pH对中国人乳β-酪蛋白二级结构的圆二色和拉曼特征影响

摘要

人乳中β-酪蛋白为婴儿的快速生长提供了最适宜的氨基酸、钙磷等。以往的研究均是对牛乳的β-酪蛋白二级结构进行分析，而牛乳β-酪蛋白对婴儿配方乳的指导意义不如人乳β-酪蛋白，且人乳β-酪蛋白难以获得合适的蛋白晶体，其三级结构不清楚。为了获得中国人乳β-酪蛋白二级结构的相关基础信息，采用圆二色光谱（CD）法和拉曼光谱（Raman）法分析在不同pH 条件下其二级结构的变化，结果表明：pH 值的改变能够诱导中国人乳β-酪蛋白二级结构的改变，CD分析表明β-酪蛋白各部分的二级结构分别是：0．5％～2％α-螺旋，16％～18％β-折叠，30％～34％β-转角，49％～51％无规则卷曲；随着pH 的增加，部分结构发生了变化，α-螺旋在pH 8处含量增加，而在pH 10处含量减小。拉曼光谱分析表明：β-酪蛋白在酰胺I的特征峰在1662 cm－1处，分析出β-酪蛋白的无规则卷曲含量较高。同时，根据I850／I830的比值计算出β-酪蛋白的酪氨酸残基趋向于“暴露式”。圆二色光谱和拉曼光谱都证明了人乳β-酪蛋白二级结构中无规则卷曲的含量最高，而β-折叠和β-转角含量保持相对的稳定。并且，在pH 8条件下，α-螺旋含量高于在其他pH 条件下的含量。%To obtain a structural basis for theβ-casein in Chinese human milk,structural transitions of theβ-casein in response to variation of pH were investigated using Raman and circular dichroism (CD)spectroscopy.Both methods indicated that the sec-ondary structures ofβ-casein in the solution were induced by the pH.Secondary structural analysis ofβ-casein by CD spectrosco-py yielded 0. 5%~2%α-helical,16%~18%β-sheet,30%~34%β-turn and 49%~51% random coil contents.Another result was that as pH increases,these structures change.Several distinct transitions were observed by circular dichroism inα-helix at pH 8 and pH 10.Raman spectrum also showed random coil as the major secondary structure in nativeβ-casein,for the charac-teristic band of theβ-casein amide I was at 1 662 cm-1 .Calculations from I850/I830 suggested that the tyrosine residues ofβ-case-in tended to “exposure”.CD and Raman spectra both showed that at neutral and alkaline pH theβ-casein existed predominantly in random coil conformation,and the proportion ofα-helix was higher at pH 8 than under other pH conditions.Over the range of pH studied,the sheet and turn areas remained relatively constant,and in the condition of pH 8,the content ofα-helical was higher than in the other pH conditions.