| Abstract
| - We have investigated the binding of bovine pancreatic trypsin inhibitor (BPTI) to bovinetrypsinogen by combining ultrasonic velocimetry, high precision densimetry, and fluorescence spectroscopy.We report the changes in volume, adiabatic compressibility, van't Hoff enthalpy, entropy, and free energythat accompany the association of the two proteins at 25 °C and pH 8.0. We have used the measuredchanges in volume and compressibility in conjunction with available structural data to characterize thebinding-induced changes in the hydration properties and intrinsic packing of the two proteins. Our estimatereveals that 110 ± 40 water molecules become released to the bulk from the hydration shells of BPTI andtrypsinogen. Furthermore, we find that the intrinsic coefficient of adiabatic compressibility of the twoproteins decreases by 14 ± 2%, which is suggestive of the binding-induced rigidification of the proteins'interior. BPTI−trypsinogen association is an entropy-driven event which proceeds with an unfavorablechange in enthalpy. The favorable change in entropy results from partial compensation between twopredominant terms. Namely, a large favorable change in hydrational entropy slightly prevails over a closein magnitude but opposite in sign change in configurational entropy. The reduction in configurationalentropy and, consequently, protein dynamics is consistent with the observed decrease in intrinsiccompressibility. In general, results of this work emphasize the vital role that water plays in modulatingprotein recognition events.
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