REFINED STRUCTURE OF BABOON ALPHA-LACTALBUMIN AT 1.7-A RESOLUTION - COMPARISON WITH C-TYPE LYSOZYME
REFINED STRUCTURE OF BABOON ALPHA-LACTALBUMIN AT 1.7-A RESOLUTION - COMPARISON WITH C-TYPE LYSOZYME
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DOI：
10.1016/0022-2836(89)90091-0
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发表时间：
1989-07-05
影响因子：
5.6
通讯作者：
PHILLIPS, DC
中科院分区：
文献类型：
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作者：
ACHARYA, KR;STUART, DI;PHILLIPS, DC
以前报道过用同晶取代法在4.5°分辨率下解决从狒狒奶(Papio Cynocephalus)中提取的α-内酰胺的结构。在这些低分辨率研究的基础上进行的初步改进并不成功，因为最好的重原子导数的同构性很差。由于溶菌酶和α-乳清蛋白的结构具有惊人的相似性，因此尝试了一种更谨慎的分子置换方法来完善该模型。以蛋清溶菌酶为初始模型，在倒易空间中采用重约束最小二乘法进行初步求精。在1.7ANG.分辨率为1141个蛋白质原子的0.22的传统结晶学残差的情况下，使用立体化学约束进一步改进了该模型。在最终模型中，键距的均方根值与理想值的偏差为0.015，角距的均方根偏差为0.027。使用人α-乳清蛋白序列进行提纯，并且在提纯过程中计算的“省略图”表明在狒狒α-乳蛋白X-射线序列中有八种可能的序列变化。在提纯过程中，已经找到了一个紧密结合的钙离子和150个水分子，其中四个是内部分子。一些水分子被模拟为无序侧链。围绕钙的配位是一个略微扭曲的五角形双金字塔。钙-氧距离从2.2°到2.6°不等，在结构中呈现出一个紧密的钙结合环。钙结合折叠只是表面上类似于“EF-Hand”，推测与其他EF-Hand结构没有进化关系。α-乳清蛋白的整体结构与溶菌酶非常相似。所有较大的偏差都发生在找到所有序列缺失和插入的循环中。与溶菌酶相比，α-乳清蛋白的C末端似乎相当灵活。实验证据支持早期对α-乳白蛋白结构的预测，该预测基于α-乳白蛋白和溶菌酶具有相似的三维结构的假设，具有最小的缺失和插入。对这两种结构的详细比较显示了显著的特征，并为这两种蛋白质作为共同前体的进化提供了一些线索。
The solution of the structure of .alpha.-lactablumin from baboon milk (Papio cynocephalus) at 4.5 .ANG. resolution using the isomorphous replacement method has been reported previously. Initial refinement on the basis of these low-resolution studies was not successful because of the poor ismorphism of the best heavy-atom derivative. Because of the striking similarity between the structure of lysozyme and .alpha.-lactalbumin, a more cautious molecular replacement approach was tried to refine the model. Using hen egg-white lysozyme as the starting model, preliminary refinement was performed using heavily constrained least-squares minimization in reciprocal space. The model was further refined using stereochemical restraints at 1.7 .ANG. resolution to a conventional crystallographic residual of 0.22 for 1141 protein atoms. In the final model, the root-mean-square deviation from ideality for bond distances is 0.015 .ANG., and for angle distances it is 0.027 .ANG.. The refinement was carried out using the human .alpha.-lactalbumin sequence and "omit maps" calculated during the course of refinement indicated eight possible sequence changes in the baboon .alpha.-lactalbumin X-ray sequence. During the refinement, a tightly bound calcium ion and 150 water molecules, of which four are internal, have been located. Some of the water molecules were modelled for disordered side-chains. The co-ordination around the calcium is a slightly distorted pentagonal bipyramid. The Ca-O distances vary from 2.2 .ANG. to 2.6 .ANG., respresenting a tight calcium-binding loop in the structure. The calcium-binding fold only superficially resembles the "EF-hand" and presumably has no evolutionary relationship withother EF-hand structures. The overall structure of .alpha.-lactalbumin is very similar to that of lysozyme. All large deviations occur in the loops where all sequence deletions and insertions are found. The C terminus appears to be rather flexible in .alpha.-lactalbumin compared to lysozyme. The experimental evidence supports the earlier predictions for the .alpha.-lactalbumin structure that were based upon the assumption that .alpha.-lacatalbumin and lysozyme have similar three-dimensional structures, with minimal deletions and insertions. A detailed comparison of the two structures shows striking features as well as throwing some light on the evolution of these two proteins for a common precursor.