The enzymatic superactivity in reverse micelles: Role of the dielectric constant

Abstract

Calculations previously carried out by the authors regarding the activity of α-chymotrypsin solubilized by the reverse micellar solution of Aerosol OT in isooctane are modified by accounting for the effect of the variation of the dielectric constant with the local field strength and local ionic concentrations in the Poisson-Boltzmann equation. The effect of the field strength on the dielectric constant is accounted for through the use of Booth's expression and that of the local ionic concentrations through Hasted's expression. As in the previous calculations, the activity passes through a maximum with the hydration ratio ( W 0) and is, in general, greater than that in a bulk aqueous solution having the substrate concentration equal to the overall concentration in the entire reverse micellar solution. The position and the value of the maximum are significantly affected by the value and the radial variation of the dielectric constant of the water pool. For relatively low degrees of dissociation of the reverse micelle, the dielectric constant does not vary significantly with the field strength and the ionic concentrations. In this case, the activities exhibit only moderate enhancement as compared to that in the bulk aqueous solution. Large degrees of dissociation result, however, in significant changes in values and radial variations of the dielectric constant. This leads to higher activities which are comparable to the experimental ones and shifts the maximum to lower values of W 0 as compared to the previous case. The higher activity is due (1) to the higher average substrate concentration in the water pool than that in the bulk aqueous solution when the partitioning of the substrate in the surfactant layer is not too large, and (2) to the pushing of the negatively charged substrate by the likewise charged reverse micellar surface toward the enzyme. The latter effect is expected to be higher when the dielectric constant is smaller because the electrostatic repulsion is higher.