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

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Abstract
The summed electrostatic free energy contributions to deoxy-Hb AO 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 4 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 .apprx. 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 about 7 kcal/mol over the range from pH 6.0-8.0, 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 8 sites. The stabilization provided by 4 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 3 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.

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