682:Magnetic Resonance Spectroscopy; D008969:Molecular Sequence Data; D011487:Protein Conformation; D011732:Pyridoxal Phosphate / Q000378:metabolism; D012486:Salmonella typhimurium / Q000201:enzymology; D014367:Tryptophan Synthase / Q000378:metabolism*
chemical_list: D011732:Pyridoxal Phosphate; D014367:Tryptophan Synthase