Abstract:

The Bohr effect in Hb, which refers to the dependence of the oxygen affinity on the pH, plays an important role in its cooperativity and physiol. function. The dominant contribution to the Bohr effect arises from the difference in the pKa values of His residues of the unliganded (deoxy) and liganded (carbonmonoxy) structures. Using recent high resoln. structures, the residue pKa values corresponding to the two structures are calcd. The method is based on detg. the electrostatic interactions between residues in the protein, relative to those of the residue in soln., by use of the linearized finite difference Poisson-​Boltzmann equation and Monte Carlo sampling of protonation states. Given that good agreement is obtained with the available exptl. values for the contribution of His residues in HbA to the Bohr effect, the calcd. results are used to det. the at. origin of the pKa shift between deoxy and carbonmonoxy HbA. The contributions to the pKa shift calcd. by means of the linear response approxn. show that the salt bridge involving His146 plays an important role in the alk. Bohr effect, as suggested by Perutz but that other interactions are significant as well. A corresponding anal. is made for the contribution of His143 to the acid Bohr effect for which there is no proposed explanation. The method used is summarized and the program by which it is implemented is described in the Appendix.