title: Dissociation equilibria of the tryptophan synthase alpha 2 beta 2 complex in saline buffer and guanidine isothiocyanate, as studied by sedimentation equilibrium.
abstract: The dissociation equilibria of Salmonella typhimurium tryptophan synthase alpha 2 beta 2 complex were studied via centrifugation of the complex to sedimentation equilibrium in neutral saline buffers containing 0 to 137 mM guanidine isothiocyanate (GuSCN). The resulting concentration gradients were analyzed in the context of an equilibrium model for sequential dissociation of two alpha subunits from a stable beta 2 subunit. Under the conditions of these experiments, the first dissociation constant alone could be evaluated at GuSCN concentrations < or = 100 mM, and the second dissociation constant alone could be evaluated at GuSCN = 137 mM. At intermediate GuSCN, both dissociation constants were sufficiently well defined to rule out the presence of a large equilibrium cooperative effect in the stepwise dissociation of the alpha subunits.
authors.affiliation: Section on Physical Biochemistry, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-0830, USA.
authors.forename: S
authors.initials: S
authors.lastname: Darawshe
authors.forename: D B
authors.initials: DB
authors.lastname: Millar
authors.forename: S A
authors.initials: SA
authors.lastname: Ahmed
authors.forename: E W
authors.initials: EW
authors.lastname: Miles
authors.forename: A P
authors.initials: AP
authors.lastname: Minton
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D002021:Buffers; D002498:Centrifugation; D006146:Guanidines / Q000737:chemistry*; D017879:Isothiocyanates / Q000737:chemistry*; D007700:Kinetics; D046911:Macromolecular Substances; D008432:Mathematical Computing; D012486:Salmonella typhimurium / Q000201:enzymology; D012965:Sodium Chloride; D014367:Tryptophan Synthase / Q000737:chemistry* / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D002021:Buffers; D006146:Guanidines; D017879:Isothiocyanates; D046911:Macromolecular Substances; C054435:guanidine isothiocyanate; D012965:Sodium Chloride; D014367:Tryptophan Synthase