Robbins Arthur H, Domsic John F, Agbandje-McKenna Mavis, McKenna Robert
Department of Biochemistry and Molecular Biology, University of Florida, Gainesville, FL 32610, USA.
Acta Crystallogr D Biol Crystallogr. 2010 May;66(Pt 5):628-34. doi: 10.1107/S0907444910006797. Epub 2010 Apr 21.
The crystal structure of human carbonic anhydrase II with a doubled a axis from that of the usually observed monoclinic unit cell has been determined and refined to 1.4 A resolution. The diffraction data with h = 2n + 1 were systematically weaker than those with h = 2n. Consequently, the scaling of the data, structure solution and refinement were challenging. The two molecules comprising the asymmetric unit are related by a noncrystallographic translation of (1/2) along a, but one of the molecules has two alternate positions related by a rotation of approximately 2 degrees. This rotation axis is located near the edge of the central beta-sheet, causing a maximum distance disparity of 1.7 A between equivalent atoms on the diametrically opposite side of the molecule. The crystal-packing contacts are similar to two sequential combined unit cells along a of the previously determined monoclinic unit cell. Abnormally high final R(cryst) and R(free) values (20.2% and 23.7%, respectively) are not unusual for structures containing pseudo-translational symmetry and probably result from poor signal to noise in the weak h-odd data.
已确定人碳酸酐酶II的晶体结构，其a轴是通常观察到的单斜晶胞的两倍，并精修至1.4 Å分辨率。h = 2n + 1的衍射数据系统地弱于h = 2n的衍射数据。因此，数据的缩放、结构解析和精修都具有挑战性。构成不对称单元的两个分子通过沿a方向(1/2)的非晶体学平移相关，但其中一个分子有两个通过约2度旋转相关的交替位置。该旋转轴位于中央β折叠的边缘附近，导致分子直径相对侧上等效原子之间的最大距离差异为1.7 Å。晶体堆积接触类似于先前确定的单斜晶胞沿a方向的两个连续组合晶胞。对于包含伪平移对称性的结构来说，最终的R(cryst)和R(free)值异常高(分别为20.2%和23.7%)并不罕见，这可能是由于弱h奇数数据中信噪比不佳所致。
