Microcalorimetric study of the binding of thiodigalactoside to the lactose permease M protein of Escherichia coli.

The energetics of the binding of thiodigalatoside onto vesicles of Escherichia coli containing M protein is described. The Kd determined from equilibrium dialysis was 5-10(-5) M. The enthalpy change (deltaH) was measured by calorimetry. The derived deltaG and deltaH values allowed estimation of the entropic change associated with the binding reaction. The control experiments were made with membranes from cells that were not induced for the lac system. All the experiments were carried out in presence of 10(-2) M sodium azide to prevent any concentration of thiodigalactoside into the vesicles. It was concluded that such membrane vesicles which are in a de-energized state are able to bine thiodigalactoside specifically with a Kd corresponding to the Km of the entry of beta-galactoside measured with intact, active cells.