Detergent binding as a sensor of hydrophobicity and polar interactions in the binding cavities of proteins

To evaluate the role of hydrophobic and electrostatic or other polar interactions for protein-ligand binding, we studied the interaction of human serum albumin (HSA) and beta-lactoglobulin with various aliphatic (C10-C14) cationic and zwitterionic detergents. We find that cationic detergents, at levels that do not cause unfolding, interact with a single site on beta-lactoglobulin and with two primary and five to six secondary sites on HSA with an affinity that is approximately the same as that with which zwitterionic (dimethylamineoxide) detergents interact, suggesting the absence of significant electrostatic interactions in the high-affinity binding of these compounds. The binding affinity for all of the groups of compounds was dependent upon hydrocarbon chain length, suggesting the predominant role of hydrophobic forces, supported by polar interactions at the protein surface. A distinct correlation between the binding energy and the propensity for micelle formation within the group of cationic or noncharged (nonionic and zwitterionic) detergents indicated that the critical micellar concentration (CMC) for each of these detergent groups, rather than the absolute length of the hydrocarbon chain, can be used to compare their hydrophobicities during their interaction with protein. Intrinsic fluorescence data suggest that the two primary binding sites on serum albumin for the zwitterionic and cationic compounds are located in the C-terminal part of the albumin molecule, possibly in the Sudlow II binding region. Comparisons with previous binding data on anionic amphiphiles emphasize the important contribution of ion bond formation and other polar interactions in the binding of fatty acids and dodecyl sulfate (SDS) by HSA but not by beta-lactoglobulin. Electrostatic interactions by cationic detergents played a significant role in destabilizing the protein structure at high binding levels, with beta-lactoglobulin being more susceptible to unfolding than HSA. Zwitterionic detergents, in contrast to the cationic detergents, had no tendency to unfold the proteins at high concentrations.