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1.  Effects of cryoprotectants on the structure and thermostability of the human carbonic anhydrase II–acetazolamide complex 
Here, a case study of the effects of cryoprotectants on the kinetics of carbonic anhydrase II (CA II) and its inhibition by the clinically used inhibitor acetazolamide (AZM) is presented.
Protein X-ray crystallography has seen a progressive shift from data collection at cool/room temperature (277–298 K) to data collection at cryotemperature (100 K) because of its ease of crystal preparation and the lessening of the detrimental effects of radiation-induced crystal damage, with 20–25%(v/v) glycerol (GOL) being the preferred choice of cryoprotectant. Here, a case study of the effects of cryoprotectants on the kinetics of carbonic anhydrase II (CA II) and its inhibition by the clinically used inhibitor acetazolamide (AZM) is presented. Comparative studies of crystal structure, kinetics, inhibition and thermostability were performed on CA II and its complex with AZM in the presence of either GOL or sucrose. These results suggest that even though the cryoprotectant GOL was previously shown to be directly bound in the active site and to interact with AZM, it affects neither the thermostability of CA II nor the binding of AZM in the crystal structure or in solution. However, addition of GOL does affect the kinetics of CA II, presumably as it displaces the water proton-transfer network in the active site.
doi:10.1107/S0907444913002771
PMCID: PMC3640473  PMID: 23633596
carbonic anhydrase; acetazolamide; cryoprotectants; glycerol; sucrose
2.  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
3.  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
4.  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 1-4 (4)