X-ray structures of carbonmonoxymyoglobin (MbCO) are available for different pH values. We used conventional electrostatic continuum methods to calculate the titration behavior of MbCO in the pH range from 3 to 7. For our calculations, we considered five different x-ray structures determined at pH values of 4, 5, and 6. We developed a Monte Carlo method to sample protonation states and conformations at the same time so that we could calculate the population of the considered MbCO structures at different pH values and the titration behavior of MbCO for an ensemble of conformers. To increase the sampling efficiency, we introduced parallel tempering in our Monte Carlo method. The calculated population probabilities show, as expected, that the x-ray structures determined at pH 4 are most populated at low pH, whereas the x-ray structure determined at pH 6 is most populated at high pH, and the population of the x-ray structures determined at pH 5 possesses a maximum at intermediate pH. The calculated titration behavior is in better agreement with experimental results compared to calculations using only a single conformation. The most striking feature of pH-dependent conformational changes in MbCO - the rotation of His-64 out of the CO binding pocket - is reproduced by our calculations and is correlated with a protonation of His-64, as proposed earlier.
Rabenstein, B., & Knapp, E. W. (2001). Calculated pH-dependent population and protonation of carbon-monoxy-myoglobin conformers. Biophysical Journal, 80(3), 1141–1150. https://doi.org/10.1016/S0006-3495(01)76091-2