Conformational Stability and Binding Properties of Porcine Odorant Binding Protein

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Abstract
Apparently homogeneous odorant binding protein purified from pig nasal mucosa (pOBP) exhibited subunit molecular masses of 17 223, 17 447, and 17 689 (major component) Da as estimated by ESI/MS. According to gel filtration, this protein, its truncated forms, and/or its variants are homodimeric under physiologic conditions (pH 6−7, 0.1 M NaCl). The dimer ⇔ monomer equilibrium shifts toward a prevalent monomeric form at pH 6 and 4.94 × 106 M-1 and enthalpies of −97.2 and −87.8 kJ/mol, respectively. The binding stoichiometry of both ligands is nearly one molecule of ligand per homodimer of pOBP. The interaction of pOBP with both ligands is enthalpically driven with an unfavorable change of entropy. The binding affinity of pOBP with IBMP does not change significantly at acidic pH, while the binding stoichiometry is nearly halved. According to HS-DSC data, the interaction with IBMP and DMO leads to a substantial stabilization of the pOBP folded structure, which is manifested by the increase in the unfolding temperature and enthalpy. The calorimetric data allow us to conclude that the mechanism of binding of the studied odorants to pOBP is not dominated by a hydrophobic effect related to any change in the hydration state of protein and ligand groups but, most likely, is driven by polar and van der Waals interactions.