Temperature effects on calcium binding to caseins

Area of Science:

• Biochemistry
• Physical Chemistry
• Food Science

Background:

• Caseins are the primary proteins in milk, crucial for calcium transport and bioavailability.
• Understanding the kinetics and thermodynamics of calcium-casein interactions is vital for food processing and nutritional science.

Purpose of the Study:

• To investigate the binding kinetics and thermodynamics of calcium ions with individual caseins (αs-casein, β-casein) and casein ingredients (sodium caseinate).
• To elucidate the reaction mechanisms, energy parameters, and structural changes during calcium binding to caseins.

Main Methods:

• Stopped-flow absorption spectroscopy to study reaction kinetics at pH 6.4.
• Temperature dependence studies (25-50°C) to determine activation energies.
• Dynamic and static light scattering to assess particle size and surface changes.
• Isothermal titration calorimetry (ITC) to determine binding thermodynamics.

Main Results:

• Calcium binding involves two consecutive reactions: a faster second-order reaction followed by a slower first-order reaction.
• Sodium caseinate exhibited faster initial calcium binding kinetics compared to αs-casein and β-casein.
• Spectroscopic analysis revealed reaction intermediates resembling final products.
• Light scattering indicated reduced particle size and increased surface area upon calcium binding, especially at lower temperatures.
• ITC confirmed the endothermic nature of calcium binding, driven by enthalpy-entropy compensation around 38°C, suggesting binding to o-phosphoserine residues.

Conclusions:

• Calcium binding to caseins is an endothermic process controlled by enthalpy-entropy compensation, with binding affinity increasing with temperature.
• The binding mechanism is consistent with calcium interacting with o-phosphoserine residues.
• The temperature-independent nature of calcium dissociation suggests its importance for calcium bioaccessibility.