Monovalent cation binding to cubic insulin crystals.
Monovalent cation binding to cubic insulin crystals.
复制标题
单价阳离子与立方胰岛素晶体结合。
DOI：
10.1016/s0006-3495(92)81865-9
复制
发表时间：
1992
影响因子：
3.4
通讯作者：
Caspar,DL
中科院分区：
文献类型：
--
作者：
Gursky,O;Li,Y;Badger,J;Caspar,DL
两个本地化的单价阳离子结合位点已被确定在立方胰岛素从2.8 A-分辨率差电子密度图比较晶体中的Na+离子已被替换为Tl+。一个阳离子被埋在胰岛素二聚体之间的封闭空腔中，并通过与晶体二分体相关的两个并列交替位置中的蛋白质羰基偶极相互作用而稳定。第二个阳离子结合位点，这也涉及与羰基偶极连接，竞争性地占据了两个交替的His B10侧链构象的一个位置。两个位点中的阳离子占有率取决于蛋白质上的净电荷，其通过在pH范围7-10中平衡晶体而变化。阳离子结合位点的详细结构从在pH 9下的钠-胰岛素晶体的精细2-A分辨率图推断。在pH 9时，局部单价阳离子占中和每个蛋白质分子上的负电荷所需的三到四个正抗衡电荷中的不到一个。大多数单价抗衡离子的移动的太多，无法显示在仅使用分辨率高于10 A的数据计算的电子密度图中。晶体稳定性需要离子半径小于1.5 A的单价阳离子。用Cs+、Mg++、Ca++或La+++取代Na+会破坏晶格有序，但在pH 9下用0.1 M Li+、K+、NH 4+、Rb+或Tl+取代晶体可达到至少2.8 A分辨率。
Two localized monovalent cation binding sites have been identified in cubic insulin from 2.8 A-resolution difference electron density maps comparing crystals in which the Na+ ions have been replaced by Tl+. One cation is buried in a closed cavity between insulin dimers and is stabilized by interaction with protein carbonyl dipoles in two juxtaposed alternate positions related by the crystal dyad. The second cation binding site, which also involves ligation with carbonyl dipoles, is competitively occupied by one position of two alternate His B10 side chain conformations. The cation occupancy in both sites depends on the net charge on the protein which was varied by equilibrating crystals in the pH range 7–10. Detailed structures of the cation binding sites were inferred from the refined 2-A resolution map of the sodium-insulin crystal at pH 9. At pH 9, the localized monovalent cations account for less than one of the three to four positive counterion charges necessary to neutralize the negative charge on each protein molecule. The majority of the monovalent counterions are too mobile to show up in the electron density maps calculated using data only at resolution higher than 10 A. Monovalent cations of ionic radius less than 1.5 A are required for crystal stability. Replacing Na+ with Cs+, Mg++, Ca++ or La+++ disrupts the lattice order, but crystals at pH 9 with 0.1 M Li+, K+, NH4+, Rb+ or Tl+ diffract to at least 2.8 A resolution.