(a) Fo − Fc OMIT maps at residues 13, 51 and 89 of AH when the three non-disordered crystal structures obtained by the molecular-replacement method were refined separately using the P212121 processed data. From solution 1, the three structural modules of AH are found at three sites in the crystal. In the different solutions, the module rotates between the three sites. Therefore, the amino-acid residues in the same site vary between solutions, e.g. Leu13→Val89→Cys51 at site 1. The densities are contoured at the 3σ level. The negative density coloured brown indicates the S atom of Cys65, suggesting that it is not fully occupied. Its residual density appears between the two hydrophobic amino-acid residues in the other modules. Local 2Fo − Fc map (contoured at the 1.5σ level) of AH (b) and an OMIT map (contoured at the 3.0σ level) of MB (c). The three disordered molecules are coloured red, green and blue, respectively.