Department of Food Science, University of Copenhagen, Rolighedsvej 26, DK-1958 Frederiksberg C, Denmark.
Food Res Int. 2022 Apr;154:110981. doi: 10.1016/j.foodres.2022.110981. Epub 2022 Feb 22.
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-casein due to lower energy of activation (ΔE = 8.2 kJ∙mol; ΔE = 18.1 kJ∙mol, respectively), is followed by a slower first-order reaction with similar energies of activation (ΔE = 25.3 kJ∙mol and ΔE = 20.7 kJ∙mol) as determined from temperature dependence of rate between 25 °C and 50 °C. Sodium caseinate reacts faster with calcium than both α-casein and β-casein in the first reaction of the two consecutive reactions, while the rate of the second falls between α-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-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-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.
使用停流吸收光谱法研究了在 pH 6.4 水性溶液中摩尔过量的钙离子与个别酪蛋白和酪蛋白成分的结合动力学。由于较低的活化能（ΔE = 8.2 kJ·mol；分别为 18.1 kJ·mol），β-酪蛋白的初始二级反应比α-酪蛋白更快，然后是较慢的一级反应，其活化能相似（ΔE = 25.3 kJ·mol 和 ΔE = 20.7 kJ·mol），这是从 25°C 到 50°C 的速率的温度依赖性确定的。与α-酪蛋白和β-酪蛋白相比，在两个连续反应中的第一个反应中，酪蛋白酸钠与钙的反应更快，而第二个反应的速率介于α-酪蛋白和β-酪蛋白之间。全局光谱分析表明，反应中间产物的紫外-可见光谱与最终产物更相似，而不是与初始酪蛋白反应物更相似。动态和静态光散射表明，在最显著的低温下，随着钙结合，颗粒尺寸减小，颗粒表面增大。等温滴定量热法表明，钙与酪蛋白的结合是吸热的。钙结合似乎受到焓/熵补偿的控制，与钙与 o-磷酸丝氨酸而不是与酪蛋白的天冬氨酸或谷氨酸侧链结合相对应的等平衡温度为 38°C。与结合的吸热性质一致，α-酪蛋白和酪蛋白酸钠的钙结合容量和亲和力随温度升高而增加。每个钙的结合焓降低表明钙结合时的热容降低。钙与α-酪蛋白结合的结合焓和活化能之间的小差异导致钙离解通过早期过渡态的结论。钙离解的速率几乎不随温度变化，这也解释了为什么钙与酪蛋白的结合对钙的生物可利用性很重要。
