Influence of Precipitants on Molecular Arrangements and Space Groups of Protein Crystals

In protein crystallization, precipitants are used to control the final protein concentration in the solution and/or to decrease the protein solubility for nucleation and growth. In this study, we obtained three crystal structures for the same kind of protein with three different crystallizing agents, in which one of the three different chemicals, ammonium sulfate, potassium sodium tartrate, and polyethylene glycol (PEG), was contained as a main precipitant. The space group of the protein crystal obtained by PEG was different from those obtained by the other two precipitants. Molecular dynamics simulations were carried out for the protein in the presence of each of the three precipitants at a concentration equivalent to the crystallizing condition or without any precipitant. The simulations showed that all of the three precipitants enhanced protein stability by decreasing the conformational fluctuation. The distribution of precipitant molecules was found to be not isotropic around the protein in every case. In the simulations with ammonium sulfate and potassium sodium tartrate, high-concentration areas of precipitants on the protein surface coincided with noncontact sites with other protein molecules in the crystals. In the simulations with PEG, low-concentration areas coincided with noncontact sites with other protein molecules in the crystal. The results suggest that precipitants play multiple roles not only of decreasing the protein solubility but also in restricting contact sites on the protein surface. This restriction is reflected in the molecular arrangement in protein crystals, thereby resulting in crystal growth with a specific space group.