Before revision:  Due to ionizable amino acid residues present on the solvent-exposed surfaces of proteins, the charge on the proteins is not constant but varies with the changes in their environment -- most notably, the pH of the surrounding solution.
After revision:  Due to ionizable amino acid residues present on the solvent-exposed surfaces of proteins, the protein charge is not constant but varies with the changes in their environment -- most notably, the pH of the surrounding solution.