Pseudo-merohedral twinning in crystals of the dihaem c-type cytochrome DHC2 from Geobacter sulfurreducens

The dihaem cytochrome DHC2 from Geobacter sulfurreducens has been described as a model system for the study of one of the conserved haem-packing motifs typically observed in multihaem cytochromes c, in which the covalent attachment of haem to the protein allows effective spatial clustering of redox centres and a high haem:protein ratio. Heterologously expressed DHC2 has been crystallized previously and the structure has been solved by multiple-wavelength anomalous dispersion (MAD) and refined to a resolution of 1.5 Å (PDB code 2czs). Crystals belonged to the monoclinic space group P2₁, with unit-cell parameters a = 39.65, b = 55.68, c = 39.64 Å, β = 105.91°. Close inspection of the data indicated the presence of pseudo-merohedral twinning; while not hindering structure solution by MAD, this resulted in high refinement R factors. A re-assessment of the diffraction data for DHC2 is presented, together with a refined structure that includes the twinning present with a twinning fraction α of 0.32.