Effects of Cooperativity in Proton Binding on the Net Charge of Proteins in Charge Ladders

Abstract

This paper addresses the effects of cooperativity of proton binding on values of net charge and the change in net charge of a protein ΔZ due to the modification of charged groups. Capillary electrophoresis (CE) and charge ladderscollections of protein derivatives that differ in chargeare used to measure the net charge of lysozyme and values of pKa of the N-terminal α-amino group of this protein with different numbers of Lys e-amino groups acetylated. Values of pKa serve as local thermodynamic reporters of electrostatic potentials and therefore provide a direct measure of cooperativity in proton binding. Measured values are compared with values calculated using a model which combines the Poisson−Boltzmann (PB) equation and atomically detailed models of the distribution of charges on lysozyme with Monte Carlo (MC) sampling of different protonation states of the protein to determine the average extent of protonation of each titratable residue as a function of pH. Calculations with the PB-MC model together wi...