Sevcík Jozef, Dauter Zbigniew, Wilson Keith S
Institute of Molecular Biology, Member of the Centre of Excellence for Molecular Medicine, Slovak Academy of Sciences, Dubravska Cesta 21, 84551 Bratislava, Slovak Republic.
Acta Crystallogr D Biol Crystallogr. 2004 Jul;60(Pt 7):1198-204. doi: 10.1107/S0907444904009035. Epub 2004 Jun 22.
Three different strains of Streptomyces aureofaciens produce the homologous ribonucleases Sa, Sa2 and Sa3. The crystal structures of ribonuclease Sa (RNase Sa) and its complexes with mononucleotides have previously been reported at high resolution. Here, the structures of two crystal forms (I and II) of ribonuclease Sa2 (RNase Sa2) are presented at 1.8 and 1.5 A resolution. The structures were determined by molecular replacement using the coordinates of RNase Sa as a search model and were refined to R factors of 17.5 and 15.0% and R(free) factors of 21.8 and 17.2%, respectively. The asymmetric unit of crystal form I contains three enzyme molecules, two of which have similar structures to those seen for ribonuclease Sa, with Tyr87 at the bottom of their active sites. In the third molecule, Tyr87 has moved substantially: the CA atom moves almost 5 A and the OH of the side chain moves 10 A, inserting itself into the active site of a neighbouring molecule at a similar position to that observed for the nucleotide base in RNase Sa complexes. The asymmetric unit of crystal form II contains two Sa2 molecules, both of which are similar to the usual Sa structures. In one molecule, two main-chain conformations were modelled in the alpha-helix. Finally, a brief comparison is made between the conformations of the Sa2 molecules and those of 34 independent molecules taken from 20 structures of ribonuclease Sa and two independent molecules taken from two structures of ribonuclease Sa3 in various crystal forms.
三种不同的金色链霉菌菌株可产生同源核糖核酸酶Sa、Sa2和Sa3。核糖核酸酶Sa（RNase Sa）及其与单核苷酸复合物的晶体结构此前已被高分辨率报道。在此，以1.8 Å和1.5 Å分辨率展示了核糖核酸酶Sa2（RNase Sa2）的两种晶体形式（I和II）的结构。结构通过以RNase Sa的坐标作为搜索模型的分子置换法确定，并分别精修至R因子为17.5%和15.0%，R（自由）因子为21.8%和17.2%。晶体形式I的不对称单元包含三个酶分子，其中两个具有与核糖核酸酶Sa所见相似的结构，其活性位点底部为Tyr87。在第三个分子中，Tyr87发生了显著移动：CA原子移动了近5 Å，侧链的OH移动了10 Å，自身插入到相邻分子的活性位点，位置与在RNase Sa复合物中观察到的核苷酸碱基相似。晶体形式II的不对称单元包含两个Sa2分子，两者均与常见的Sa结构相似。在一个分子中，α-螺旋中有两种主链构象被建模。最后，对Sa2分子的构象与从20种核糖核酸酶Sa结构中选取的34个独立分子以及从两种不同晶体形式的核糖核酸酶Sa3结构中选取的两个独立分子的构象进行了简要比较。
