Temperature effects on calcium binding to caseins

The kinetics of binding of calcium ions in molar excess to individual caseins and casein ingredients was studied in pH 6.4 aqueous solutions using stopped-flow absorption spectroscopy. An initial second-order reaction, faster for β-casein than for α_s-casein due to lower energy of activation (ΔE_a1,β = 8.2 kJ∙mol^-1; ΔE_a1,α = 18.1 kJ∙mol^-1, respectively), is followed by a slower first-order reaction with similar energies of activation (ΔE_a2,β = 25.3 kJ∙mol^-1 and ΔE_a2,α = 20.7 kJ∙mol^-1) as determined from temperature dependence of rate between 25 °C and 50 °C. Sodium caseinate reacts faster with calcium than both α_s-casein and β-casein in the first reaction of the two consecutive reactions, while the rate of the second falls between α_s-casein and β-casein. Global spectral analysis showed the UV-visible spectra of the reaction intermediates of the caseins to be more similar to the final products than to the initial casein reactants. Dynamic and static light scattering indicated decreasing particle sizes and increasing particle surface upon calcium-binding most significantly at low temperatures. The calcium binding to casein was found endothermic by isothermal titration calorimetry. Calcium binding seems to be controlled by enthalpy/entropy compensation corresponding to an isoequilibrium temperature of 38 °C in agreement with binding of calcium to o-phosphoserine rather than to aspartate or glutamate side chains of the caseins. Binding capacity and affinity for calcium to α_s-casein and sodium caseinate both increased with increasing temperature in agreement with the endothermic nature of the binding. Decreasing enthalpy of binding for each calcium indicating a decrease in heat capacity of the caseins upon calcium-binding. The small difference between binding enthalpy and energy of activation for association of calcium to α_s-casein lead to the conclusion that calcium dissociation goes through an early transition state. The rate of calcium dissociation hardly depends on temperature also explaining why calcium binding to caseins is important for calcium bioaccessibility.