Sjöberg B, Pap S
Department of Medical Biochemistry, University of Göteborg, Sweden.
J Biol Chem. 1989 Sep 5;264(25):14686-90.
The molecular organization of human plasma alpha 2-macroglobulin (alpha 2M), and its 1:1 and 1:2 trypsin complexes, have been investigated using the small-angle x-ray scattering method. All the experimental data can be explained by the same basic model, consisting of three oblate-shaped domains arranged in a sandwich-like structure. Each of the larger peripheral domains consists of two parallel elliptic cylinders associated side-by-side, whereas the smaller central domain consists of just one elliptic cylinder. In the native molecule the three domains are separated by regions of low protein density. Upon trypsin binding the dimensions of the four peripheral cylinders remain unchanged, but their positioning in space is reorganized so that the whole molecule becomes more compact. The model thus offers a plausible explanation for the mechanism of inactivating of the protease by entrapping it between the two larger domains. By comparing the shape and dimensions of the total molecule with those determined for the half-molecular fragment, obtained after reducing the intersubunit disulfide bonds, we propose that the fragment consists of just one of the peripheral domains plus half of the central domain. Different projections of the model are consistent with the electron micrographs of alpha 2M given in the literature. The model can also explain many of the physical and chemical properties recorded for alpha 2M and its protease complexes.
利用小角X射线散射法研究了人血浆α2-巨球蛋白（α2M）及其1:1和1:2胰蛋白酶复合物的分子结构。所有实验数据都可以用同一个基本模型来解释，该模型由三个扁球形结构域组成，呈三明治状排列。每个较大的外围结构域由两个并排的平行椭圆圆柱体组成，而较小的中央结构域仅由一个椭圆圆柱体组成。在天然分子中，这三个结构域被低蛋白质密度区域隔开。胰蛋白酶结合后，四个外围圆柱体的尺寸保持不变，但它们在空间中的位置重新排列，使得整个分子变得更加紧凑。因此，该模型为蛋白酶通过被困在两个较大结构域之间而失活的机制提供了一个合理的解释。通过将整个分子的形状和尺寸与还原亚基间二硫键后得到的半分子片段的形状和尺寸进行比较，我们提出该片段仅由一个外围结构域加半个中央结构域组成。该模型的不同投影与文献中给出的α2M的电子显微镜图像一致。该模型还可以解释记录到的α2M及其蛋白酶复合物的许多物理和化学性质。
