In silico physicochemical characterization and comparison of two intrinsically disordered phosphoproteins: β-casein and acidic PRP-1

A coarse-grained model has been implemented using the Metropolis-Hastings Monte Carlo method to simulate β-casein and acidic proline-rich protein 1 (PRP-1). The aim of the study is to directly compare the properties and behavior of β-casein and PRP-1, in both bulk solution and in the presence of a negatively charged surface, in order to evaluate the possibility of using β-casein as a replacement for PRP-1 in, e.g. saliva substitutes and, possibly, dental products. The results are obtained by studying the effect of varying pH, monovalent salt concentration and with/without charge saturation of the phosphorylated serines. The electrostatic properties in bulk are found to be very similar for the two proteins, especially at physiological pH and with simulated calcium saturation. When studying surface adsorption it is observed that both proteins attach to the surface in similar ways, relatively to their size. Surface adsorption seems to be stronger for PRP-1, but β-casein is shown to adsorb closer to the surface. The adsorption of both proteins onto the negatively charged surface is strikingly similar under physiological pH and higher, near physiological, salt concentrations. The effect of calcium saturation on surface adsorption is almost negligible for both proteins at high salt concentration.