ABSTRACT
Globular proteins that are not integrated into the membranes are usually soluble, while brilar proteins are essentially nonsoluble in water solutions. Solubility of globular proteins depends on several factors related to the protein structure and the composition of the solvent. Interactions with water molecules that keep the protein in solution are provided by the charged and polar residues on the protein surface. Water around the protein forms a cage composed of a layer of water molecules that are hydrogen bonded to the hydrophilic groups of the surface residues; however, the exchange of water molecules in the cage with the ones in bulk water is very fast. The solubility of proteins is highly dependent on their charge and the concentration of salts in the solution. All molecules of a protein carry the same net charge, which depends on the protein structure and on the pH of the solution. This causes electrostatic repulsion between protein molecules, which additionally prevents protein aggregation and increases protein solubility. In pH that equals the value of the isoelectric point, pI, the net charge of the protein is zero, and repulsion is lost. This is the reason why, at this pH, proteins are the least soluble.
