Do Macromolecular Crowding Agents Exert Only an Excluded Volume Effect? A Protein Solvation Study

The effect of macromolecular crowding on protein structure and dynamics has mostly been explained on the basis of the excluded volume effect, its origin being entropic. In recent times a progressive shift in this view has been taking place with increasing emphasis on soft interactions that are enthalpic by nature. Using very low concentrations (1-10 g/L) of both synthetic (dextran- and poly(ethylene glycol) (PEG)-based) and protein (alpha-synuclein and myoglobin)-based crowders, we have shown that the solvation of probe molecule ANS (1-anilinonapthalene-8-sulfonate) bound to serum proteins bovine serum albumin (BSA) and human serum albumin (HSA) is significantly modulated in both a protein- and crowder-dependent fashion. Since under such conditions the effect of excluded volume is appreciably low, we propose that our observations are direct evidence of soft interactions between the macromolecular crowding agents used and the serum proteins. Moreover, our data reveal, that since at these low crowder concentrations major perturbations to the protein structure are unlikely to take place while minor perturbations might not be readily visible, protein solvation provides a unique spectral signature for capturing such local dynamics, thereby allowing one to decouple hard-sphere interactions from soft sphere ones. Furthermore, since fast fluctuations are known to play a major role in determining the functional characteristics of proteins and enzymes, our results suggest that such motions are prone to be modulated even when the cellular crowding conditions are quite relaxed. In other words, by the time the excluded volume effects come into the picture in the physiological milieu, modulations of functionally important protein motions that need a relatively lower activation energy have already taken place as a result of the aforementioned enthalpic (soft) interactions.