On the charge regulation of proteins.

It is known that the overall charge of a protein can change as the molecule approaches a charged object like another protein or a cell membrane. We have formalized this mechanism using a statistical mechanical framework and show how this rather overlooked interaction increases the attraction between protein molecules. From the theory, we can identify a unique property, the protein charge capacitance, that contains all information needed to describe the charge regulation mechanism. The capacitance can be obtained from experiment or theory and is a function of pH, salt concentration, and the number of titrating residues. For a range of different protein molecules, we calculate the capacitance and demonstrate how it can be used to quantify the charge regulation interaction. With minimal effort, the derived formulas can be used to improve existing models by including a charge regulation term. Good agreement is found between theory, simulations, and experimental data.