Electrostatic stabilization in myoglobin. Interactive free energies between individual sites.

Abstract

The pattern of electrostatic interactions between pairs of charge sites in sperm whale ferrimyoglobin was examined as a function of pH in terms of proton site occupancy, static solvent accessibility, and distance of separation. By grouping all examples of the most stabilizing interactions and all examples of the most destabilizing interactions, we can easily show that at pH 7.50 the former is much stronger; that is, the negative contributions to electrostatic free energy far outweigh the positive contributions. Much of the electrostatic energy of stabilization in native myoglobin is provided by specific charge-pair partners that are very highly conserved among 53 mammalian myoglobin species and is invariant substantially from pH 8.5 to 3.5. Destablizing interactions that become most significant, but not actually dominant, near the acid unfolding pH range can be recognized in emerging clusters of uncompensated positive charges. Binding of azide ion by the heme iron effectively reduces the most prominent destabilizing set of such interactions. In general, thoe charged residues that experience the largest summed stabilizing interactions with other groups are the most conserved between species. The histidine residues, however, show their best correlation of conservation with low values of static accessibility. Although histidine residue 64 has an effective pK corresponding to the midpoint of the unfolding transition near pH 4.2 at an ionic strength of 0.10 M and so might be called a "trigger group", its interactions contribute only a modest fraction of the overall pH-dependent free energy change. An examination of the primary stabilizing interactions represented by the charge-pair partners indicates a probably major role of electrostatic interactions in the nucleation and docking stages of the condensation of the polypeptide chain into the compact native structure.