Westbrook E M, Piro O E, Sigler P B
J Biol Chem. 1984 Jul 25;259(14):9096-103.
The crystal structure of the dimeric steroid metabolizing enzyme, delta 5-3-ketosteroid isomerase (EC 5.3.3.1), has been solved to 6-A resolution by multiple isomorphous replacement, augmented by real space direct methods. The unit cell is hexagonal (space group P6122) with dimensions a = b = 65.4 A, c = 504 A, and contains four identical 13,400-dalton protomers in each of its 12 asymmetric units. The 504-A c axis required double focusing mirrors (Franks optics) to resolve the reflections. The complexity of the combined local and lattice symmetry necessitated direct methods to establish the positions of heavy atoms in even the simplest of the isomorphous derivatives. The electron density map clearly showed both (a) the elaborate packing scheme of protomers, which accounts for this large and complicated unit cell, and (b) the coarse features of the functional dimer. The steroid-binding site has been established by imaging the bound inhibitor, 4-acetoxymercuriestradiol, in a difference Fourier map. Each of the dimer's two steroid-binding sites lies completely within one subunit but close enough to the opposing subunit that functional interactions may be possible.
通过多重同晶置换法并结合实空间直接法，已将二聚体类固醇代谢酶δ5-3-酮类固醇异构酶（EC 5.3.3.1）的晶体结构解析到6埃的分辨率。晶胞为六方晶胞（空间群P6122），其尺寸为a = b = 65.4埃，c = 504埃，在其12个不对称单元中每个单元都包含四个相同的13,400道尔顿的原体。504埃的c轴需要双聚焦镜（弗兰克斯光学系统）来解析反射。局部对称性和晶格对称性相结合的复杂性使得即使在最简单的同晶衍生物中也必须采用直接法来确定重原子的位置。电子密度图清楚地显示了（a）原体的精细堆积方式，这解释了这个大而复杂的晶胞，以及（b）功能性二聚体的粗略特征。通过在差值傅里叶图中对结合的抑制剂4-乙酰氧基汞雌二醇进行成像，确定了类固醇结合位点。二聚体的两个类固醇结合位点中的每一个都完全位于一个亚基内，但与相对的亚基距离足够近，可能存在功能相互作用。
