The Effects of pH on Orientations of Adsorbed Lysozyme on Negatively Charged Surfaces

The adsorption of proteins on charged surfaces is essential for a number of applications, such as biosensors and biocatalytic systems. An important aspect here is the orientation of the adsorbed proteins on the surface, determined in part by their pH-dependent patchy charge distribution, which can also be affected by the presence of charged surfaces in proximity (due to the charge-regulation (CR) mechanism). In this study, the hen-egg white lysozyme (HEWL) is adopted as a model protein to study the features of the protein adsorption into a negatively charged surface via employing a coarse-grained model and constant-pH Monte Carlo simulations. Our goal is to study the CR effects on the charge-patch distribution and how this affects the orientation of the adsorbed protein molecules. We observe an orientation transition in which the amino-acid residue HIS15 stops being in contact with the surface and starts being positioned on the opposite protein face, whereas the LYS33 residue undergoes the opposite inversion. This transition takes place at lower pH values, when the protein charges are constant in the course of simulations, because the CR mechanism promotes the persistence of positive-charge patches and the emergence of negative-patches at higher pH values. This promotes the stability of the orientation observed at low pH.