Emerging from pseudo-symmetry: the redetermination of human carbonic anhydrase II in monoclinic P2₁ with a doubled a axis

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.