Coordination complexes and catalytic properties of proteins and related substances. 104. Electrostatic effects in hemoglobin: hydrogen ion equilibriums in human deoxy- and oxyhemoglobin A

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Abstract
The modified Tanford-Kirkwood theory of Shire et al. for electrostatic interactions was applied to the hydrogen ion equilibria of human deoxyhemoglobin and HbO2. Atomic coordinates for HbO2 were generated by the application of the appropriate rigid rotation function to .alpha. and .beta. chains of the deoxyhemoglobin structure. The model employs 2 sets of parameters derived from the crystalline protein structures, the atomic coordinates of charged amino acid residues and static solvent accessibility factors to reflect their individual degrees of exposure to solvent. Theoretical titration curves based on a consistent set of pKint values compared closely with experimental potentiometric curves. Theoretical pK values at half-titration for individual protein sites corresponded to available observed values for both quaternary states. The cumulative effects of numerous electrostatic interactions in the tetrameric structures and the major effects of the quaternary transition that result from changes in static solvent accessibility of certain ionizable groups were noted.