光谱学与光谱分析, 2020, 40 (3): 770, 网络出版: 2020-03-25  

圆二色光谱、 红外光谱法解析羊乳和牛乳β-酪蛋白结构及性质差异

Analyzing Structure and Properties of Goat Milk β-Casein and Bovine Milk β-Casein by Circular Dichroism and Fourier Transformation Infrared Spectroscopy
作者单位
1 乳品科学教育部重点实验室(东北农业大学), 黑龙江 哈尔滨 150030
2 东北农业大学医院, 黑龙江 哈尔滨 150030
摘要
羊乳β-酪蛋白比牛乳β-酪蛋白更容易被婴幼儿消化吸收, 主要原因是二者结构的不同。 目前对牛乳β-酪蛋白结构的研究较多, 但对羊乳β-酪蛋白的结构以及羊乳和牛乳β-酪蛋白结构差异的研究还鲜有报道。 蛋白质二级结构的信息可由光谱获得, 其中圆二色光谱是利用蛋白质分子中具有光学活性的生色基团对左、 右平面圆偏振光吸收不同, 对蛋白质结构进行表征的方法, 可以测定溶液状态下的蛋白质样品, 使蛋白质构象更接近其生理状态, 而且具有快速简便, 对构象变化灵敏等优点; 红外光谱则是利用蛋白质分子在振动过程中不同化学键或官能团对红外光吸收频率不同, 对蛋白质结构进行表征的方法, 可以测定固体状态下的蛋白质样品, 具有扫描速度快、 分辨率高、 可测波长范围广、 不易受蛋白质样品的分子大小和外界条件影响等优点。 圆二色光谱和红外光谱已被广泛应用于蛋白质构象的研究中, 但是结合使用这两种方法分析β-酪蛋白结构的研究还鲜有报道。 因此, 该研究采用圆二色光谱和红外光谱比较羊乳和牛乳β-酪蛋白的结构特点, 并利用分光光度法对二者的巯基含量及溶解性进行了分析, 从功能性质方面的不同对两种蛋白结构的差异进行更好的说明。 圆二色光谱测得羊乳和牛乳β-酪蛋白二级结构中主要以无规卷曲为主, 但羊乳β-酪蛋白的无规卷曲含量(50.2%±0.16%)显著高于牛乳β-酪蛋白(43.8%±0.14%), 其有序结构中α-螺旋含量(2.7%±0.21%)、 β-折叠含量(15.3%±0.08%)显著低于牛乳β-酪蛋白(4.3%±0.13%, 19.5%±0.12%), β-转角含量分别为31.8%±0.11%和32.4%±0.09%, 差异不显著; 红外光谱测得羊乳β-酪蛋白二级结构中α-螺旋、 β-折叠、 β-转角含量分别比牛乳β-酪蛋白低18%~20%, 9%~10%, 0.6%~1%, 无规卷曲含量比牛乳β-酪蛋白高17%~19%。 对两种蛋白功能性质的研究表明, 羊乳β-酪蛋白与牛乳β-酪蛋白表面巯基含量基本一致19~20 μmol·g-1, 但羊乳β-酪蛋白总巯基含量[(28.35±0.13) μmol·g-1]显著低于牛乳β-酪蛋白[(46.72±0.21) μmol·g-1]; 羊乳β-酪蛋白与牛乳β-酪蛋白的等电点较为接近(pH为4~5), 且在等电点附近前者的溶解性低于后者, 而远离等电点时前者溶解性则高于后者。 研究结果说明与牛乳β-酪蛋白相比, 羊乳β-酪蛋白分子的无序性和柔韧性更高, 胶束内部结构更加的柔软疏松。
Abstract
Goat milk β-casein is more easily digested and absorbed by infants than bovine milk β-casein, and the principal reason for this difference is the diversity of their structure. Recently, many investigations have reported the structure of bovine milk β-casein, however, the structure of goat milk β-casein and the difference between the structure of goat milk β-casein and bovine milk β-casein still needs to be studied in detail. The information of protein secondary structure can be obtained by spectroscopy technique, Circular dichroism is a method to characterize the structure of protein in solution state by using different optical active chromophores to absorb circularly polarized light in the left and right planes, which can make the protein conformation closer to its physiological state, and has the advantages of being rapid, simple and sensitive to conformational changes; Fourier transformation infrared spectroscopy is a method to characterize the structure of protein in solid state by using different chemical bonds or functional groups in the process of the vibration, which has the advantages of fast scanning speed, high resolution, wide measuring wavelength range, and is not easily affected by the molecular size and external conditions of protein samples. Circular dichroism and Fourier transformation infrared spectroscopy have been widely used in the study of protein conformation, but these two spectroscopy techniques to analyze the structure of β-casein has been rarely reported. Thus, this study used Circular dichroism and Fourier transformation infrared spectroscopy to compare the structural characteristics of the goat milk β-casein and bovine milk β-casein, and the sulfhydryl content and solubility of the two proteins were analyzed by spectrophotometry. Circular dichroism showed that random coil was the main secondary structure of goat milk β-casein and bovine milk β-casein, but the content of random coil of goat milk β-casein (50.2%±0.16%) was significantly higher than bovine milk β-casein (43.8%±0.14%), the content of α-helix (2.7%±0.21%) and β-fold (15.3%±0.08%) in the ordered structure were significantly lower than bovine milk β-casein (4.3%±0.13%, 19.5%±0.12%), the content of β-turn was 31.8%±0.11%, 32.4%±0.09%, respectively and the difference was not significant; Fourier transformation infrared spectroscopy showed that the content of α-helix, β-fold, β-turn in the secondary structure of goat milk β-casein were lower than bovine milk β-casein by 18%~20%, 9%~10%, 0.6%~1%, respectively and the content of random coil was higher than bovine milk β-casein by 17%~19%. The functional properties of the two proteins showed that the surface sulfhydryl content of goat milk β-casein and bovine milk β-casein were basically consistent with 19~20 μmol·g-1, but the total sulfhydryl content of goat milk β-casein (28.35±0.13 μmol·g-1) was significantly lower than bovine milk β-casein (46.72±0.21 μmol·g-1); the isoelectric point of goat milk β-casein was similar to bovine milk β-casein (pH 4~5), and the solubility of goat milk β-casein was lower than bovine milk β-casein near the isoelectric point, but higher than bovine milk β-casein far from the isoelectric point. The results showed that compared with the bovine milk β-casein, the disorder and flexibility of the goat milk β-casein were higher, and the internal structure of micelle was softer and looser.

李萌, 王娟, 魏子凯, 康佳欣, 张玲, Tabys Dina, 刘宁, 张爽. 圆二色光谱、 红外光谱法解析羊乳和牛乳β-酪蛋白结构及性质差异[J]. 光谱学与光谱分析, 2020, 40(3): 770. LI Meng, WANG Juan, WEI Zi-kai, KANG Jia-xin, ZHANG Ling, Tabys Dina, LIU Ning, ZHANG Shuang. Analyzing Structure and Properties of Goat Milk β-Casein and Bovine Milk β-Casein by Circular Dichroism and Fourier Transformation Infrared Spectroscopy[J]. Spectroscopy and Spectral Analysis, 2020, 40(3): 770.

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