Emerging from pseudo‐symmetry: the redetermination of human carbonic anhydrase II in monoclinic P 2 1 with a doubled a axis

The crystal structure of human carbonic anhydrase II in the monoclinic P 2 1 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 . D 66 , 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.