Including Side Chain Flexibility in Continuum Electrostatic Calculations of Protein Titration

Abstract

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_as. The results suggest that well-solvated orientations of amino acid side chains are an important factor in determining proton binding characteristics of proteins.