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Thermodynamic distinction between hydrophobic anesthetic association and site-specific fatty acid binding in serum
Makoto Nishimoto1, Kazunobu Tsudura2, Masaki Goto3
1Department of Applied Chemistry and Biochemistry, National Institute of Technology (KOSEN), Wakayama College, 77 Noshima, Nada-cho, Gobo, Wakayama 644-0023, Japan.
Abstract:
Interactions between volatile anesthetics and bovine serum albumin (BSA) were thermodynamically characterized using isothermal titration calorimetry (ITC). Because weak ligand binding requires appropriately matched protein and ligand concentrations for reliable thermodynamic analysis, measurements were performed at several protein concentrations to characterize the concentration-dependent titration behavior. Binding constants and thermodynamic parameters were evaluated by enthalpy-based analysis combined with Scatchard analysis of the resulting ITC data. For comparison, the binding of medium-chain fatty acids to BSA was analyzed using the same approach to contrast the thermodynamic behavior of weak- and strong-binding ligands. Fatty acid binding exhibited clear saturation behavior and could be described as finite-site binding with a well-defined number of binding sites (approximately 7-9) and relatively high binding constants (on the order of 105-106). In contrast, anesthetic binding showed pronounced protein concentration dependence, and the resulting Scatchard plots were nonlinear, indicating that the interaction cannot be explained by a simple finite-site binding model. The apparent binding constants and binding numbers were broadly consistent with trends in anesthetic hydrophobicity and potency. Although the binding enthalpy changed sign depending on the anesthetic species, this sign did not determine the essential binding mode and likely reflects solvent reorganization and hydrophobic interactions. These results indicate that fatty acids bind to serum albumin through site-specific interactions at a finite number of binding sites, whereas anesthetic interactions are better described as hydrophobic, partition-like multi-site associations on the protein surface. The present study provides a thermodynamic framework for distinguishing site-specific binding from hydrophobic association in protein-ligand interactions.
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