REFINEMENT OF THE CRYSTAL-STRUCTURE OF RIBONUCLEASE-S - COMPARISON WITH AND BETWEEN THE VARIOUS RIBONUCLEASE-A STRUCTURES
REFINEMENT OF THE CRYSTAL-STRUCTURE OF RIBONUCLEASE-S - COMPARISON WITH AND BETWEEN THE VARIOUS RIBONUCLEASE-A STRUCTURES
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DOI：
10.1021/bi00164a004
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发表时间：
1992-12-15
期刊：
影响因子：
2.9
通讯作者：
RICHARDS, FM
中科院分区：
文献类型：
--
作者：
KIM, EE;VARADARAJAN, R;RICHARDS, FM
核糖核酸酶S（RNase-S）是由牛胰腺核糖核酸酶A（RNase-A）的两个蛋白水解片段组成的复合物：S-肽（残基1-20）和S-蛋白（残基21-124）。我们已经完善了三个RNase-S复合物的晶体结构。前两个含有全长20个残基的S-肽，并在4.75和5.5的pH下进行了研究。第三个由S-肽的截短形式（残基1-15）组成，并在pH 4.75下研究，作为一系列突变肽复合物的参考结构，分别报告。除了缺失（在S15复合物中）或无序（在两种S20复合物中）的残基16-23之外，在1.6埃分辨率下精制的所有三种结构在坐标的估计误差内是相同的（对于骨架原子为0.048埃）。R值（残差）范围为17.4%-18.6%。S20的最终模型，pH 4.75，包括1个硫酸盐和84个水分子。11个残基的侧链在两个离散的构象建模。最终结构与用作起始模型的特定RNase-A或RNase-S无关。与RNA酶-A的精细晶体结构的广泛比较表明，在所有情况下，与广泛的氢键结合在一起的分子核心都是相同的模式。然而，环区域从一种结构到另一种结构是不同的，并且通常以高B因子为特征。热参数的模式似乎是依赖于晶体包装和相关性以及与计算的可访问性在晶体中。Gln 60是迄今为止已知的这类核糖核酸酶的所有序列中的保守残基。然而，它是唯一的残基，明确界定在一个不利的位置（φ = -100度，psi = -130度）的拉马钱德兰图。RNase-A和RNase-S在对酸和温度变性的稳定性以及在中性pH下对蛋白水解的敏感性方面的实质性差异的起源在我们对这两种结构的视觉比较中并不明显。
Ribonuclease S (RNase-S) is a complex that consists of two proteolytic fragments of bovine pancreatic ribonuclease A (RNase-A): the S-peptide (residues 1-20) and S-protein (residues 21-124). We have refined the crystal structures of three RNase-S complexes. The first two contain the full-length 20-residue S-peptide and were studied at pHs of 4.75 and 5.5. The third one consists of a truncated form of S-peptide (residues 1-15) and was studied at pH 4.75 as the reference structure for a series of mutant peptide complexes to be reported separately. Excluding residues 16-23 which are either missing (in the S15 complex) or disordered (in both S20 complexes), all three structures refined at 1.6-angstrom resolution are identical within the estimated errors in the coordinates (0.048 angstrom for the backbone atoms). The R-values, residual error, range from 17.4% to 18.6%. The final model of S20, pH 4.75, includes 1 sulfate and 84 water molecules. The side chains of 11 residues were modeled in two discrete conformations. The final structures were independent of the particular RNase-A or RNase-S used as a starting model. An extensive comparison with refined crystal structures of RNase-A reveals that the core of the molecule which is held together with extensive hydrogen bonds is in identical pattern in all cases. However, the loop regions vary from one structure to another and are often characterized by high B-factors. The pattern of thermal parameters appears to be dependent on crystal packing and correlates well with the accessibility calculated in the crystal. Gln60 is a conserved residue in all sequences known to date for this class of ribonucleases. However, it is the only residue that is clearly defined in an unfavorable position (phi = -100-degrees, psi = -130-degrees) on the Ramachandran plot. The origin of the substantial differences between RNase-A and RNase-S in stability to both acid and temperature denaturation and in susceptibility to proteolysis at neutral pH is not obvious in our visual comparison of these two structures.