INCLUDING SIDE-CHAIN FLEXIBILITY IN CONTINUUM ELECTROSTATIC CALCULATIONS OF PROTEIN TITRATION

We have extended Monte Carlo procedures for computing statistical aver ages over protonation states of a protein to include conformational st ates of the titrating amino acid side chains. This computational metho d couples side chain motion and protonation with changes in solution p H. Using a continuum electrostatic model for protein titration, we app lied 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 thes e additional conformations improved agreement with experimental measur ements for both overall titration and individual pK(a)s. The results s uggest that well-solvated orientations of amino acid side chains are a n important factor in determining proton binding characteristics of pr oteins.