Regulation of tryptophan synthase by temperature, monovalent cations, and an allosteric ligand. Evidence from Arrhenius plots, absorption spectra, and primary kinetic isotope effects

To investigate the linkage between enzyme conformation and catalysis, we have determined the effects of temperature on catalytic properties of the tryptophan synthase alpha(2)beta(2) complex and beta(2) subunit in the absence or presence of different monovalent cations (Cs(+), Na(+), and GuH(+)) and of an allosteric ligand, alpha-glycerol 3-phosphate. Arrhenius plots of the activity data between 5 and 50 degrees C are nonlinear in the presence of certain ligands but not others. The conditions that yield nonlinear Arrhenius plots also yield temperature-dependent changes in the equilibrium distribution of enzyme-substrate intermediates and in primary kinetic isotope effects. The results provide evidence that the nonlinear Arrhenius plots are caused by a temperature-dependent conformational change that precedes the rate-limiting step in catalysis. Thermodynamic analysis of the data associated with the conformational change shows that the activation energies are much higher at low temperatures than at high temperatures. We correlate the results with a model in which the enzyme is converted by increased temperature under certain conditions from a low-activity "open" conformation to a high-activity "closed" conformation. The allosteric ligand and different monovalent cations, including GuH(+), which also acts as a chaotropic agent, affect the equilibrium between the open and closed forms. The large positive entropy changes in the conformational conversion suggest that the closed conformation results from tightened hydrophobic interactions that exclude water from the active site of the beta subunit.