Dissociation equilibria of the tryptophan synthase α2β2 complex in saline buffer and guanidine isothiocyanate, as studied by sedimentation equilibrium

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 cu subunits from a stable beta(2) subunit. Under the conditions of these experiments, the first dissociation constant alone could be evaluated at GuSCN concentrations less than or equal to 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. Published by Elsevier Science B.V.