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1.  Emerging from pseudo-symmetry: the redetermination of human carbonic anhydrase II in monoclinic P21 with a doubled a axis 
The structure of human carbonic anhydrase II in the monoclinic P21 space group with a doubled a axis from that of the usually observed unit cell has been re-determined and shown that the choice for how the four molecules in the unit cell are grouped (based on their coordinates) into pairs that represent a single asymmetric unit determines whether or not rotational disorder is observed/created during refinement.
The crystal structure of human carbonic anhydrase II in the monoclinic P21 space group with a doubled a axis from that of the usually observed unit cell has recently been reported, with one of the two molecules in the asymmetric unit demonstrating rotational disorder [Robbins et al. (2010 ▶), Acta Cryst. D66, 628–634]. The structure has been redetermined, with the coordinates of both pseudo-symmetrically related molecules in the crystallographic asymmetric unit translated by x′ = x ± 1/4, and no rotational disorder is observed. This corresponds to a different choice of how the four molecules in the unit cell should be grouped into pairs that represent a single asymmetric unit.
doi:10.1107/S0907444910023723
PMCID: PMC2917278  PMID: 20693695
doubled axis; systematically weak data; pseudo-translational symmetry; redetermination
2.  Structure of a monoclinic polymorph of human carbonic anhydrase II with a doubled a axis 
The crystal structure of human carbonic anhydrase II with a doubled a axis from that of the usually observed monoclinic cell has been determined and refined to 1.4 Å resolution. The two molecules comprising the asymmetric unit are related by a noncrystallographic translation of ½ along a, but one of the molecules has two alternate orientations related by a rotation of approximately 2°.
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 Å 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 ½ along a, but one of the molecules has two alternate positions related by a rotation of approximately 2°. This rotation axis is located near the edge of the central β-sheet, causing a maximum distance disparity of 1.7 Å 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.
doi:10.1107/S0907444910006797
PMCID: PMC2865368  PMID: 20445238
doubled axis; systematically weak data; crystal contacts; translational symmetry; rotational disorder
3.  Structure of the unbound form of HIV-1 subtype A protease: comparison with unbound forms of proteases from other HIV subtypes 
The crystal structure of the unbound form of HIV-1 subtype A protease has been determined to 1.7 Å resolution. A detailed structural analysis and comparison of the unbound subtype A, B and C protease structures is presented. The results showed that although no inhibitor is present in the active site, the subtype A protease has flaps in the closed position.
The crystal structure of the unbound form of HIV-1 subtype A protease (PR) has been determined to 1.7 Å resolution and refined as a homodimer in the hexagonal space group P61 to an R cryst of 20.5%. The structure is similar in overall shape and fold to the previously determined subtype B, C and F PRs. The major differences lie in the conformation of the flap region. The flaps in the crystal structures of the unbound subtype B and C PRs, which were crystallized in tetragonal space groups, are either semi-open or wide open. In the present structure of subtype A PR the flaps are found in the closed position, a conformation that would be more anticipated in the structure of HIV protease complexed with an inhibitor. The amino-acid differences between the subtypes and their respective crystal space groups are discussed in terms of the differences in the flap conformations.
doi:10.1107/S0907444909054298
PMCID: PMC2827345  PMID: 20179334
HIV-1 proteases; HIV-1 protease subtype A; unbound; crystal packing

Results 1-3 (3)