Protein-protein interactions: nature of the electrostatic stabilization of deoxyhemoglobin tetramer formation

The summed electrostatic free energy contributions to deoxyhemoglobin A0 tetramer formation were computed at a series of pH and ionic strength values as the difference between the computed values for the tetramer and for the sum of the four individual chains. The electrostatic stabilization of each monomer is similar and close to that for myoglobin. At ionic strength 0.10 M the electrostatic contribution to the stability of the tetramer is approximately 35 kcal/mol at pH 6.0 and 18 kcal/mol at pH 9.6. The specific contribution to the stabilization of the tetramer, (sigma delta G"i,el)tet, is obtained by difference and shows a broad plateau above 7 kcal/mol over the range from pH 6.0 to 8.0, which is nearly obliterated by pH 9.6. By examination of the contributions of individual sites under the above summation, it is found that sites in the alpha chains are responsible for virtually the entire stabilizing effects in tetramer formation. The major differences on tetramer formation are sensed at eight sites. The stabilization provided by four of these sites results simply from changes in solvent exposure of sites in the given monomers as the tetramer is assembled. They are offset in part by changes at three sites that sense the greatest destabilization and that are responsible for the near cancellation of effects among the beta-chain sites. The general implications for the stabilization of molecular assemblies are considered.