Electrostatic free energy of lysozyme.

The electrostatic free energies of native and acetylated lysozymes were computed by the fixed charge model (Tanford, C., and J. G. Kirkwood, 1957, J. Am. Chem. Soc., 79:5333-5339). For each computation, the charges were transformed into a sphere of fixed radius without changing their depths or the distance between charges. The depths of charges were assumed proportional to one minus accessibility. When the conversion factor was set to 1.62 A, the computed titration curve fitted best to the experimental data. The calculated electrostatic free energies for native and acetylated lysozymes were consistent with our earlier finding that acetylated lysozyme is less stable than native around neutral pH (Imoto, T., K. Fukuda, and K. Yagishita, 1976, J. Biochem. [Tokyo], 80:1313-1318; Imoto, T., S. Moriyama, and K. Yagishita, 1976, J. Biochem. [Tokyo], 80:1319-1325). The contribution of each charge to the stabilization of the protein and the apparent pK's of ionizable groups were computed by this method.