Interaction of Heparin with Proteins: Hydration Effects

Area of Science:

• Biophysical Chemistry
• Thermodynamics of Biomolecular Interactions
• Polyelectrolyte Science

Background:

• Heparin and lysozyme are key biomolecules with significant biological roles.
• Understanding their interaction is crucial for various biochemical and medical applications.
• The influence of salts on biomolecular complexation is a complex phenomenon involving hydration and ion effects.

Purpose of the Study:

• To thermodynamically investigate the interaction between heparin and lysozyme.
• To elucidate the role of potassium glutamate (KGlu) concentration and temperature on this interaction.
• To compare the effects of KGlu with NaCl, focusing on hydration and ion-specific (Hofmeister) effects.

Main Methods:

• Isothermal Titration Calorimetry (ITC) was employed to measure binding constants (Kb).
• Experiments were conducted across a temperature range (288–310 K) and varying KGlu concentrations (25–175 mM).
• Thermodynamic parameters (ΔGb, Δw, Δcp) were analyzed using model-free approaches like the generalized van't Hoff equation.

Main Results:

• Binding free energy (ΔGb) decreased significantly with increasing KGlu concentration, primarily due to counterion release and entropy increase.
• Temperature dependence of ΔGb was minimal, attributed to hydration changes quantified by a characteristic temperature (T0).
• KGlu demonstrated distinct hydration effects compared to NaCl, with a lower T0, and specific heat (Δcp) became more negative with increasing salt concentration.

Conclusions:

• The study quantifies the significant, albeit small, contribution of hydration changes to the binding free energy of heparin-lysozyme complexation.
• Ion-specific Hofmeister effects are demonstrated and can be quantitatively modeled using T0 and Δcp dependence on salt concentration.
• The findings provide a deeper understanding of salt effects in biomolecular recognition and polyelectrolyte interactions.