Kolodziej S J, Klueppelberg H U, Nolasco N, Ehses W, Strickland D K, Stoops J K
Department of Pathology and Laboratory Medicine, The University of Texas Health Science Center, Houston, Texas, 77030, USA.
J Struct Biol. 1998 Oct;123(2):124-33. doi: 10.1006/jsbi.1998.4027.
The three-dimensional reconstructions of the human plasmin alpha2-macroglobulin binary complex were computed from electron microscopy images of stain and frozen-hydrated specimens. The structures show excellent agreement and reveal a molecule with approximate dimensions of 170 (length) x 140 (width) x 140 A (depth). The asymmetric plasmin structure imparts significant asymmetry to the plasmin alpha2-macroglobulin complex not seen in the structures resulting from the reaction of alpha2-macroglobulin with methylamine or chymotrypsin. The structure shows, when combined with other studies, that the C-terminal catalytic domain of the rod-shaped plasmin molecule is entrapped inside of the alpha2-macroglobulin cavity, whereas its N-terminal kringle domains protrude outside one end between the two arm-like features of the transformed alpha2-macroglobulin structure. This arrangement ensures that the catalytic site of plasmin is prevented from degrading plasma proteins. The internalized C-terminal portion of the plasmin structure resides primarily on the major axis of alpha2-macroglobulin, suggesting that after the initial cleavage of the two bait domains and the thiol esters, the rod-shaped plasmin molecule enters the alpha2-macroglobulin cavity through the large openings afforded by the half-transformed structure. This mode of entrapment requires the untwisting and the separation of the two strands that constitute the alpha2-macroglobulin structure.
人纤溶酶α2-巨球蛋白二元复合物的三维重建是根据染色和冷冻水合标本的电子显微镜图像计算得出的。这些结构显示出极佳的一致性，并揭示了一个尺寸约为170（长）×140（宽）×140埃（深）的分子。纤溶酶的不对称结构赋予了纤溶酶α2-巨球蛋白复合物显著的不对称性，这在α2-巨球蛋白与甲胺或胰凝乳蛋白酶反应产生的结构中未见。结合其他研究来看，该结构表明杆状纤溶酶分子的C端催化结构域被困在α2-巨球蛋白腔内，而其N端kringle结构域则从转化后的α2-巨球蛋白结构的两个臂状特征之间的一端向外突出。这种排列确保了纤溶酶的催化位点不会降解血浆蛋白。纤溶酶结构内化的C端部分主要位于α2-巨球蛋白的主轴上，这表明在两个诱饵结构域和硫酯最初裂解后，杆状纤溶酶分子通过半转化结构提供的大开口进入α2-巨球蛋白腔。这种捕获模式需要构成α2-巨球蛋白结构的两条链解捻和分离。
