van Antwerpen Rik
Department of Biochemistry, Medical College of Virginia Campus, Virginia Commonwealth University, P.O. Box 980614, Richmond, VA 23298, USA.
Arch Biochem Biophys. 2004 Dec 1;432(1):122-7. doi: 10.1016/j.abb.2004.08.031.
The structure of the human low-density lipoprotein (LDL) was analyzed in vitreous ice using cryo-electron microscopy (cryo-EM). In relatively thick cryo-EM preparations, random orientation of LDL particles produced various types of projections on the microscope screen, including circular projections with a high-density ring and rectangular projections with two high-density bands. However, in especially thin preparations, preferred, non-random orientations of the LDL particle produced only circular projections of the lipoprotein structure. In preparations with high LDL concentrations, ordered two-dimensional arrays, including hexagonal arrangements of circular projections and short stacks of rectangular projections, were observed. These observations are consistent with a discoid shape of the LDL particle, and suggest that surface tension forces may influence orientation of the LDL disc in thin aqueous films. Face-on orientation of LDL in especially thin cryo-EM preparations may explain earlier difficulties in identifying discoid features of the lipoprotein particle, and illustrates that some caution is warranted when attempts are made to reconstruct the three-dimensional structure of LDL from cryo-electron micrographs.
利用冷冻电子显微镜（cryo-EM）在玻璃态冰中分析了人低密度脂蛋白（LDL）的结构。在相对较厚的冷冻电镜样品中，LDL颗粒的随机取向在显微镜屏幕上产生了各种类型的投影，包括具有高密度环的圆形投影和具有两条高密度带的矩形投影。然而，在特别薄的样品中，LDL颗粒的优先、非随机取向仅产生脂蛋白结构的圆形投影。在LDL浓度较高的样品中，观察到有序的二维阵列，包括圆形投影的六边形排列和矩形投影的短堆叠。这些观察结果与LDL颗粒的盘状形状一致，并表明表面张力可能会影响LDL盘在薄水膜中的取向。在特别薄的冷冻电镜样品中LDL的正面取向可能解释了早期在识别脂蛋白颗粒盘状特征方面的困难，并表明在试图从冷冻电子显微照片重建LDL的三维结构时需要谨慎。
