Electrostatic parameters of the theoretical quaternary structure of bovine α-crystallin

By changing the ionic strength, the effects of charge modification on the electrostatic properties of our predicted ‘open’ micellar quaternary structure composed of bovine αA subunits were determined. The electrostatic potential values (ø) at 6 arbitrary points surrounding the protein and at all atomic sites of the protein were calculated using the non-linear Poisson-Boltzmann equation. The effective charge ( q) of our predicted aggregate ranged from 16 at 0.0022 M to 45 at 0.1472 M ionic strengths. The variation of potential (ø), as well as charge, is a hyperbolic function of ionic strength ( R 2, 0.901). Plotting the inverse charge (l/ q) against inverse ionic strength (1/ I) it is possible to calculate maximum charge ( q max) (-48) at saturation. This value is close to previously reported experimental (50±5), and our theoretical charge (45), values at physiological ionic strength (0.145 M). These data indicate that maximal repulsion among the α-crystallin aggregates occurs at or near physiological ionic strength. Also, half-maximal charge ( q max/2) at 0.003-0.004 M indicates a transition state at very low ionic strength. The calculated volume available for the mobile solvent in our quaternary structure is -70%. These data are in good agreement with experimental values for bovine α-crystallin in solution reported by Xia et al. (Biophys. J., 1994; 66: 861-872). This agreement provides support for our predicted models of α-crystallin and a level of confidence in the reliability of the theoretical calculations. Since an ionic gradient exists between the lens cortical and nuclear regions, the modification of charge on α-crystallin by varying ionic strength could contribute to the function of α-crystallin as a modulator of lens supermolecular order during fiber cell maturation.