Human heat shock protein 90 (Hsp90) is a key player in the homeostasis of the proteome and plays a role in numerous diseases, such as cancer. For the design of Hsp90 ATPase activity inhibitors, it is important to understand the relationship between an inhibitor structure and its inhibition potential. The volume of inhibitor binding is one of the most important such parameters that are rarely being studied. Here, the volumes of binding of several ligands to recombinant Hsp90 were obtained by three independent experimental techniques: fluorescent pressure shift assay, vibrating tube densitometry, and high-pressure NMR. Within the error range, all techniques provided similar volumetric parameters for the investigated protein-ligand systems. Protein-ligand binding volumes were negative, suggesting that the protein-ligand complex, together with its hydration shell, occupies less volume than the separate constituents with their hydration shells. Binding volumes of tightly binding, subnanomolar ligands were significantly more negative than those of weakly binding, millimolar ligands. The volumes of binding could be useful for designing inhibitors with desired recognition properties and further development as drugs.

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Interactions between charges play a role in protein stability and contribute to the energetics of binding between various charged ligands. Ionic surfactants are charged molecules, whose interactions with proteins are still rather poorly understood despite their wide applications. Here, we show by isothermal titration calorimetry that cationic alkylammonium surfactants bind to negatively charged polyaspartate and polyglutamate homopolymers stoichiometrically, i.e., one surfactant molecule per charged amino acid. Similarly, negatively charged alkyl sulfates (e.g., sodium dodecyl sulfate) and alkane sulfonates bind stoichiometrically to positively charged polylysine, polyornithine, and polyarginine homopolymers. In these reactions, the interacting counterparts form ion pairs and the resulting electrostatically neutral complex coprecipitates from solution. The enthalpies and heat capacities are determined for various pairs of ionic surfactants and charged amino acid homopolymers. These results show the energetic contributions of ionic headgroups and the CH2 group to surfactant interactions with proteins.

Proteins undergo changes in their partial volumes in numerous biological processes such as enzymatic catalysis, unfolding–refolding, and ligand binding. The change in the protein volume upon ligand binding—a parameter termed the protein–ligand binding volume—can be extensively studied by high-pressure NMR spectroscopy. In this study, we developed a method to determine the protein–ligand binding volume from a single two-dimensional (2D) 1 H– 15 N heteronuclear single quantum coherence (HSQC) spectrum at different pressures, if the exchange between ligand-free and ligand-bound states of a protein is slow in the NMR time-scale. This approach required a significantly lower amount of protein and NMR time to determine the protein–ligand binding volume of two carbonic anhydrase isozymes upon binding their ligands. The proposed method can be used in other protein–ligand systems and expand the knowledge about protein volume changes upon small-molecule binding.

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