Including side chain flexibility in continuum electrostatic calculations of protein titration

We have extended Monte Carlo procedures for computing statistical averages over protonation states of a protein to include conformational states of the titrating amino acid side chains. This computational method couples side chain motion and protonation with changes in solution pH. Using a continuum electrostatic model for protein titration, we applied this sampling method to calculate titration curves for lysozyme, myoglobin, and hemoglobin. In addition to the X-ray conformation, each titrating site was allowed to reorient to a conformation with maximum solvent accessibility. For all proteins considered, inclusion of these additional conformations improved agreement with experimental measurements for both overall titration and individual pK(a)s. The results suggest that well-solvated orientations of amino acid side chains are an important factor in determining proton binding characteristics of proteins.