Protein-Protein Interactions in Solution and Crystallization

Interactions between biomacromolecules in solution are a key factor determining the phase behavior of biological systems. Also, the phase behavior determines whether one can get good quality protein crystals for X-ray diffraction, which is critical in obtaining the proteins three-dimensional structure and further to elucidate its biochemical role. [1-3] On the other hand, the protein interaction and aggregation processes are also very important in understanding many physiological problems, for example, diseases like Alzheimer or Kreutzfeld-Jacob and Parkinson, which are caused by protein or peptide association phenomena. Among the various factors, the electrostatic effects play a major role in protein structure, interactions, and dynamics. They depend on the protein itself, the surrounding water, and ions present (see detail).

George and Wilson [4] proposed a relation between protein crystallization behavior and the osmotic second virial coefficient, A2, which represents the interaction potential between a pair of macromolecules in solution. A positive value of A2 implies a repulsive interaction and a negative value indicates an attractive interaction. Based on measurements of a variety of proteins, they found that protein crystallization occurs only when A2 lies within a narrow window. These studies provide a way to understand the mechanism of protein crystallization and a guide for optimization of conditions for protein crystallization. [5-7] On the other hand, the protein interaction and aggregation processes are also very important in understanding many physiological problems, for example, diseases like Alzheimer or Kreutzfeld-Jacob and Parkinson, which are caused by protein or peptide association phenomena, and the short-range order of (-crystalline accounts for the eye lens transparency. [3,8] In vivo, the biochemical function of proteins cannot perform without the cooperation of ions around them. Therefore, studies on the effect of ionic strength and the nature of ions on the protein interaction have attracted much attention in biophysics. [5,6,9] Studies show that the interaction strongly depends on the nature of salt used at a fixed ionic strength which is known as the "Hofmeister effect". [10]

This project addresses the important question of

• how ions surrounding a protein interact with the protein and its hydration shell
• how ions influence specific and unspecific protein-protein interactions, and
• how ions influence flexibility and dynamics of proteins and protein-complexes.

To answer these questions, the density and location of ions around the protein, their specific interaction with surface charges and the dynamics of these ions and the solvent will be determined. These effects influence protein-protein aggregation, the stability of protein-protein complexes, and flexibility of individual protein side chains or whole loop regions.
Typical interaction potential is described by DLVO theory which includes a screened Coulomb repulsion with a hard sphere core and an attraction of van der Waals interaction.

By the combination of theoretical and experimental studies we gain a comprehensive picture of charge effects in this context. [11,12] The techniques complement each other, revealing a number of different aspects of charge effects. Experimentally, optical microscopy (view real-time video), spectroscopic lab-based techniques (IR, UV/vis, CD) as well as neutron and synchrotron-based scattering techniques (SAXS, ASAXS, QENS) are employed to determine the structure, the salt-ion distribution, and various aspects of the dynamics of individual proteins and protein complexes. Theoretical studies based on local and nonlocal continuum electrostatics calculations and molecular dynamics simulations complement the spectroscopic techniques. These techniques reveal counter ion distribution, the effect of ions on electrostatic shielding and molecular recognition, counter ion mobility and the influence of charge on local protein motion at atomic resolution.