Curley D M, Kumosinski T F, Unruh J J, Farrell H M
USDA, Agricultural Research Service, Eastern Regional Research Center, Wyndmoor, PA 19038, USA.
J Dairy Sci. 1998 Dec;81(12):3154-62. doi: 10.3168/jds.S0022-0302(98)75881-3.
Bovine casein submicelles and reformed micelles, produced by addition of Ca2+, were examined by Fourier transform infrared spectroscopy at 15 and 37 degrees C in aqueous salt solutions of K+ and Na+. Previous measurements of caseins, made in D2O and in the solid form, can now be made in a more realistic environment of H2O. When analyzed in detail, data obtained by Fourier transform infrared spectroscopy have the potential to show subtle changes in secondary structural elements that are associated with changes in protein environment. Electrostatic binding of Ca2+ to casein resulted in a redistribution of the components of the infrared spectra. Addition of Ca2+ in salt solutions of K+ and Na+ led to apparent decreases in large loop or helical structures at 37 degrees C with concomitant increases in the percentage of structures having greater bond energy, such as turns and extended helical structures. At 15 degrees C, Na+ and K+ have differential effects on the Ca(2+)-casein complexes. All of these observations are in accordance with the important role of serine phosphate side chains as sites for Ca2+ binding in caseins and the swelling of the casein structure upon incorporation into reformed micelles at 37 degrees C. This new open, hydrated structure is buttressed by a change in backbone as evidenced by a shift in absorbance to higher wave numbers (greater bond energies) as colloidal micelles are reformed.
通过添加Ca2+产生的牛酪蛋白亚微团和重组微团，在15℃和37℃下于K+和Na+的盐水溶液中用傅里叶变换红外光谱进行了检测。以前对酪蛋白在D2O和固态下所做的测量，现在可以在更接近实际的H2O环境中进行。当进行详细分析时，傅里叶变换红外光谱获得的数据有可能显示与蛋白质环境变化相关的二级结构单元的细微变化。Ca2+与酪蛋白的静电结合导致红外光谱成分的重新分布。在K+和Na+的盐溶液中添加Ca2+，在37℃时导致大环或螺旋结构明显减少，同时具有更高键能的结构（如转角和延伸螺旋结构）的百分比增加。在15℃时，Na+和K+对Ca(2+) - 酪蛋白复合物有不同的影响。所有这些观察结果都与丝氨酸磷酸侧链作为酪蛋白中Ca2+结合位点的重要作用以及在37℃时酪蛋白结构在掺入重组微团时的膨胀相一致。这种新的开放的、水合的结构通过主链的变化得到支撑，这一点可通过在重组胶态微团时吸光度向更高波数（更高键能）的移动来证明。
