The structure of F₄₂₀-dependent methylenetetrahydromethanopterin dehydrogenase: a crystallographic `superstructure' of the selenomethionine-labelled protein crystal structure

The diffraction pattern of native protein crystals of F₄₂₀-dependent methylenetetrahydromethanopterin dehydrogenase from Methanopyrus kandleri shows weak additional reflections compared with the selenomethionine-labelled protein crystals, indicating a doubled c unit-cell parameter. These reflections indicate small reorientations of the hexameric structural units, breaking the translational symmetry. TLS refinement of the selenomethionine-labelled protein structure at 1.55 Å resolution revealed an anisotropic rigid-body libration of the hexameric units. The anisotropy is consistent with the static reorientation in the native protein crystals. These results are discussed as related to the crystal packing. The relation between the two structures suggests an analogy to structural changes during certain kinds of phase transitions that have been well studied in inorganic structural chemistry.