Anisotropic Distribution of Ammonium Sulfate Ions in Protein Crystallization

Some proteins are easily crystallized by utilizing ammonium sulfate (AS) as a precipitant, while others are not. To investigate the difference of AS behavior in protein crystallization between both types of proteins, crystals were grown for two proteins in the former type; carbonic anhydrase II (CAII) and myoglobin (Mb), and also for two proteins in the latter one; hen egg white lysozyme (HEWL) and human serum albumin (HSA). In particular, CAII and Mb were crystallized at high AS concentrations around 3.0 M. In contrast, single crystals were grown at a lower AS concentration of 1.2 M both for HEWL and HSA. Molecular dynamics simulations were carried out for all the proteins with calculation models, including AS at the concentrations of the respective crystallization conditions. The motion of the protein during the simulation was reduced in the presence of AS for all the proteins. Ammonium and sulfate ions (AS ions were anisotropically distributed around the protein molecules, especially for the proteins in the former type, CAII and Mb, under the condition of high AS concentrations. The electrostatic potential around CAII and Mb was almost equally divided into the positive and negative areas, and the AS anisotropic distributions observed in the simulations were compatible with the shape of the iso-surface of the electrostatic potential. In contrast, AS ions were sparsely distributed under the low AS concentration for HEWL and HSA. Either positive or negative area of the electrostatic potential was dominant for HEWL and HSA. Hence, the surrounding space of the latter-type protein was not so distinctively polarized as that of the former-type one. AS ions were anisotropically distributed even for HEWL and HSA, when simulations were performed at high AS concentrations corresponding to 2.0 and 3.0 M in precipitant solution. The AS distributions were, however, different between the former-type proteins and the latter-type ones. Two AS dense areas appeared around CAII and Mb, while AS ions were crowded at one area for HEWL and HSA.