Role of hydrophobic effect in the salt-induced dimerization of bovine beta-lactoglobulin at pH 3.

beta-Lactoglobulin is a dimeric protein around neutral pH, but the monomer becomes the dominant species at pH 3.0 due to strong electrostatic repulsions between the positively charged molecules. It has been found that the addition of salts to water at pH 3.0 favors the dimerization of beta-lactoglobulin. In particular, the dimer is the dominant species at 1M NaCl, 1M GuHCl, and 25 mM NaClO(4) [Sakurai, Oobatake, and Goto, Protein Sci 2001, 10, 2325-2335]. The effect of these salt conditions on the strength of hydrophobic interaction has been calculated by means of a simple but physically sound approach. The obtained estimates indicate that: (a) the hydrophobic interaction contribution is strengthened in 1M NaCl and 1M GuHCl with respect to pure water, but not in 25 mM NaClO(4); (b) anion binding on the positively charged surface of protein molecules has to be the major factor for the salt-induced dimerization.