DISSOCIATION EQUILIBRIA OF THE TRYPTOPHAN SYNTHASE ALPHA(2)BETA(2) COMPLEX IN SALINE BUFFER AND GUANIDINE ISOTHIOCYANATE, AS STUDIED BY SEDIMENTATION EQUILIBRIUM

The dissociation equilibria of Salmonella typhimurium tryptophan synth ase alpha(2) beta(2) complex were studied via centrifugation of the co mplex to sedimentation equilibrium in neutral saline buffers containin g 0 to 137 mM guanidine isothiocyanate (GuSCN). The resulting concentr ation gradients were analyzed in the context of an equilibrium model f or sequential dissociation of two cu subunits from a stable beta(2) su bunit. Under the conditions of these experiments, the first dissociati on 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 dissociati on constants were sufficiently well defined to rule out the presence o f a large equilibrium cooperative effect in the stepwise dissociation of the alpha subunits. Published by Elsevier Science B.V.