Smith C A, Vogel C W, Müller-Eberhard H J
J Biol Chem. 1982 Sep 10;257(17):9879-82.
The molecular architecture of the cobra venom factor (CVF)-dependent C3/C5 convertase (EC 3.4.21.47) of human complement was deduced from electron microscopy of the purified bimolecular complex (CVF,Bb) and its isolated subunits, CVF and Bb. Negatively stained CVF imaged as an irregularly shaped cylindrical structure, with approximate dimensions of 137 A x 82 A (length x diameter). The zymogen Factor B appeared globular with a diameter of about 80 A and its image suggested a bipartite structure of two compact, closely associated regions, one about twice as large as the other. Bb, the larger, catalytic site-bearing cleavage fragment of Factor B, revealed two globular domains, each 40 A in diameter, connected by a short linker region about 10 A long and thick. CVF and Bb could be distinguished in micrographs of the CVF,Bb complex: Bb attached to one end of the CVF molecule through only one of its two domains, and in an orientation making the long axes of both molecules approximately orthogonal. It is suggested the naturally occurring C3 convertases (C3b,Bb and C4b,2a) have a similar morphology.
通过对纯化的双分子复合物（眼镜蛇毒因子[CVF]，Bb）及其分离的亚基CVF和Bb进行电子显微镜观察，推导了人补体中依赖眼镜蛇毒因子（CVF）的C3/C5转化酶（EC 3.4.21.47）的分子结构。经负染的CVF呈现为不规则形状的圆柱形结构，大致尺寸为137 Å×82 Å（长×直径）。酶原B因子呈球形，直径约80 Å，其图像显示为两个紧密相连的紧密区域组成的二分结构，其中一个区域约为另一个区域的两倍大。Bb是B因子较大的、带有催化位点的裂解片段，显示出两个直径均为40 Å的球形结构域，由一个长约10 Å且粗细适中的短连接区域相连。在CVF，Bb复合物的显微照片中可以区分CVF和Bb：Bb仅通过其两个结构域之一附着于CVF分子的一端，且其取向使两个分子的长轴大致正交。有人提出，天然存在的C3转化酶（C3b，Bb和C4b，2a）具有相似的形态。
